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elisp参考手册

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标签: elisp

最新版elisp参考手册

GNU Emacs Lisp Reference Manual For Emacs Version 24.4 Revision 3.1, October 2014 by Bil Lewis, Dan LaLiberte, Richard Stallman, the GNU Manual Group, et al. This is edition 3.1 of the GNU Emacs Lisp Reference Manual, corresponding to Emacs version 24.4. Copyright c 1990–1996, 1998–2014 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “GNU General Public License,” with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.” (a) The FSF’s Back-Cover Text is: “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.” Published by the Free Software Foundation 51 Franklin St, Fifth Floor Boston, MA 02110-1301 USA ISBN 1-882114-74-4 Cover art by Etienne Suvasa. i Short Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 Lisp Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4 Strings and Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 5 Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6 Sequences, Arrays, and Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 88 7 Hash Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 8 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 9 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 10 Control Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 11 Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 12 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 13 Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 14 Customization Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 15 Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 16 Byte Compilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 17 Debugging Lisp Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 18 Reading and Printing Lisp Objects . . . . . . . . . . . . . . . . . . . . . 280 19 Minibuffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 20 Command Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 21 Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 22 Major and Minor Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 23 Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 24 Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470 25 Backups and Auto-Saving . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 26 Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 27 Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540 28 Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 29 Positions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633 30 Markers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 646 31 Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655 32 Non-ASCII Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 717 33 Searching and Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744 34 Syntax Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 769 35 Abbrevs and Abbrev Expansion . . . . . . . . . . . . . . . . . . . . . . . . 785 ii 36 Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 792 37 Emacs Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834 38 Operating System Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . 924 39 Preparing Lisp code for distribution . . . . . . . . . . . . . . . . . . . . 960 A Emacs 23 Antinews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966 B GNU Free Documentation License . . . . . . . . . . . . . . . . . . . . . . 968 C GNU General Public License . . . . . . . . . . . . . . . . . . . . . . . . . . 976 D Tips and Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 987 E GNU Emacs Internals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000 F Standard Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024 G Standard Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1028 H Standard Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035 iii Table of Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Caveats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Lisp History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.3 Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3.1 Some Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3.2 nil and t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3.3 Evaluation Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3.4 Printing Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3.5 Error Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3.6 Buffer Text Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3.7 Format of Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3.7.1 A Sample Function Description . . . . . . . . . . . . . . . . . . . . . . . 4 1.3.7.2 A Sample Variable Description. . . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Version Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2 Lisp Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.1 Printed Representation and Read Syntax . . . . . . . . . . . . . . . . . . . . . . . 8 2.2 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3 Programming Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3.1 Integer Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3.2 Floating-Point Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3.3 Character Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3.3.1 Basic Char Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3.3.2 General Escape Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3.3.3 Control-Character Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.3.4 Meta-Character Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.3.5 Other Character Modifier Bits . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.4 Symbol Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3.5 Sequence Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.6 Cons Cell and List Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.6.1 Drawing Lists as Box Diagrams . . . . . . . . . . . . . . . . . . . . . . 15 2.3.6.2 Dotted Pair Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3.6.3 Association List Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.3.7 Array Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.3.8 String Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.3.8.1 Syntax for Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.3.8.2 Non-ASCII Characters in Strings . . . . . . . . . . . . . . . . . . . . . 19 2.3.8.3 Nonprinting Characters in Strings . . . . . . . . . . . . . . . . . . . 19 2.3.8.4 Text Properties in Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.3.9 Vector Type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.3.10 Char-Table Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.3.11 Bool-Vector Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 iv 2.3.12 Hash Table Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.3.13 Function Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.3.14 Macro Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.3.15 Primitive Function Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.3.16 Byte-Code Function Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.3.17 Autoload Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4 Editing Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4.1 Buffer Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4.2 Marker Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4.3 Window Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4.4 Frame Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.4.5 Terminal Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.4.6 Window Configuration Type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.4.7 Frame Configuration Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.4.8 Process Type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.4.9 Stream Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4.10 Keymap Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4.11 Overlay Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4.12 Font Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.5 Read Syntax for Circular Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.6 Type Predicates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.7 Equality Predicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.1 Integer Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2 Floating-Point Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.3 Type Predicates for Numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.4 Comparison of Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.5 Numeric Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.6 Arithmetic Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.7 Rounding Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.8 Bitwise Operations on Integers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.9 Standard Mathematical Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.10 Random Numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 4 Strings and Characters. . . . . . . . . . . . . . . . . . . . . . . . 48 4.1 String and Character Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.2 Predicates for Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.3 Creating Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.4 Modifying Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.5 Comparison of Characters and Strings . . . . . . . . . . . . . . . . . . . . . . . . . 53 4.6 Conversion of Characters and Strings . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.7 Formatting Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.8 Case Conversion in Lisp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.9 The Case Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 v 5 Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 5.1 Lists and Cons Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 5.2 Predicates on Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 5.3 Accessing Elements of Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 5.4 Building Cons Cells and Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 5.5 Modifying List Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.6 Modifying Existing List Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 5.6.1 Altering List Elements with setcar . . . . . . . . . . . . . . . . . . . . . . 74 5.6.2 Altering the CDR of a List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.6.3 Functions that Rearrange Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5.7 Using Lists as Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5.8 Association Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 5.9 Property Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.9.1 Property Lists and Association Lists . . . . . . . . . . . . . . . . . . . . . . 86 5.9.2 Property Lists Outside Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . 86 6 Sequences, Arrays, and Vectors . . . . . . . . . . . . . . 88 6.1 Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 6.2 Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 6.3 Functions that Operate on Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 6.4 Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 6.5 Functions for Vectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 6.6 Char-Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 6.7 Bool-vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 6.8 Managing a Fixed-Size Ring of Objects . . . . . . . . . . . . . . . . . . . . . . . . 97 7 Hash Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 7.1 Creating Hash Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 7.2 Hash Table Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 7.3 Defining Hash Comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 7.4 Other Hash Table Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 8 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 8.1 Symbol Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 8.2 Defining Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 8.3 Creating and Interning Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 8.4 Symbol Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 8.4.1 Accessing Symbol Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 8.4.2 Standard Symbol Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 vi 9 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 9.1 Kinds of Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 9.1.1 Self-Evaluating Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 9.1.2 Symbol Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 9.1.3 Classification of List Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 9.1.4 Symbol Function Indirection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 9.1.5 Evaluation of Function Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 9.1.6 Lisp Macro Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 9.1.7 Special Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 9.1.8 Autoloading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 9.2 Quoting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 9.3 Backquote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 9.4 Eval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 10 Control Structures . . . . . . . . . . . . . . . . . . . . . . . . . . 124 10.1 Sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 10.2 Conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 10.2.1 Pattern matching case statement . . . . . . . . . . . . . . . . . . . . . . . 127 10.3 Constructs for Combining Conditions . . . . . . . . . . . . . . . . . . . . . . . 129 10.4 Iteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 10.5 Nonlocal Exits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 10.5.1 Explicit Nonlocal Exits: catch and throw . . . . . . . . . . . . . . 131 10.5.2 Examples of catch and throw . . . . . . . . . . . . . . . . . . . . . . . . . . 133 10.5.3 Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 10.5.3.1 How to Signal an Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 10.5.3.2 How Emacs Processes Errors . . . . . . . . . . . . . . . . . . . . . . 136 10.5.3.3 Writing Code to Handle Errors . . . . . . . . . . . . . . . . . . . . 136 10.5.3.4 Error Symbols and Condition Names . . . . . . . . . . . . . . 140 10.5.4 Cleaning Up from Nonlocal Exits . . . . . . . . . . . . . . . . . . . . . . . 141 11 Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 11.1 Global Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 11.2 Variables that Never Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 11.3 Local Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 11.4 When a Variable is “Void” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 11.5 Defining Global Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 11.6 Tips for Defining Variables Robustly . . . . . . . . . . . . . . . . . . . . . . . . 149 11.7 Accessing Variable Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 11.8 Setting Variable Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 11.9 Scoping Rules for Variable Bindings . . . . . . . . . . . . . . . . . . . . . . . . . 152 11.9.1 Dynamic Binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 11.9.2 Proper Use of Dynamic Binding . . . . . . . . . . . . . . . . . . . . . . . . 154 11.9.3 Lexical Binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 11.9.4 Using Lexical Binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 11.10 Buffer-Local Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 11.10.1 Introduction to Buffer-Local Variables . . . . . . . . . . . . . . . . . 157 11.10.2 Creating and Deleting Buffer-Local Bindings . . . . . . . . . . 158 vii 11.10.3 The Default Value of a Buffer-Local Variable . . . . . . . . . . 161 11.11 File Local Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 11.12 Directory Local Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 11.13 Variable Aliases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 11.14 Variables with Restricted Values . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 11.15 Generalized Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 11.15.1 The setf Macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 11.15.2 Defining new setf forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 12 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 12.1 What Is a Function? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 12.2 Lambda Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 12.2.1 Components of a Lambda Expression . . . . . . . . . . . . . . . . . . . 174 12.2.2 A Simple Lambda Expression Example . . . . . . . . . . . . . . . . . 174 12.2.3 Other Features of Argument Lists . . . . . . . . . . . . . . . . . . . . . . 175 12.2.4 Documentation Strings of Functions . . . . . . . . . . . . . . . . . . . . 176 12.3 Naming a Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 12.4 Defining Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 12.5 Calling Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 12.6 Mapping Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 12.7 Anonymous Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 12.8 Accessing Function Cell Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 12.9 Closures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 12.10 Advising Emacs Lisp Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 12.10.1 Primitives to manipulate advices . . . . . . . . . . . . . . . . . . . . . . 186 12.10.2 Advising Named Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 12.10.3 Ways to compose advices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 12.10.4 Adapting code using the old defadvice. . . . . . . . . . . . . . . . . 191 12.11 Declaring Functions Obsolete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 12.12 Inline Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 12.13 The declare Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 12.14 Telling the Compiler that a Function is Defined . . . . . . . . . . . . 195 12.15 Determining whether a Function is Safe to Call . . . . . . . . . . . . 196 12.16 Other Topics Related to Functions . . . . . . . . . . . . . . . . . . . . . . . . . 196 13 Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 13.1 A Simple Example of a Macro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 13.2 Expansion of a Macro Call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 13.3 Macros and Byte Compilation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 13.4 Defining Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 13.5 Common Problems Using Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 13.5.1 Wrong Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 13.5.2 Evaluating Macro Arguments Repeatedly . . . . . . . . . . . . . . . 201 13.5.3 Local Variables in Macro Expansions . . . . . . . . . . . . . . . . . . . 203 13.5.4 Evaluating Macro Arguments in Expansion . . . . . . . . . . . . . 203 13.5.5 How Many Times is the Macro Expanded? . . . . . . . . . . . . . 204 13.6 Indenting Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 viii 14 Customization Settings . . . . . . . . . . . . . . . . . . . . . 206 14.1 Common Item Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 14.2 Defining Customization Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 14.3 Defining Customization Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 14.4 Customization Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 14.4.1 Simple Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 14.4.2 Composite Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 14.4.3 Splicing into Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 14.4.4 Type Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 14.4.5 Defining New Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 14.5 Applying Customizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 14.6 Custom Themes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 15 Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 15.1 How Programs Do Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 15.2 Load Suffixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 15.3 Library Search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 15.4 Loading Non-ASCII Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 15.5 Autoload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 15.6 Repeated Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 15.7 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 15.8 Which File Defined a Certain Symbol . . . . . . . . . . . . . . . . . . . . . . . 236 15.9 Unloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 15.10 Hooks for Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 16 Byte Compilation . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 16.1 Performance of Byte-Compiled Code . . . . . . . . . . . . . . . . . . . . . . . . 239 16.2 Byte-Compilation Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 16.3 Documentation Strings and Compilation . . . . . . . . . . . . . . . . . . . . 242 16.4 Dynamic Loading of Individual Functions . . . . . . . . . . . . . . . . . . . 242 16.5 Evaluation During Compilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 16.6 Compiler Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 16.7 Byte-Code Function Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 16.8 Disassembled Byte-Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 17 Debugging Lisp Programs . . . . . . . . . . . . . . . . . . 249 17.1 The Lisp Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 17.1.1 Entering the Debugger on an Error . . . . . . . . . . . . . . . . . . . . . 249 17.1.2 Debugging Infinite Loops. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 17.1.3 Entering the Debugger on a Function Call . . . . . . . . . . . . . . 251 17.1.4 Explicit Entry to the Debugger . . . . . . . . . . . . . . . . . . . . . . . . . 252 17.1.5 Using the Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 17.1.6 Debugger Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 17.1.7 Invoking the Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 17.1.8 Internals of the Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 17.2 Edebug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 17.2.1 Using Edebug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 ix 17.2.2 Instrumenting for Edebug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 17.2.3 Edebug Execution Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 17.2.4 Jumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 17.2.5 Miscellaneous Edebug Commands . . . . . . . . . . . . . . . . . . . . . . 261 17.2.6 Breaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 17.2.6.1 Edebug Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 17.2.6.2 Global Break Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 17.2.6.3 Source Breakpoints. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 17.2.7 Trapping Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 17.2.8 Edebug Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 17.2.9 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 17.2.10 Evaluation List Buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 17.2.11 Printing in Edebug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 17.2.12 Trace Buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 17.2.13 Coverage Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 17.2.14 The Outside Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 17.2.14.1 Checking Whether to Stop . . . . . . . . . . . . . . . . . . . . . . . 269 17.2.14.2 Edebug Display Update . . . . . . . . . . . . . . . . . . . . . . . . . . 269 17.2.14.3 Edebug Recursive Edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 17.2.15 Edebug and Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 17.2.15.1 Instrumenting Macro Calls . . . . . . . . . . . . . . . . . . . . . . . 270 17.2.15.2 Specification List. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 17.2.15.3 Backtracking in Specifications . . . . . . . . . . . . . . . . . . . . 274 17.2.15.4 Specification Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 17.2.16 Edebug Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 17.3 Debugging Invalid Lisp Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 17.3.1 Excess Open Parentheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 17.3.2 Excess Close Parentheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 17.4 Test Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 17.5 Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 18 Reading and Printing Lisp Objects . . . . . . . . 280 18.1 Introduction to Reading and Printing . . . . . . . . . . . . . . . . . . . . . . . 280 18.2 Input Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 18.3 Input Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 18.4 Output Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 18.5 Output Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 18.6 Variables Affecting Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 x 19 Minibuffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 19.1 Introduction to Minibuffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 19.2 Reading Text Strings with the Minibuffer . . . . . . . . . . . . . . . . . . . 291 19.3 Reading Lisp Objects with the Minibuffer . . . . . . . . . . . . . . . . . . . 294 19.4 Minibuffer History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 19.5 Initial Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 19.6 Completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 19.6.1 Basic Completion Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 19.6.2 Completion and the Minibuffer . . . . . . . . . . . . . . . . . . . . . . . . . 301 19.6.3 Minibuffer Commands that Do Completion . . . . . . . . . . . . . 303 19.6.4 High-Level Completion Functions . . . . . . . . . . . . . . . . . . . . . . 305 19.6.5 Reading File Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 19.6.6 Completion Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 19.6.7 Programmed Completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 19.6.8 Completion in Ordinary Buffers . . . . . . . . . . . . . . . . . . . . . . . . 313 19.7 Yes-or-No Queries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 19.8 Asking Multiple Y-or-N Questions . . . . . . . . . . . . . . . . . . . . . . . . . . 315 19.9 Reading a Password . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 19.10 Minibuffer Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 19.11 Minibuffer Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 19.12 Minibuffer Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 19.13 Recursive Minibuffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 19.14 Minibuffer Miscellany . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 20 Command Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 20.1 Command Loop Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 20.2 Defining Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322 20.2.1 Using interactive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322 20.2.2 Code Characters for interactive . . . . . . . . . . . . . . . . . . . . . . 324 20.2.3 Examples of Using interactive . . . . . . . . . . . . . . . . . . . . . . . 327 20.2.4 Select among Command Alternatives . . . . . . . . . . . . . . . . . . . 328 20.3 Interactive Call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328 20.4 Distinguish Interactive Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 20.5 Information from the Command Loop . . . . . . . . . . . . . . . . . . . . . . . 331 20.6 Adjusting Point After Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 20.7 Input Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 20.7.1 Keyboard Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 20.7.2 Function Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 20.7.3 Mouse Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 20.7.4 Click Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 20.7.5 Drag Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 20.7.6 Button-Down Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 20.7.7 Repeat Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 20.7.8 Motion Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 20.7.9 Focus Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 20.7.10 Miscellaneous System Events . . . . . . . . . . . . . . . . . . . . . . . . . . 341 20.7.11 Event Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 20.7.12 Classifying Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 xi 20.7.13 Accessing Mouse Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 20.7.14 Accessing Scroll Bar Events . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 20.7.15 Putting Keyboard Events in Strings . . . . . . . . . . . . . . . . . . . 348 20.8 Reading Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 20.8.1 Key Sequence Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 20.8.2 Reading One Event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 20.8.3 Modifying and Translating Input Events . . . . . . . . . . . . . . . . 353 20.8.4 Invoking the Input Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 20.8.5 Quoted Character Input. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 20.8.6 Miscellaneous Event Input Features . . . . . . . . . . . . . . . . . . . . 355 20.9 Special Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 20.10 Waiting for Elapsed Time or Input . . . . . . . . . . . . . . . . . . . . . . . . . 357 20.11 Quitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358 20.12 Prefix Command Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360 20.13 Recursive Editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362 20.14 Disabling Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 20.15 Command History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364 20.16 Keyboard Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 21 Keymaps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 21.1 Key Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 21.2 Keymap Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368 21.3 Format of Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368 21.4 Creating Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 21.5 Inheritance and Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 21.6 Prefix Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 21.7 Active Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374 21.8 Searching the Active Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 21.9 Controlling the Active Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 21.10 Key Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379 21.11 Functions for Key Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380 21.12 Changing Key Bindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 21.13 Remapping Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 21.14 Keymaps for Translating Sequences of Events . . . . . . . . . . . . . . 386 21.14.1 Interaction with normal keymaps . . . . . . . . . . . . . . . . . . . . . . 388 21.15 Commands for Binding Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 21.16 Scanning Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 21.17 Menu Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 21.17.1 Defining Menus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 21.17.1.1 Simple Menu Items . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 21.17.1.2 Extended Menu Items . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 21.17.1.3 Menu Separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 21.17.1.4 Alias Menu Items . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 21.17.2 Menus and the Mouse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 21.17.3 Menus and the Keyboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 21.17.4 Menu Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 21.17.5 The Menu Bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 21.17.6 Tool bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400 xii 21.17.7 Modifying Menus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 21.17.8 Easy Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 22 Major and Minor Modes . . . . . . . . . . . . . . . . . . . 406 22.1 Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 22.1.1 Running Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 22.1.2 Setting Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 22.2 Major Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 22.2.1 Major Mode Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 22.2.2 How Emacs Chooses a Major Mode . . . . . . . . . . . . . . . . . . . . 412 22.2.3 Getting Help about a Major Mode . . . . . . . . . . . . . . . . . . . . . 414 22.2.4 Defining Derived Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415 22.2.5 Basic Major Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416 22.2.6 Mode Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 22.2.7 Tabulated List mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 22.2.8 Generic Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420 22.2.9 Major Mode Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420 22.3 Minor Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 22.3.1 Conventions for Writing Minor Modes . . . . . . . . . . . . . . . . . . 423 22.3.2 Keymaps and Minor Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 22.3.3 Defining Minor Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 22.4 Mode Line Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 22.4.1 Mode Line Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 22.4.2 The Data Structure of the Mode Line . . . . . . . . . . . . . . . . . . 428 22.4.3 The Top Level of Mode Line Control . . . . . . . . . . . . . . . . . . . 430 22.4.4 Variables Used in the Mode Line . . . . . . . . . . . . . . . . . . . . . . . 431 22.4.5 %-Constructs in the Mode Line . . . . . . . . . . . . . . . . . . . . . . . . . 433 22.4.6 Properties in the Mode Line. . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 22.4.7 Window Header Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 22.4.8 Emulating Mode Line Formatting . . . . . . . . . . . . . . . . . . . . . . 435 22.5 Imenu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436 22.6 Font Lock Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438 22.6.1 Font Lock Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439 22.6.2 Search-based Fontification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440 22.6.3 Customizing Search-Based Fontification . . . . . . . . . . . . . . . . 443 22.6.4 Other Font Lock Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444 22.6.5 Levels of Font Lock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 22.6.6 Precalculated Fontification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 22.6.7 Faces for Font Lock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446 22.6.8 Syntactic Font Lock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 22.6.9 Multiline Font Lock Constructs . . . . . . . . . . . . . . . . . . . . . . . . 448 22.6.9.1 Font Lock Multiline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448 22.6.9.2 Region to Fontify after a Buffer Change . . . . . . . . . . . 449 22.7 Automatic Indentation of code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 22.7.1 Simple Minded Indentation Engine . . . . . . . . . . . . . . . . . . . . . 450 22.7.1.1 SMIE Setup and Features . . . . . . . . . . . . . . . . . . . . . . . . . 451 22.7.1.2 Operator Precedence Grammars . . . . . . . . . . . . . . . . . . . 451 22.7.1.3 Defining the Grammar of a Language . . . . . . . . . . . . . . 453 xiii 22.7.1.4 Defining Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454 22.7.1.5 Living With a Weak Parser. . . . . . . . . . . . . . . . . . . . . . . . 454 22.7.1.6 Specifying Indentation Rules . . . . . . . . . . . . . . . . . . . . . . 456 22.7.1.7 Helper Functions for Indentation Rules . . . . . . . . . . . . 457 22.7.1.8 Sample Indentation Rules . . . . . . . . . . . . . . . . . . . . . . . . . 458 22.7.1.9 Customizing Indentation . . . . . . . . . . . . . . . . . . . . . . . . . . 459 22.8 Desktop Save Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 23 Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 23.1 Documentation Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 23.2 Access to Documentation Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462 23.3 Substituting Key Bindings in Documentation . . . . . . . . . . . . . . . . 464 23.4 Describing Characters for Help Messages . . . . . . . . . . . . . . . . . . . . 465 23.5 Help Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 24 Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470 24.1 Visiting Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470 24.1.1 Functions for Visiting Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470 24.1.2 Subroutines of Visiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 24.2 Saving Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 24.3 Reading from Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 24.4 Writing to Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477 24.5 File Locks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 24.6 Information about Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 24.6.1 Testing Accessibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 24.6.2 Distinguishing Kinds of Files . . . . . . . . . . . . . . . . . . . . . . . . . . . 482 24.6.3 Truenames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483 24.6.4 File Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 24.6.5 Extended File Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 24.6.6 Locating Files in Standard Places . . . . . . . . . . . . . . . . . . . . . . 488 24.7 Changing File Names and Attributes . . . . . . . . . . . . . . . . . . . . . . . . 488 24.8 File Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 24.8.1 File Name Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 24.8.2 Absolute and Relative File Names . . . . . . . . . . . . . . . . . . . . . . 494 24.8.3 Directory Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495 24.8.4 Functions that Expand Filenames . . . . . . . . . . . . . . . . . . . . . . 496 24.8.5 Generating Unique File Names . . . . . . . . . . . . . . . . . . . . . . . . . 498 24.8.6 File Name Completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 24.8.7 Standard File Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 24.9 Contents of Directories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 24.10 Creating, Copying and Deleting Directories . . . . . . . . . . . . . . . . 503 24.11 Making Certain File Names “Magic” . . . . . . . . . . . . . . . . . . . . . . . 504 24.12 File Format Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508 24.12.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508 24.12.2 Round-Trip Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508 24.12.3 Piecemeal Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510 xiv 25 Backups and Auto-Saving . . . . . . . . . . . . . . . . . . 513 25.1 Backup Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 25.1.1 Making Backup Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 25.1.2 Backup by Renaming or by Copying?. . . . . . . . . . . . . . . . . . . 515 25.1.3 Making and Deleting Numbered Backup Files . . . . . . . . . . 516 25.1.4 Naming Backup Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516 25.2 Auto-Saving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 25.3 Reverting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521 26 Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 26.1 Buffer Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 26.2 The Current Buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 26.3 Buffer Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526 26.4 Buffer File Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527 26.5 Buffer Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529 26.6 Buffer Modification Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 530 26.7 Read-Only Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531 26.8 The Buffer List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 26.9 Creating Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535 26.10 Killing Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536 26.11 Indirect Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537 26.12 Swapping Text Between Two Buffers . . . . . . . . . . . . . . . . . . . . . . . 538 26.13 The Buffer Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539 27 Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540 27.1 Basic Concepts of Emacs Windows . . . . . . . . . . . . . . . . . . . . . . . . . . 540 27.2 Windows and Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541 27.3 Window Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 27.4 Resizing Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549 27.5 Splitting Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552 27.6 Deleting Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554 27.7 Recombining Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556 27.8 Selecting Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561 27.9 Cyclic Ordering of Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562 27.10 Buffers and Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 564 27.11 Switching to a Buffer in a Window . . . . . . . . . . . . . . . . . . . . . . . . . 566 27.12 Choosing a Window for Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567 27.13 Action Functions for display-buffer . . . . . . . . . . . . . . . . . . . . . . 569 27.14 Additional Options for Displaying Buffers . . . . . . . . . . . . . . . . . . 572 27.15 Window History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574 27.16 Dedicated Windows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 576 27.17 Quitting Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 576 27.18 Windows and Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578 27.19 The Window Start and End Positions . . . . . . . . . . . . . . . . . . . . . . 579 27.20 Textual Scrolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582 27.21 Vertical Fractional Scrolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585 27.22 Horizontal Scrolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 xv 27.23 27.24 27.25 27.26 Coordinates and Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588 Window Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590 Window Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593 Hooks for Window Scrolling and Changes . . . . . . . . . . . . . . . . . . 595 28 Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 28.1 Creating Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 598 28.2 Multiple Terminals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 598 28.3 Frame Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602 28.3.1 Access to Frame Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 602 28.3.2 Initial Frame Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603 28.3.3 Window Frame Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604 28.3.3.1 Basic Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604 28.3.3.2 Position Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604 28.3.3.3 Size Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 606 28.3.3.4 Layout Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 606 28.3.3.5 Buffer Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 607 28.3.3.6 Window Management Parameters . . . . . . . . . . . . . . . . . 608 28.3.3.7 Cursor Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 609 28.3.3.8 Font and Color Parameters . . . . . . . . . . . . . . . . . . . . . . . . 609 28.3.4 Frame Size And Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 611 28.3.5 Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614 28.4 Terminal Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614 28.5 Frame Titles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615 28.6 Deleting Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615 28.7 Finding All Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616 28.8 Minibuffers and Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617 28.9 Input Focus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617 28.10 Visibility of Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 619 28.11 Raising and Lowering Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620 28.12 Frame Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620 28.13 Mouse Tracking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621 28.14 Mouse Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621 28.15 Pop-Up Menus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622 28.16 Dialog Boxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623 28.17 Pointer Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624 28.18 Window System Selections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625 28.19 Drag and Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625 28.20 Color Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 626 28.21 Text Terminal Colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627 28.22 X Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 628 28.23 Display Feature Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 629 xvi 29 Positions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633 29.1 Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633 29.2 Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634 29.2.1 Motion by Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634 29.2.2 Motion by Words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635 29.2.3 Motion to an End of the Buffer. . . . . . . . . . . . . . . . . . . . . . . . . 635 29.2.4 Motion by Text Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 636 29.2.5 Motion by Screen Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 637 29.2.6 Moving over Balanced Expressions . . . . . . . . . . . . . . . . . . . . . 640 29.2.7 Skipping Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 641 29.3 Excursions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642 29.4 Narrowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643 30 Markers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 646 30.1 Overview of Markers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 646 30.2 Predicates on Markers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647 30.3 Functions that Create Markers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647 30.4 Information from Markers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 30.5 Marker Insertion Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 30.6 Moving Marker Positions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650 30.7 The Mark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650 30.8 The Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654 31 Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655 31.1 Examining Text Near Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655 31.2 Examining Buffer Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 656 31.3 Comparing Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 658 31.4 Inserting Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 659 31.5 User-Level Insertion Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 661 31.6 Deleting Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 662 31.7 User-Level Deletion Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663 31.8 The Kill Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665 31.8.1 Kill Ring Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665 31.8.2 Functions for Killing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 666 31.8.3 Yanking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 666 31.8.4 Functions for Yanking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 667 31.8.5 Low-Level Kill Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 668 31.8.6 Internals of the Kill Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 670 31.9 Undo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671 31.10 Maintaining Undo Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673 31.11 Filling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674 31.12 Margins for Filling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677 31.13 Adaptive Fill Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 31.14 Auto Filling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 31.15 Sorting Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 31.16 Counting Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 31.17 Indentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 xvii 31.17.1 Indentation Primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 31.17.2 Indentation Controlled by Major Mode . . . . . . . . . . . . . . . . 685 31.17.3 Indenting an Entire Region . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 31.17.4 Indentation Relative to Previous Lines . . . . . . . . . . . . . . . . 687 31.17.5 Adjustable “Tab Stops” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 688 31.17.6 Indentation-Based Motion Commands . . . . . . . . . . . . . . . . . 688 31.18 Case Changes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689 31.19 Text Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 690 31.19.1 Examining Text Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 690 31.19.2 Changing Text Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 692 31.19.3 Text Property Search Functions . . . . . . . . . . . . . . . . . . . . . . . 694 31.19.4 Properties with Special Meanings . . . . . . . . . . . . . . . . . . . . . 696 31.19.5 Formatted Text Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 701 31.19.6 Stickiness of Text Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 702 31.19.7 Lazy Computation of Text Properties . . . . . . . . . . . . . . . . . 703 31.19.8 Defining Clickable Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 703 31.19.9 Defining and Using Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706 31.19.10 Why Text Properties are not Intervals . . . . . . . . . . . . . . . 708 31.20 Substituting for a Character Code . . . . . . . . . . . . . . . . . . . . . . . . . 708 31.21 Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709 31.22 Transposition of Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711 31.23 Dealing With Compressed Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711 31.24 Base 64 Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711 31.25 Checksum/Hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712 31.26 Parsing HTML and XML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713 31.27 Atomic Change Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714 31.28 Change Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715 32 Non-ASCII Characters . . . . . . . . . . . . . . . . . . . . . . 717 32.1 Text Representations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 717 32.2 Disabling Multibyte Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 718 32.3 Converting Text Representations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719 32.4 Selecting a Representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 720 32.5 Character Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 721 32.6 Character Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722 32.7 Character Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 725 32.8 Scanning for Character Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726 32.9 Translation of Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727 32.10 Coding Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728 32.10.1 Basic Concepts of Coding Systems . . . . . . . . . . . . . . . . . . . . 728 32.10.2 Encoding and I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 730 32.10.3 Coding Systems in Lisp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 731 32.10.4 User-Chosen Coding Systems . . . . . . . . . . . . . . . . . . . . . . . . . 734 32.10.5 Default Coding Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735 32.10.6 Specifying a Coding System for One Operation . . . . . . . . 738 32.10.7 Explicit Encoding and Decoding . . . . . . . . . . . . . . . . . . . . . . 739 32.10.8 Terminal I/O Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741 32.11 Input Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741 xviii 32.12 Locales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742 33 Searching and Matching . . . . . . . . . . . . . . . . . . . . 744 33.1 Searching for Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744 33.2 Searching and Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746 33.3 Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747 33.3.1 Syntax of Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . 747 33.3.1.1 Special Characters in Regular Expressions . . . . . . . . . 747 33.3.1.2 Character Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750 33.3.1.3 Backslash Constructs in Regular Expressions . . . . . . 752 33.3.2 Complex Regexp Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755 33.3.3 Regular Expression Functions . . . . . . . . . . . . . . . . . . . . . . . . . . 755 33.4 Regular Expression Searching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 757 33.5 POSIX Regular Expression Searching . . . . . . . . . . . . . . . . . . . . . . . 759 33.6 The Match Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 760 33.6.1 Replacing the Text that Matched. . . . . . . . . . . . . . . . . . . . . . . 760 33.6.2 Simple Match Data Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 761 33.6.3 Accessing the Entire Match Data . . . . . . . . . . . . . . . . . . . . . . . 763 33.6.4 Saving and Restoring the Match Data . . . . . . . . . . . . . . . . . . 764 33.7 Search and Replace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 765 33.8 Standard Regular Expressions Used in Editing . . . . . . . . . . . . . . 768 34 Syntax Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 769 34.1 Syntax Table Concepts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 769 34.2 Syntax Descriptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 770 34.2.1 Table of Syntax Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 770 34.2.2 Syntax Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773 34.3 Syntax Table Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774 34.4 Syntax Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 34.5 Motion and Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 34.6 Parsing Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 34.6.1 Motion Commands Based on Parsing . . . . . . . . . . . . . . . . . . . 778 34.6.2 Finding the Parse State for a Position . . . . . . . . . . . . . . . . . . 778 34.6.3 Parser State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779 34.6.4 Low-Level Parsing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 34.6.5 Parameters to Control Parsing . . . . . . . . . . . . . . . . . . . . . . . . . 781 34.7 Syntax Table Internals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781 34.8 Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782 35 Abbrevs and Abbrev Expansion . . . . . . . . . . . 785 35.1 Abbrev Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785 35.2 Defining Abbrevs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786 35.3 Saving Abbrevs in Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 787 35.4 Looking Up and Expanding Abbreviations . . . . . . . . . . . . . . . . . . 788 35.5 Standard Abbrev Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790 35.6 Abbrev Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790 35.7 Abbrev Table Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791 xix 36 Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 792 36.1 Functions that Create Subprocesses . . . . . . . . . . . . . . . . . . . . . . . . . 792 36.2 Shell Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793 36.3 Creating a Synchronous Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795 36.4 Creating an Asynchronous Process . . . . . . . . . . . . . . . . . . . . . . . . . . 799 36.5 Deleting Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801 36.6 Process Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801 36.7 Sending Input to Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804 36.8 Sending Signals to Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805 36.9 Receiving Output from Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 807 36.9.1 Process Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807 36.9.2 Process Filter Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808 36.9.3 Decoding Process Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810 36.9.4 Accepting Output from Processes . . . . . . . . . . . . . . . . . . . . . . 810 36.10 Sentinels: Detecting Process Status Changes . . . . . . . . . . . . . . . 811 36.11 Querying Before Exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 36.12 Accessing Other Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 36.13 Transaction Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815 36.14 Network Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 36.15 Network Servers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818 36.16 Datagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819 36.17 Low-Level Network Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819 36.17.1 make-network-process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820 36.17.2 Network Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822 36.17.3 Testing Availability of Network Features . . . . . . . . . . . . . . 823 36.18 Misc Network Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824 36.19 Communicating with Serial Ports . . . . . . . . . . . . . . . . . . . . . . . . . . 824 36.20 Packing and Unpacking Byte Arrays . . . . . . . . . . . . . . . . . . . . . . . 827 36.20.1 Describing Data Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 827 36.20.2 Functions to Unpack and Pack Bytes . . . . . . . . . . . . . . . . . . 829 36.20.3 Examples of Byte Unpacking and Packing . . . . . . . . . . . . . 830 37 Emacs Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834 37.1 Refreshing the Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834 37.2 Forcing Redisplay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834 37.3 Truncation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835 37.4 The Echo Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 836 37.4.1 Displaying Messages in the Echo Area . . . . . . . . . . . . . . . . . . 836 37.4.2 Reporting Operation Progress . . . . . . . . . . . . . . . . . . . . . . . . . . 838 37.4.3 Logging Messages in *Messages* . . . . . . . . . . . . . . . . . . . . . . . 840 37.4.4 Echo Area Customization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 840 37.5 Reporting Warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 841 37.5.1 Warning Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 841 37.5.2 Warning Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 842 37.5.3 Warning Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843 37.5.4 Delayed Warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844 37.6 Invisible Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844 37.7 Selective Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 847 xx 37.8 Temporary Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 848 37.9 Overlays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 851 37.9.1 Managing Overlays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 851 37.9.2 Overlay Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 854 37.9.3 Searching for Overlays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 857 37.10 Size of Displayed Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 858 37.11 Line Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 860 37.12 Faces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 861 37.12.1 Face Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 862 37.12.2 Defining Faces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865 37.12.3 Face Attribute Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 867 37.12.4 Displaying Faces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 870 37.12.5 Face Remapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 871 37.12.6 Functions for Working with Faces . . . . . . . . . . . . . . . . . . . . . 872 37.12.7 Automatic Face Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . 873 37.12.8 Basic Faces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873 37.12.9 Font Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874 37.12.10 Looking Up Fonts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 876 37.12.11 Fontsets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 876 37.12.12 Low-Level Font Representation. . . . . . . . . . . . . . . . . . . . . . . 878 37.13 Fringes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880 37.13.1 Fringe Size and Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 881 37.13.2 Fringe Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 881 37.13.3 Fringe Cursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 883 37.13.4 Fringe Bitmaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 883 37.13.5 Customizing Fringe Bitmaps . . . . . . . . . . . . . . . . . . . . . . . . . . 884 37.13.6 The Overlay Arrow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885 37.14 Scroll Bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 886 37.15 Window Dividers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 887 37.16 The display Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 888 37.16.1 Display Specs That Replace The Text . . . . . . . . . . . . . . . . . 888 37.16.2 Specified Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 889 37.16.3 Pixel Specification for Spaces . . . . . . . . . . . . . . . . . . . . . . . . . 890 37.16.4 Other Display Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 891 37.16.5 Displaying in the Margins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 892 37.17 Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893 37.17.1 Image Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893 37.17.2 Image Descriptors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 894 37.17.3 XBM Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 897 37.17.4 XPM Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 898 37.17.5 PostScript Images. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 898 37.17.6 ImageMagick Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 898 37.17.7 Other Image Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 899 37.17.8 Defining Images. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 900 37.17.9 Showing Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 902 37.17.10 Multi-Frame Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903 37.17.11 Image Cache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904 37.18 Buttons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 905 xxi 37.18.1 Button Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 905 37.18.2 Button Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 906 37.18.3 Making Buttons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 906 37.18.4 Manipulating Buttons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 907 37.18.5 Button Buffer Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 908 37.19 Abstract Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 909 37.19.1 Abstract Display Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 910 37.19.2 Abstract Display Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 912 37.20 Blinking Parentheses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914 37.21 Character Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914 37.21.1 Usual Display Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 915 37.21.2 Display Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 916 37.21.3 Active Display Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 917 37.21.4 Glyphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 918 37.21.5 Glyphless Character Display . . . . . . . . . . . . . . . . . . . . . . . . . . 918 37.22 Beeping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 919 37.23 Window Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 920 37.24 Bidirectional Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 921 38 Operating System Interface . . . . . . . . . . . . . . . . 924 38.1 Starting Up Emacs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 924 38.1.1 Summary: Sequence of Actions at Startup . . . . . . . . . . . . . . 924 38.1.2 The Init File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 927 38.1.3 Terminal-Specific Initialization . . . . . . . . . . . . . . . . . . . . . . . . . 928 38.1.4 Command-Line Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 929 38.2 Getting Out of Emacs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 930 38.2.1 Killing Emacs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 930 38.2.2 Suspending Emacs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 931 38.3 Operating System Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933 38.4 User Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 936 38.5 Time of Day . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 938 38.6 Time Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 939 38.7 Parsing and Formatting Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 940 38.8 Processor Run time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 943 38.9 Time Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 944 38.10 Timers for Delayed Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 944 38.11 Idle Timers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946 38.12 Terminal Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 948 38.12.1 Input Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 948 38.12.2 Recording Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949 38.13 Terminal Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949 38.14 Sound Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 950 38.15 Operating on X11 Keysyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 951 38.16 Batch Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 952 38.17 Session Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 952 38.18 Desktop Notifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953 38.19 Notifications on File Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956 38.20 Dynamically Loaded Libraries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 958 xxii 39 Preparing Lisp code for distribution . . . . . . 960 39.1 Packaging Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 960 39.2 Simple Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 961 39.3 Multi-file Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 962 39.4 Creating and Maintaining Package Archives . . . . . . . . . . . . . . . . . 963 Appendix A Emacs 23 Antinews . . . . . . . . . . . . . 966 A.1 Old Lisp Features in Emacs 23 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966 Appendix B GNU Free Documentation License . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 968 Appendix C GNU General Public License. . . 976 Appendix D Tips and Conventions . . . . . . . . . . . 987 D.1 Emacs Lisp Coding Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 987 D.2 Key Binding Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 989 D.3 Emacs Programming Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 990 D.4 Tips for Making Compiled Code Fast . . . . . . . . . . . . . . . . . . . . . . . . 992 D.5 Tips for Avoiding Compiler Warnings . . . . . . . . . . . . . . . . . . . . . . . . 992 D.6 Tips for Documentation Strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993 D.7 Tips on Writing Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996 D.8 Conventional Headers for Emacs Libraries . . . . . . . . . . . . . . . . . . . 997 Appendix E GNU Emacs Internals . . . . . . . . . 1000 E.1 Building Emacs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000 E.2 Pure Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 E.3 Garbage Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 E.4 Memory Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1007 E.5 C Dialect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1007 E.6 Writing Emacs Primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1007 E.7 Object Internals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 E.7.1 Buffer Internals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1012 E.7.2 Window Internals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1017 E.7.3 Process Internals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1021 E.8 C Integer Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1022 Appendix F Standard Errors . . . . . . . . . . . . . . . . 1024 Appendix G Standard Keymaps . . . . . . . . . . . . 1028 Appendix H Standard Hooks . . . . . . . . . . . . . . . . 1031 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035 Chapter 1: Introduction 1 1 Introduction Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. You can write new code in Emacs Lisp and install it as an extension to the editor. However, Emacs Lisp is more than a mere “extension language”; it is a full computer programming language in its own right. You can use it as you would any other programming language. Because Emacs Lisp is designed for use in an editor, it has special features for scanning and parsing text as well as features for handling files, buffers, displays, subprocesses, and so on. Emacs Lisp is closely integrated with the editing facilities; thus, editing commands are functions that can also conveniently be called from Lisp programs, and parameters for customization are ordinary Lisp variables. This manual attempts to be a full description of Emacs Lisp. For a beginner’s introduction to Emacs Lisp, see An Introduction to Emacs Lisp Programming, by Bob Chassell, also published by the Free Software Foundation. This manual presumes considerable familiarity with the use of Emacs for editing; see The GNU Emacs Manual for this basic information. Generally speaking, the earlier chapters describe features of Emacs Lisp that have counterparts in many programming languages, and later chapters describe features that are peculiar to Emacs Lisp or relate specifically to editing. This is edition 3.1 of the GNU Emacs Lisp Reference Manual, corresponding to Emacs version 24.4. 1.1 Caveats This manual has gone through numerous drafts. It is nearly complete but not flawless. There are a few topics that are not covered, either because we consider them secondary (such as most of the individual modes) or because they are yet to be written. Because we are not able to deal with them completely, we have left out several parts intentionally. The manual should be fully correct in what it does cover, and it is therefore open to criticism on anything it says—from specific examples and descriptive text, to the ordering of chapters and sections. If something is confusing, or you find that you have to look at the sources or experiment to learn something not covered in the manual, then perhaps the manual should be fixed. Please let us know. As you use this manual, we ask that you mark pages with corrections so you can later look them up and send them to us. If you think of a simple, real-life example for a function or group of functions, please make an effort to write it up and send it in. Please reference any comments to the chapter name, section name, and function name, as appropriate, since page numbers and chapter and section numbers will change and we may have trouble finding the text you are talking about. Also state the version of the edition you are criticizing. Please send comments and corrections using M-x report-emacs-bug. 1.2 Lisp History Lisp (LISt Processing language) was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it ideal for other purposes as well, such as writing editing commands. Chapter 1: Introduction 2 Dozens of Lisp implementations have been built over the years, each with its own idiosyncrasies. Many of them were inspired by Maclisp, which was written in the 1960s at MIT’s Project MAC. Eventually the implementers of the descendants of Maclisp came together and developed a standard for Lisp systems, called Common Lisp. In the meantime, Gerry Sussman and Guy Steele at MIT developed a simplified but very powerful dialect of Lisp, called Scheme. GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common Lisp. If you know Common Lisp, you will notice many similarities. However, many features of Common Lisp have been omitted or simplified in order to reduce the memory requirements of GNU Emacs. Sometimes the simplifications are so drastic that a Common Lisp user might be very confused. We will occasionally point out how GNU Emacs Lisp differs from Common Lisp. If you don’t know Common Lisp, don’t worry about it; this manual is self-contained. A certain amount of Common Lisp emulation is available via the cl-lib library. See Section “Overview” in Common Lisp Extensions. Emacs Lisp is not at all influenced by Scheme; but the GNU project has an implementation of Scheme, called Guile. We use it in all new GNU software that calls for extensibility. 1.3 Conventions This section explains the notational conventions that are used in this manual. You may want to skip this section and refer back to it later. 1.3.1 Some Terms Throughout this manual, the phrases “the Lisp reader” and “the Lisp printer” refer to those routines in Lisp that convert textual representations of Lisp objects into actual Lisp objects, and vice versa. See Section 2.1 [Printed Representation], page 8, for more details. You, the person reading this manual, are thought of as “the programmer” and are addressed as “you”. “The user” is the person who uses Lisp programs, including those you write. Examples of Lisp code are formatted like this: (list 1 2 3). Names that represent metasyntactic variables, or arguments to a function being described, are formatted like this: first-number. 1.3.2 nil and t In Emacs Lisp, the symbol nil has three separate meanings: it is a symbol with the name ‘nil’; it is the logical truth value false; and it is the empty list—the list of zero elements. When used as a variable, nil always has the value nil. As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are identical: they stand for the same object, the symbol nil. The different ways of writing the symbol are intended entirely for human readers. After the Lisp reader has read either ‘()’ or ‘nil’, there is no way to determine which representation was actually written by the programmer. In this manual, we write () when we wish to emphasize that it means the empty list, and we write nil when we wish to emphasize that it means the truth value false. That is a good convention to use in Lisp programs also. (cons ’foo ()) (setq foo-flag nil) ; Emphasize the empty list ; Emphasize the truth value false Chapter 1: Introduction 3 In contexts where a truth value is expected, any non-nil value is considered to be true. However, t is the preferred way to represent the truth value true. When you need to choose a value which represents true, and there is no other basis for choosing, use t. The symbol t always has the value t. In Emacs Lisp, nil and t are special symbols that always evaluate to themselves. This is so that you do not need to quote them to use them as constants in a program. An attempt to change their values results in a setting-constant error. See Section 11.2 [Constant Variables], page 143. booleanp object [Function] Return non-nil if object is one of the two canonical boolean values: t or nil. 1.3.3 Evaluation Notation A Lisp expression that you can evaluate is called a form. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with ‘⇒’: (car ’(1 2)) ⇒1 You can read this as “(car ’(1 2)) evaluates to 1”. When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with ‘→’. We may or may not show the result of the evaluation of the expanded form. (third ’(a b c)) → (car (cdr (cdr ’(a b c)))) ⇒c To help describe one form, we sometimes show another form that produces identical results. The exact equivalence of two forms is indicated with ‘ ≡ ’. (make-sparse-keymap) ≡ (list ’keymap) 1.3.4 Printing Notation Many of the examples in this manual print text when they are evaluated. If you execute example code in a Lisp Interaction buffer (such as the buffer *scratch*), the printed text is inserted into the buffer. If you execute the example by other means (such as by evaluating the function eval-region), the printed text is displayed in the echo area. Examples in this manual indicate printed text with ‘ ’, irrespective of where that text goes. The value returned by evaluating the form follows on a separate line with ‘⇒’. (progn (prin1 ’foo) (princ "\n") (prin1 ’bar)) foo bar ⇒ bar 1.3.5 Error Messages Some examples signal errors. This normally displays an error message in the echo area. We show the error message on a line starting with ‘ error ’. Note that ‘ error ’ itself does not appear in the echo area. (+ 23 ’x) error Wrong type argument: number-or-marker-p, x Chapter 1: Introduction 4 1.3.6 Buffer Text Notation Some examples describe modifications to the contents of a buffer, by showing the “before” and “after” versions of the text. These examples show the contents of the buffer in question between two lines of dashes containing the buffer name. In addition, ‘ ’ indicates the location of point. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place between two characters where point is currently located.) ---------- Buffer: foo ---------This is the contents of foo. ---------- Buffer: foo ---------- (insert "changed ") ⇒ nil ---------- Buffer: foo ---------This is the changed contents of foo. ---------- Buffer: foo ---------- 1.3.7 Format of Descriptions Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category—function, variable, or whatever—is printed next to the right margin. The description follows on succeeding lines, sometimes with examples. 1.3.7.1 A Sample Function Description In a function description, the name of the function being described appears first. It is followed on the same line by a list of argument names. These names are also used in the body of the description, to stand for the values of the arguments. The appearance of the keyword &optional in the argument list indicates that the subsequent arguments may be omitted (omitted arguments default to nil). Do not write &optional when you call the function. The keyword &rest (which must be followed by a single argument name) indicates that any number of arguments can follow. The single argument name following &rest receives, as its value, a list of all the remaining arguments passed to the function. Do not write &rest when you call the function. Here is a description of an imaginary function foo: foo integer1 &optional integer2 &rest integers [Function] The function foo subtracts integer1 from integer2, then adds all the rest of the arguments to the result. If integer2 is not supplied, then the number 19 is used by default. (foo 1 5 3 9) ⇒ 16 (foo 5) ⇒ 14 Chapter 1: Introduction 5 More generally, (foo w x y...) ≡ (+ (- x w) y...) By convention, any argument whose name contains the name of a type (e.g., integer, integer1 or buffer) is expected to be of that type. A plural of a type (such as buffers) often means a list of objects of that type. An argument named object may be of any type. (For a list of Emacs object types, see Chapter 2 [Lisp Data Types], page 8.) An argument with any other sort of name (e.g., new-file) is specific to the function; if the function has a documentation string, the type of the argument should be described there (see Chapter 23 [Documentation], page 461). See Section 12.2 [Lambda Expressions], page 174, for a more complete description of arguments modified by &optional and &rest. Command, macro, and special form descriptions have the same format, but the word ‘Function’ is replaced by ‘Command’, ‘Macro’, or ‘Special Form’, respectively. Commands are simply functions that may be called interactively; macros process their arguments differently from functions (the arguments are not evaluated), but are presented the same way. The descriptions of macros and special forms use a more complex notation to specify optional and repeated arguments, because they can break the argument list down into separate arguments in more complicated ways. ‘[optional-arg]’ means that optional-arg is optional and ‘repeated-args...’ stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure. Here is an example: count-loop (var [from to [inc]]) body. . . [Special Form] This imaginary special form implements a loop that executes the body forms and then increments the variable var on each iteration. On the first iteration, the variable has the value from; on subsequent iterations, it is incremented by one (or by inc if that is given). The loop exits before executing body if var equals to. Here is an example: (count-loop (i 0 10) (prin1 i) (princ " ") (prin1 (aref vector i)) (terpri)) If from and to are omitted, var is bound to nil before the loop begins, and the loop exits if var is non-nil at the beginning of an iteration. Here is an example: (count-loop (done) (if (pending) (fixit) (setq done t))) In this special form, the arguments from and to are optional, but must both be present or both absent. If they are present, inc may optionally be specified as well. These arguments are grouped with the argument var into a list, to distinguish them from body, which includes all remaining elements of the form. Chapter 1: Introduction 6 1.3.7.2 A Sample Variable Description A variable is a name that can be bound (or set) to an object. The object to which a variable is bound is called a value; we say also that variable holds that value. Although nearly all variables can be set by the user, certain variables exist specifically so that users can change them; these are called user options. Ordinary variables and user options are described using a format like that for functions, except that there are no arguments. Here is a description of the imaginary electric-future-map variable. electric-future-map [Variable] The value of this variable is a full keymap used by Electric Command Future mode. The functions in this map allow you to edit commands you have not yet thought about executing. User option descriptions have the same format, but ‘Variable’ is replaced by ‘User Option’. 1.4 Version Information These facilities provide information about which version of Emacs is in use. emacs-version &optional here [Command] This function returns a string describing the version of Emacs that is running. It is useful to include this string in bug reports. (emacs-version) ⇒ "GNU Emacs 23.1 (i686-pc-linux-gnu, GTK+ Version 2.14.4) of 2009-06-01 on cyd.mit.edu" If here is non-nil, it inserts the text in the buffer before point, and returns nil. When this function is called interactively, it prints the same information in the echo area, but giving a prefix argument makes here non-nil. emacs-build-time [Variable] The value of this variable indicates the time at which Emacs was built. It is a list of four integers, like the value of current-time (see Section 38.5 [Time of Day], page 938). emacs-build-time ⇒ (20614 63694 515336 438000) emacs-version [Variable] The value of this variable is the version of Emacs being run. It is a string such as "23.1.1". The last number in this string is not really part of the Emacs release version number; it is incremented each time Emacs is built in any given directory. A value with four numeric components, such as "22.0.91.1", indicates an unreleased test version. emacs-major-version [Variable] The major version number of Emacs, as an integer. For Emacs version 23.1, the value is 23. emacs-minor-version [Variable] The minor version number of Emacs, as an integer. For Emacs version 23.1, the value is 1. Chapter 1: Introduction 7 1.5 Acknowledgments This manual was originally written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert J. Chassell helped to review and edit the manual, with the support of the Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by Warren A. Hunt, Jr. of Computational Logic, Inc. Additional sections have since been written by Miles Bader, Lars Brinkhoff, Chong Yidong, Kenichi Handa, Lute Kamstra, Juri Linkov, Glenn Morris, Thien-Thi Nguyen, Dan Nicolaescu, Martin Rudalics, Kim F. Storm, Luc Teirlinck, and Eli Zaretskii, and others. Corrections were supplied by Drew Adams, Juanma Barranquero, Karl Berry, Jim Blandy, Bard Bloom, Stephane Boucher, David Boyes, Alan Carroll, Richard Davis, Lawrence R. Dodd, Peter Doornbosch, David A. Duff, Chris Eich, Beverly Erlebacher, David Eckelkamp, Ralf Fassel, Eirik Fuller, Stephen Gildea, Bob Glickstein, Eric Hanchrow, Jesper Harder, George Hartzell, Nathan Hess, Masayuki Ida, Dan Jacobson, Jak Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, Glenn M. Lewis, K. Richard Magill, Brian Marick, Roland McGrath, Stefan Monnier, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potort`ı, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Jason Rumney, Per Starb¨ack, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Eduard Wiebe, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn. For a more complete list of contributors, please see the relevant ChangeLog file in the Emacs sources. Chapter 2: Lisp Data Types 8 2 Lisp Data Types A Lisp object is a piece of data used and manipulated by Lisp programs. For our purposes, a type or data type is a set of possible objects. Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for “the” type of an object. A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called primitive types. Each object belongs to one and only one primitive type. These types include integer, float, cons, symbol, string, vector, hash-table, subr, and byte-code function, plus several special types, such as buffer, that are related to editing. (See Section 2.4 [Editing Types], page 23.) Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type. Lisp is unlike many other languages in that its objects are self-typing: the primitive type of each object is implicit in the object itself. For example, if an object is a vector, nothing can treat it as a number; Lisp knows it is a vector, not a number. In most languages, the programmer must declare the data type of each variable, and the type is known by the compiler but not represented in the data. Such type declarations do not exist in Emacs Lisp. A Lisp variable can have any type of value, and it remembers whatever value you store in it, type and all. (Actually, a small number of Emacs Lisp variables can only take on values of a certain type. See Section 11.14 [Variables with Restricted Values], page 168.) This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters. 2.1 Printed Representation and Read Syntax The printed representation of an object is the format of the output generated by the Lisp printer (the function prin1) for that object. Every data type has a unique printed representation. The read syntax of an object is the format of the input accepted by the Lisp reader (the function read) for that object. This is not necessarily unique; many kinds of object have more than one syntax. See Chapter 18 [Read and Print], page 280. In most cases, an object’s printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in hash notation, which consists of the characters ‘#<’, a descriptive string (typically the type name followed by the name of the object), and a closing ‘>’. For example: (current-buffer) ⇒ # Hash notation cannot be read at all, so the Lisp reader signals the error invalid-readsyntax whenever it encounters ‘#<’. Chapter 2: Lisp Data Types 9 In other languages, an expression is text; it has no other form. In Lisp, an expression is primarily a Lisp object and only secondarily the text that is the object’s read syntax. Often there is no need to emphasize this distinction, but you must keep it in the back of your mind, or you will occasionally be very confused. When you evaluate an expression interactively, the Lisp interpreter first reads the textual representation of it, producing a Lisp object, and then evaluates that object (see Chapter 9 [Evaluation], page 113). However, evaluation and reading are separate activities. Reading returns the Lisp object represented by the text that is read; the object may or may not be evaluated later. See Section 18.3 [Input Functions], page 282, for a description of read, the basic function for reading objects. 2.2 Comments A comment is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a semicolon (‘;’) starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system. The ‘#@count’ construct, which skips the next count characters, is useful for programgenerated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (see Chapter 16 [Byte Compilation], page 239). It isn’t meant for source files, however. See Section D.7 [Comment Tips], page 996, for conventions for formatting comments. 2.3 Programming Types There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former exist in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp. 2.3.1 Integer Type The range of values for an integer depends on the machine. The minimum range is −536,870,912 to 536,870,911 (30 bits; i.e., −229 to 229 − 1) but many machines provide a wider range. Emacs Lisp arithmetic functions do not check for integer overflow. Thus (1+ 536870911) is −536,870,912 if Emacs integers are 30 bits. The read syntax for integers is a sequence of (base ten) digits with an optional sign at the beginning and an optional period at the end. The printed representation produced by the Lisp interpreter never has a leading ‘+’ or a final ‘.’. -1 ; The integer −1. 1 ; The integer 1. 1. ; Also the integer 1. +1 ; Also the integer 1. As a special exception, if a sequence of digits specifies an integer too large or too small to be a valid integer object, the Lisp reader reads it as a floating-point number (see Section 2.3.2 [Floating-Point Type], page 10). For instance, if Emacs integers are 30 bits, 536870912 is read as the floating-point number 536870912.0. See Chapter 3 [Numbers], page 33, for more information. Chapter 2: Lisp Data Types 10 2.3.2 Floating-Point Type Floating-point numbers are the computer equivalent of scientific notation; you can think of a floating-point number as a fraction together with a power of ten. The precise number of significant figures and the range of possible exponents is machine-specific; Emacs uses the C data type double to store the value, and internally this records a power of 2 rather than a power of 10. The printed representation for floating-point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘1500.0’, ‘+15e2’, ‘15.0e+2’, ‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent. See Chapter 3 [Numbers], page 33, for more information. 2.3.3 Character Type A character in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character A is represented as the integer 65. Individual characters are used occasionally in programs, but it is more common to work with strings, which are sequences composed of characters. See Section 2.3.8 [String Type], page 18. Characters in strings and buffers are currently limited to the range of 0 to 4194303— twenty two bits (see Section 32.5 [Character Codes], page 721). Codes 0 through 127 are ASCII codes; the rest are non-ASCII (see Chapter 32 [Non-ASCII Characters], page 717). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift. There are special functions for producing a human-readable textual description of a character for the sake of messages. See Section 23.4 [Describing Characters], page 465. 2.3.3.1 Basic Char Syntax Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is not clear programming. You should always use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark. The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, ‘?A’ for the character A, ‘?B’ for the character B, and ‘?a’ for the character a. For example: ?Q ⇒ 81 ?q ⇒ 113 You can use the same syntax for punctuation characters, but it is often a good idea to add a ‘\’ so that the Emacs commands for editing Lisp code don’t get confused. For example, ‘?\(’ is the way to write the open-paren character. If the character is ‘\’, you must use a second ‘\’ to quote it: ‘?\\’. You can express the characters control-g, backspace, tab, newline, vertical tab, formfeed, space, return, del, and escape as ‘?\a’, ‘?\b’, ‘?\t’, ‘?\n’, ‘?\v’, ‘?\f’, ‘?\s’, ‘?\r’, ‘?\d’, Chapter 2: Lisp Data Types 11 and ‘?\e’, respectively. (‘?\s’ followed by a dash has a different meaning—it applies the “super” modifier to the following character.) Thus, ?\a ⇒ 7 ?\b ⇒ 8 ?\t ⇒ 9 ?\n ⇒ 10 ?\v ⇒ 11 ?\f ⇒ 12 ?\r ⇒ 13 ?\e ⇒ 27 ?\s ⇒ 32 ?\\ ⇒ 92 ?\d ⇒ 127 ; control-g, C-g ; backspace, BS, C-h ; tab, TAB, C-i ; newline, C-j ; vertical tab, C-k ; formfeed character, C-l ; carriage return, RET, C-m ; escape character, ESC, C-[ ; space character, SPC ; backslash character, \ ; delete character, DEL These sequences which start with backslash are also known as escape sequences, because backslash plays the role of an “escape character”; this terminology has nothing to do with the character ESC. ‘\s’ is meant for use in character constants; in string constants, just write the space. A backslash is allowed, and harmless, preceding any character without a special escape meaning; thus, ‘?\+’ is equivalent to ‘?+’. There is no reason to add a backslash before most characters. However, you should add a backslash before any of the characters ‘()\|;’‘"#.,’ to avoid confusing the Emacs commands for editing Lisp code. You can also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of the easily readable escape sequences, such as ‘\t’ or ‘\s’, instead of an actual whitespace character such as a tab or a space. (If you do write backslash followed by a space, you should write an extra space after the character constant to separate it from the following text.) 2.3.3.2 General Escape Syntax In addition to the specific escape sequences for special important control characters, Emacs provides several types of escape syntax that you can use to specify non-ASCII text characters. Firstly, you can specify characters by their Unicode values. ?\unnnn represents a character with Unicode code point ‘U+nnnn’, where nnnn is (by convention) a hexadecimal number with exactly four digits. The backslash indicates that the subsequent characters form an escape sequence, and the ‘u’ specifies a Unicode escape sequence. There is a slightly different syntax for specifying Unicode characters with code points higher than U+ffff: ?\U00nnnnnn represents the character with code point ‘U+nnnnnn’, where nnnnnn is a six-digit hexadecimal number. The Unicode Standard only defines code points up to ‘U+10ffff’, so if you specify a code point higher than that, Emacs signals an error. Secondly, you can specify characters by their hexadecimal character codes. A hexadecimal escape sequence consists of a backslash, ‘x’, and the hexadecimal character code. Thus, ‘?\x41’ is the character A, ‘?\x1’ is the character C-a, and ?\xe0 is the character ‘`a’. You can use any number of hex digits, so you can represent any character code in this way. Thirdly, you can specify characters by their character code in octal. An octal escape sequence consists of a backslash followed by up to three octal digits; thus, ‘?\101’ for the Chapter 2: Lisp Data Types 12 character A, ‘?\001’ for the character C-a, and ?\002 for the character C-b. Only characters up to octal code 777 can be specified this way. These escape sequences may also be used in strings. See Section 2.3.8.2 [Non-ASCII in Strings], page 19. 2.3.3.3 Control-Character Syntax Control characters can be represented using yet another read syntax. This consists of a question mark followed by a backslash, caret, and the corresponding non-control character, in either upper or lower case. For example, both ‘?\^I’ and ‘?\^i’ are valid read syntax for the character C-i, the character whose value is 9. Instead of the ‘^’, you can use ‘C-’; thus, ‘?\C-i’ is equivalent to ‘?\^I’ and to ‘?\^i’: ?\^I ⇒ 9 ?\C-I ⇒ 9 In strings and buffers, the only control characters allowed are those that exist in ASCII; but for keyboard input purposes, you can turn any character into a control character with ‘C-’. The character codes for these non-ASCII control characters include the 226 bit as well as the code for the corresponding non-control character. Ordinary text terminals have no way of generating non-ASCII control characters, but you can generate them straightforwardly using X and other window systems. For historical reasons, Emacs treats the DEL character as the control equivalent of ?: ?\^? ⇒ 127 ?\C-? ⇒ 127 As a result, it is currently not possible to represent the character Control-?, which is a meaningful input character under X, using ‘\C-’. It is not easy to change this, as various Lisp files refer to DEL in this way. For representing control characters to be found in files or strings, we recommend the ‘^’ syntax; for control characters in keyboard input, we prefer the ‘C-’ syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it. 2.3.3.4 Meta-Character Syntax A meta character is a character typed with the META modifier key. The integer that represents such a character has the 227 bit set. We use high bits for this and other modifiers to make possible a wide range of basic character codes. In a string, the 27 bit attached to an ASCII character indicates a meta character; thus, the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters. See Section 20.7.15 [Strings of Events], page 348, for details about META-handling in strings. The read syntax for meta characters uses ‘\M-’. For example, ‘?\M-A’ stands for MA. You can use ‘\M-’ together with octal character codes (see below), with ‘\C-’, or with any other syntax for a character. Thus, you can write M-A as ‘?\M-A’, or as ‘?\M-\101’. Likewise, you can write C-M-b as ‘?\M-\C-b’, ‘?\C-\M-b’, or ‘?\M-\002’. 2.3.3.5 Other Character Modifier Bits The case of a graphic character is indicated by its character code; for example, ASCII distinguishes between the characters ‘a’ and ‘A’. But ASCII has no way to represent whether Chapter 2: Lisp Data Types 13 a control character is upper case or lower case. Emacs uses the 225 bit to indicate that the shift key was used in typing a control character. This distinction is possible only when you use X terminals or other special terminals; ordinary text terminals do not report the distinction. The Lisp syntax for the shift bit is ‘\S-’; thus, ‘?\C-\S-o’ or ‘?\C-\S-O’ represents the shifted-control-o character. The X Window System defines three other modifier bits that can be set in a character: hyper, super and alt. The syntaxes for these bits are ‘\H-’, ‘\s-’ and ‘\A-’. (Case is significant in these prefixes.) Thus, ‘?\H-\M-\A-x’ represents Alt-Hyper-Meta-x. (Note that ‘\s’ with no following ‘-’ represents the space character.) Numerically, the bit values are 222 for alt, 223 for super and 224 for hyper. 2.3.4 Symbol Type A symbol in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary Lisp use, with one single obarray (see Section 8.3 [Creating Symbols], page 107), a symbol’s name is unique—no two symbols have the same name. A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended. But you can use one symbol in all of these ways, independently. A symbol whose name starts with a colon (‘:’) is called a keyword symbol. These symbols automatically act as constants, and are normally used only by comparing an unknown symbol with a few specific alternatives. See Section 11.2 [Constant Variables], page 143. A symbol name can contain any characters whatever. Most symbol names are written with letters, digits, and the punctuation characters ‘-+=*/’. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a ‘\’ at the beginning of the name to force interpretation as a symbol.) The characters ‘_~!@$%^&:<>{}?’ are less often used but also require no special punctuation. Any other characters may be included in a symbol’s name by escaping them with a backslash. In contrast to its use in strings, however, a backslash in the name of a symbol simply quotes the single character that follows the backslash. For example, in a string, ‘\t’ represents a tab character; in the name of a symbol, however, ‘\t’ merely quotes the letter ‘t’. To have a symbol with a tab character in its name, you must actually use a tab (preceded with a backslash). But it’s rare to do such a thing. Common Lisp note: In Common Lisp, lower case letters are always “folded” to upper case, unless they are explicitly escaped. In Emacs Lisp, upper case and lower case letters are distinct. Here are several examples of symbol names. Note that the ‘+’ in the fourth example is escaped to prevent it from being read as a number. This is not necessary in the sixth example because the rest of the name makes it invalid as a number. foo ; A symbol named ‘foo’. FOO ; A symbol named ‘FOO’, different from ‘foo’. 1+ ; A symbol named ‘1+’ ; (not ‘+1’, which is an integer). \+1 ; A symbol named ‘+1’ ; (not a very readable name). Chapter 2: Lisp Data Types 14 \(*\ 1\ 2\) ; A symbol named ‘(* 1 2)’ (a worse name). +-*/_~!@$%^&=:<>{} ; A symbol named ‘+-*/_~!@$%^&=:<>{}’. ; These characters need not be escaped. As an exception to the rule that a symbol’s name serves as its printed representation, ‘##’ is the printed representation for an interned symbol whose name is an empty string. Furthermore, ‘#:foo’ is the printed representation for an uninterned symbol whose name is foo. (Normally, the Lisp reader interns all symbols; see Section 8.3 [Creating Symbols], page 107.) 2.3.5 Sequence Types A sequence is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp: lists and arrays. Lists are the most commonly-used sequences. A list can hold elements of any type, and its length can be easily changed by adding or removing elements. See the next subsection for more about lists. Arrays are fixed-length sequences. They are further subdivided into strings, vectors, char-tables and bool-vectors. Vectors can hold elements of any type, whereas string elements must be characters, and bool-vector elements must be t or nil. Char-tables are like vectors except that they are indexed by any valid character code. The characters in a string can have text properties like characters in a buffer (see Section 31.19 [Text Properties], page 690), but vectors do not support text properties, even when their elements happen to be characters. Lists, strings and the other array types also share important similarities. For example, all have a length l, and all have elements which can be indexed from zero to l minus one. Several functions, called sequence functions, accept any kind of sequence. For example, the function length reports the length of any kind of sequence. See Chapter 6 [Sequences Arrays Vectors], page 88. It is generally impossible to read the same sequence twice, since sequences are always created anew upon reading. If you read the read syntax for a sequence twice, you get two sequences with equal contents. There is one exception: the empty list () always stands for the same object, nil. 2.3.6 Cons Cell and List Types A cons cell is an object that consists of two slots, called the car slot and the cdr slot. Each slot can hold any Lisp object. We also say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr. A list is a series of cons cells, linked together so that the cdr slot of each cons cell holds either the next cons cell or the empty list. The empty list is actually the symbol nil. See Chapter 5 [Lists], page 64, for details. Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a list structure. A note to C programmers: a Lisp list thus works as a linked list built up of cons cells. Because pointers in Lisp are implicit, we do not distinguish between a cons cell slot “holding” a value versus “pointing to” the value. Because cons cells are so central to Lisp, we also have a word for “an object which is not a cons cell”. These objects are called atoms. Chapter 2: Lisp Data Types 15 The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a right parenthesis. Here are examples of lists: (A 2 "A") () nil ("A ()") (A ()) (A nil) ((A B C)) ; A list of three elements. ; A list of no elements (the empty list). ; A list of no elements (the empty list). ; A list of one element: the string "A ()". ; A list of two elements: A and the empty list. ; Equivalent to the previous. ; A list of one element ; (which is a list of three elements). Upon reading, each object inside the parentheses becomes an element of the list. That is, a cons cell is made for each element. The car slot of the cons cell holds the element, and its cdr slot refers to the next cons cell of the list, which holds the next element in the list. The cdr slot of the last cons cell is set to hold nil. The names car and cdr derive from the history of Lisp. The original Lisp implementation ran on an IBM 704 computer which divided words into two parts, called the “address” part and the “decrement”; car was an instruction to extract the contents of the address part of a register, and cdr an instruction to extract the contents of the decrement. By contrast, “cons cells” are named for the function cons that creates them, which in turn was named for its purpose, the construction of cells. 2.3.6.1 Drawing Lists as Box Diagrams A list can be illustrated by a diagram in which the cons cells are shown as pairs of boxes, like dominoes. (The Lisp reader cannot read such an illustration; unlike the textual notation, which can be understood by both humans and computers, the box illustrations can be understood only by humans.) This picture represents the three-element list (rose violet buttercup): --- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup In this diagram, each box represents a slot that can hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell. In this example, the first box, which holds the car of the first cons cell, refers to or “holds” rose (a symbol). The second box, holding the cdr of the first cons cell, refers to the next pair of boxes, the second cons cell. The car of the second cons cell is violet, and its cdr is the third cons cell. The cdr of the third (and last) cons cell is nil. Here is another diagram of the same list, (rose violet buttercup), sketched in a different manner: Chapter 2: Lisp Data Types 16 --------------- ---------------- ------------------- | car | cdr | | car | cdr | | car | cdr | | rose | o-------->| violet | o-------->| buttercup | nil | | | || | | | | | --------------- ---------------- ------------------- A list with no elements in it is the empty list; it is identical to the symbol nil. In other words, nil is both a symbol and a list. Here is the list (A ()), or equivalently (A nil), depicted with boxes and arrows: --- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> A --> nil Here is a more complex illustration, showing the three-element list, ((pine needles) oak maple), the first element of which is a two-element list: --- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | | --> oak --> maple | | --- --- --- --- --> | | |--> | | |--> nil --- --- --- --- | | | | --> pine --> needles The same list represented in the second box notation looks like this: -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | o | o------->| oak | o------->| maple | nil | ||| | | | || | | -- | --------- -------------- -------------- | | | -------------- ---------------- | | car | cdr | | car | cdr | ------>| pine | o------->| needles | nil | | | | | | | -------------- ---------------- 2.3.6.2 Dotted Pair Notation Dotted pair notation is a general syntax for cons cells that represents the car and cdr explicitly. In this syntax, (a . b) stands for a cons cell whose car is the object a and whose Chapter 2: Lisp Data Types 17 cdr is the object b. Dotted pair notation is more general than list syntax because the cdr does not have to be a list. However, it is more cumbersome in cases where list syntax would work. In dotted pair notation, the list ‘(1 2 3)’ is written as ‘(1 . (2 . (3 . nil)))’. For nil-terminated lists, you can use either notation, but list notation is usually clearer and more convenient. When printing a list, the dotted pair notation is only used if the cdr of a cons cell is not a list. Here’s an example using boxes to illustrate dotted pair notation. This example shows the pair (rose . violet): --- --| | |--> violet --- --| | --> rose You can combine dotted pair notation with list notation to represent conveniently a chain of cons cells with a non-nil final cdr. You write a dot after the last element of the list, followed by the cdr of the final cons cell. For example, (rose violet . buttercup) is equivalent to (rose . (violet . buttercup)). The object looks like this: --- --- --- --- | | |--> | | |--> buttercup --- --- --- --- | | | | --> rose --> violet The syntax (rose . violet . buttercup) is invalid because there is nothing that it could mean. If anything, it would say to put buttercup in the cdr of a cons cell whose cdr is already used for violet. The list (rose violet) is equivalent to (rose . (violet)), and looks like this: --- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> rose --> violet Similarly, the three-element list (rose violet buttercup) is equivalent to (rose . (violet . (buttercup))). 2.3.6.3 Association List Type An association list or alist is a specially-constructed list whose elements are cons cells. In each element, the car is considered a key, and the cdr is considered an associated value. (In some cases, the associated value is stored in the car of the cdr.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list. For example, (setq alist-of-colors Chapter 2: Lisp Data Types 18 ’((rose . red) (lily . white) (buttercup . yellow))) sets the variable alist-of-colors to an alist of three elements. In the first element, rose is the key and red is the value. See Section 5.8 [Association Lists], page 82, for a further explanation of alists and for functions that work on alists. See Chapter 7 [Hash Tables], page 100, for another kind of lookup table, which is much faster for handling a large number of keys. 2.3.7 Array Type An array is composed of an arbitrary number of slots for holding or referring to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.) Emacs defines four types of array: strings, vectors, bool-vectors, and char-tables. A string is an array of characters and a vector is an array of arbitrary objects. A boolvector can hold only t or nil. These kinds of array may have any length up to the largest integer. Char-tables are sparse arrays indexed by any valid character code; they can hold arbitrary objects. The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed. All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details. The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table type. 2.3.8 String Type A string is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string. See Chapter 4 [Strings and Characters], page 48, for functions that operate on strings. 2.3.8.1 Syntax for Strings The read syntax for a string is a double-quote, an arbitrary number of characters, and another double-quote, "like this". To include a double-quote in a string, precede it with a backslash; thus, "\"" is a string containing just a single double-quote character. Likewise, you can include a backslash by preceding it with another backslash, like this: "this \\ is a single embedded backslash". The newline character is not special in the read syntax for strings; if you write a new line between the double-quotes, it becomes a character in the string. But an escaped newline— Chapter 2: Lisp Data Types 19 one that is preceded by ‘\’—does not become part of the string; i.e., the Lisp reader ignores an escaped newline while reading a string. An escaped space ‘\ ’ is likewise ignored. "It is useful to include newlines in documentation strings, but the newline is \ ignored if escaped." ⇒ "It is useful to include newlines in documentation strings, but the newline is ignored if escaped." 2.3.8.2 Non-ASCII Characters in Strings There are two text representations for non-ASCII characters in Emacs strings: multibyte and unibyte (see Section 32.1 [Text Representations], page 717). Roughly speaking, unibyte strings store raw bytes, while multibyte strings store human-readable text. Each character in a unibyte string is a byte, i.e., its value is between 0 and 255. By contrast, each character in a multibyte string may have a value between 0 to 4194303 (see Section 2.3.3 [Character Type], page 10). In both cases, characters above 127 are non-ASCII. You can include a non-ASCII character in a string constant by writing it literally. If the string constant is read from a multibyte source, such as a multibyte buffer or string, or a file that would be visited as multibyte, then Emacs reads each non-ASCII character as a multibyte character and automatically makes the string a multibyte string. If the string constant is read from a unibyte source, then Emacs reads the non-ASCII character as unibyte, and makes the string unibyte. Instead of writing a character literally into a multibyte string, you can write it as its character code using an escape sequence. See Section 2.3.3.2 [General Escape Syntax], page 11, for details about escape sequences. If you use any Unicode-style escape sequence ‘\uNNNN’ or ‘\U00NNNNNN’ in a string constant (even for an ASCII character), Emacs automatically assumes that it is multibyte. You can also use hexadecimal escape sequences (‘\xn’) and octal escape sequences (‘\n’) in string constants. But beware: If a string constant contains hexadecimal or octal escape sequences, and these escape sequences all specify unibyte characters (i.e., less than 256), and there are no other literal non-ASCII characters or Unicode-style escape sequences in the string, then Emacs automatically assumes that it is a unibyte string. That is to say, it assumes that all non-ASCII characters occurring in the string are 8-bit raw bytes. In hexadecimal and octal escape sequences, the escaped character code may contain a variable number of digits, so the first subsequent character which is not a valid hexadecimal or octal digit terminates the escape sequence. If the next character in a string could be interpreted as a hexadecimal or octal digit, write ‘\ ’ (backslash and space) to terminate the escape sequence. For example, ‘\xe0\ ’ represents one character, ‘a’ with grave accent. ‘\ ’ in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate any preceding hex escape. 2.3.8.3 Nonprinting Characters in Strings You can use the same backslash escape-sequences in a string constant as in character literals (but do not use the question mark that begins a character constant). For example, you can Chapter 2: Lisp Data Types 20 write a string containing the nonprinting characters tab and C-a, with commas and spaces between them, like this: "\t, \C-a". See Section 2.3.3 [Character Type], page 10, for a description of the read syntax for characters. However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the ASCII control characters. Strings do not distinguish case in ASCII control characters. Properly speaking, strings cannot hold meta characters; but when a string is to be used as a key sequence, there is a special convention that provides a way to represent meta versions of ASCII characters in a string. If you use the ‘\M-’ syntax to indicate a meta character in a string constant, this sets the 27 bit of the character in the string. If the string is used in define-key or lookup-key, this numeric code is translated into the equivalent meta character. See Section 2.3.3 [Character Type], page 10. Strings cannot hold characters that have the hyper, super, or alt modifiers. 2.3.8.4 Text Properties in Strings A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text’s properties with no special effort. See Section 31.19 [Text Properties], page 690, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax: #("characters" property-data...) where property-data consists of zero or more elements, in groups of three as follows: beg end plist The elements beg and end are integers, and together specify a range of indices in the string; plist is the property list for that range. For example, #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic)) represents a string whose textual contents are ‘foo bar’, in which the first three characters have a face property with value bold, and the last three have a face property with value italic. (The fourth character has no text properties, so its property list is nil. It is not actually necessary to mention ranges with nil as the property list, since any characters not mentioned in any range will default to having no properties.) 2.3.9 Vector Type A vector is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.) The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation. [1 "two" (three)] ; A vector of three elements. ⇒ [1 "two" (three)] See Section 6.4 [Vectors], page 92, for functions that work with vectors. Chapter 2: Lisp Data Types 21 2.3.10 Char-Table Type A char-table is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes—for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set. The printed representation of a char-table is like a vector except that there is an extra ‘#^’ at the beginning.1 See Section 6.6 [Char-Tables], page 94, for special functions to operate on char-tables. Uses of char-tables include: • Case tables (see Section 4.9 [Case Tables], page 61). • Character category tables (see Section 34.8 [Categories], page 782). • Display tables (see Section 37.21.2 [Display Tables], page 916). • Syntax tables (see Chapter 34 [Syntax Tables], page 769). 2.3.11 Bool-Vector Type A bool-vector is a one-dimensional array whose elements must be t or nil. The printed representation of a bool-vector is like a string, except that it begins with ‘#&’ followed by the length. The string constant that follows actually specifies the contents of the bool-vector as a bitmap—each “character” in the string contains 8 bits, which specify the next 8 elements of the bool-vector (1 stands for t, and 0 for nil). The least significant bits of the character correspond to the lowest indices in the bool-vector. (make-bool-vector 3 t) ⇒ #&3"^G" (make-bool-vector 3 nil) ⇒ #&3"^@" These results make sense, because the binary code for ‘C-g’ is 111 and ‘C-@’ is the character with code 0. If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. For instance, in the next example, the two bool-vectors are equal, because only the first 3 bits are used: (equal #&3"\377" #&3"\007") ⇒t 2.3.12 Hash Table Type A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. The printed representation of a hash table specifies its properties and contents, like this: (make-hash-table) ⇒ #s(hash-table size 65 test eql rehash-size 1.5 rehash-threshold 0.8 data ()) See Chapter 7 [Hash Tables], page 100, for more information about hash tables. 1 You may also encounter ‘#^^’, used for “sub-char-tables”. Chapter 2: Lisp Data Types 22 2.3.13 Function Type Lisp functions are executable code, just like functions in other programming languages. In Lisp, unlike most languages, functions are also Lisp objects. A non-compiled function in Lisp is a lambda expression: that is, a list whose first element is the symbol lambda (see Section 12.2 [Lambda Expressions], page 174). In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression can be called as a function even though it has no name; to emphasize this, we also call it an anonymous function (see Section 12.7 [Anonymous Functions], page 182). A named function in Lisp is just a symbol with a valid function in its function cell (see Section 12.4 [Defining Functions], page 178). Most of the time, functions are called when their names are written in Lisp expressions in Lisp programs. However, you can construct or obtain a function object at run time and then call it with the primitive functions funcall and apply. See Section 12.5 [Calling Functions], page 179. 2.3.14 Macro Type A Lisp macro is a user-defined construct that extends the Lisp language. It is represented as an object much like a function, but with different argument-passing semantics. A Lisp macro has the form of a list whose first element is the symbol macro and whose cdr is a Lisp function object, including the lambda symbol. Lisp macro objects are usually defined with the built-in defmacro function, but any list that begins with macro is a macro as far as Emacs is concerned. See Chapter 13 [Macros], page 198, for an explanation of how to write a macro. Warning: Lisp macros and keyboard macros (see Section 20.16 [Keyboard Macros], page 365) are entirely different things. When we use the word “macro” without qualification, we mean a Lisp macro, not a keyboard macro. 2.3.15 Primitive Function Type A primitive function is a function callable from Lisp but written in the C programming language. Primitive functions are also called subrs or built-in functions. (The word “subr” is derived from “subroutine”.) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a special form (see Section 9.1.7 [Special Forms], page 117). It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, we discourage redefinition of primitive functions. The term function refers to all Emacs functions, whether written in Lisp or C. See Section 2.3.13 [Function Type], page 22, for information about the functions written in Lisp. Primitive functions have no read syntax and print in hash notation with the name of the subroutine. Chapter 2: Lisp Data Types 23 (symbol-function ’car) ⇒ # (subrp (symbol-function ’car)) ⇒t ; Access the function cell ; of the symbol. ; Is this a primitive function? ; Yes. 2.3.16 Byte-Code Function Type Byte-code function objects are produced by byte-compiling Lisp code (see Chapter 16 [Byte Compilation], page 239). Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears in a function call. See Section 16.7 [Byte-Code Objects], page 245. The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional ‘#’ before the opening ‘[’. 2.3.17 Autoload Type An autoload object is a list whose first element is the symbol autoload. It is stored as the function definition of a symbol, where it serves as a placeholder for the real definition. The autoload object says that the real definition is found in a file of Lisp code that should be loaded when necessary. It contains the name of the file, plus some other information about the real definition. After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user’s point of view, the function call works as expected, using the function definition in the loaded file. An autoload object is usually created with the function autoload, which stores the object in the function cell of a symbol. See Section 15.5 [Autoload], page 230, for more details. 2.4 Editing Types The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing. 2.4.1 Buffer Type A buffer is an object that holds text that can be edited (see Chapter 26 [Buffers], page 523). Most buffers hold the contents of a disk file (see Chapter 24 [Files], page 470) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (see Chapter 27 [Windows], page 540). But a buffer need not be displayed in any window. Each buffer has a designated position called point (see Chapter 29 [Positions], page 633); most editing commands act on the contents of the current buffer in the neighborhood of point. At any time, one buffer is the current buffer. The contents of a buffer are much like a string, but buffers are not used like strings in Emacs Lisp, and the available operations are different. For example, you can insert text Chapter 2: Lisp Data Types 24 efficiently into an existing buffer, altering the buffer’s contents, whereas “inserting” text into a string requires concatenating substrings, and the result is an entirely new string object. Many of the standard Emacs functions manipulate or test the characters in the current buffer; a whole chapter in this manual is devoted to describing these functions (see Chapter 31 [Text], page 655). Several other data structures are associated with each buffer: • a local syntax table (see Chapter 34 [Syntax Tables], page 769); • a local keymap (see Chapter 21 [Keymaps], page 367); and, • a list of buffer-local variable bindings (see Section 11.10 [Buffer-Local Variables], page 157). • overlays (see Section 37.9 [Overlays], page 851). • text properties for the text in the buffer (see Section 31.19 [Text Properties], page 690). The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs. A buffer may be indirect, which means it shares the text of another buffer, but presents it differently. See Section 26.11 [Indirect Buffers], page 537. Buffers have no read syntax. They print in hash notation, showing the buffer name. (current-buffer) ⇒ # 2.4.2 Marker Type A marker denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer’s text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer. Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer. (point-marker) ⇒ # See Chapter 30 [Markers], page 646, for information on how to test, create, copy, and move markers. 2.4.3 Window Type A window describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows. Though many windows may exist simultaneously, at any time one window is designated the selected window. This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer, but this is not necessarily the case. Windows are grouped on the screen into frames; each window belongs to one and only one frame. See Section 2.4.4 [Frame Type], page 25. Chapter 2: Lisp Data Types 25 Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently. (selected-window) ⇒ # See Chapter 27 [Windows], page 540, for a description of the functions that work on windows. 2.4.4 Frame Type A frame is a screen area that contains one or more Emacs windows; we also use the term “frame” to refer to the Lisp object that Emacs uses to refer to the screen area. Frames have no read syntax. They print in hash notation, giving the frame’s title, plus its address in core (useful to identify the frame uniquely). (selected-frame) ⇒ # See Chapter 28 [Frames], page 597, for a description of the functions that work on frames. 2.4.5 Terminal Type A terminal is a device capable of displaying one or more Emacs frames (see Section 2.4.4 [Frame Type], page 25). Terminals have no read syntax. They print in hash notation giving the terminal’s ordinal number and its TTY device file name. (get-device-terminal nil) ⇒ # 2.4.6 Window Configuration Type A window configuration stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later. Window configurations do not have a read syntax; their print syntax looks like ‘#’. See Section 27.24 [Window Configurations], page 590, for a description of several functions related to window configurations. 2.4.7 Frame Configuration Type A frame configuration stores information about the positions, sizes, and contents of the windows in all frames. It is not a primitive type—it is actually a list whose car is frame-configuration and whose cdr is an alist. Each alist element describes one frame, which appears as the car of that element. See Section 28.12 [Frame Configurations], page 620, for a description of several functions related to frame configurations. 2.4.8 Process Type The word process usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running Chapter 2: Lisp Data Types 26 in subprocesses of Emacs, extend the capabilities of Emacs. An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess. Process objects have no read syntax. They print in hash notation, giving the name of the process: (process-list) ⇒ (#) See Chapter 36 [Processes], page 792, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes. 2.4.9 Stream Type A stream is an object that can be used as a source or sink for characters—either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a *Help* buffer, or to the echo area. The object nil, in addition to its other meanings, may be used as a stream. It stands for the value of the variable standard-input or standard-output. Also, the object t as a stream specifies input using the minibuffer (see Chapter 19 [Minibuffers], page 290) or output in the echo area (see Section 37.4 [The Echo Area], page 836). Streams have no special printed representation or read syntax, and print as whatever primitive type they are. See Chapter 18 [Read and Print], page 280, for a description of functions related to streams, including parsing and printing functions. 2.4.10 Keymap Type A keymap maps keys typed by the user to commands. This mapping controls how the user’s command input is executed. A keymap is actually a list whose car is the symbol keymap. See Chapter 21 [Keymaps], page 367, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings. 2.4.11 Overlay Type An overlay specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays have no read syntax, and print in hash notation, giving the buffer name and range of positions. See Section 37.9 [Overlays], page 851, for information on how you can create and use overlays. 2.4.12 Font Type A font specifies how to display text on a graphical terminal. There are actually three separate font types—font objects, font specs, and font entities—each of which has slightly different properties. None of them have a read syntax; their print syntax Chapter 2: Lisp Data Types 27 looks like ‘#’, ‘#’, and ‘#’ respectively. See Section 37.12.12 [Low-Level Font], page 878, for a description of these Lisp objects. 2.5 Read Syntax for Circular Objects To represent shared or circular structures within a complex of Lisp objects, you can use the reader constructs ‘#n=’ and ‘#n#’. Use #n= before an object to label it for later reference; subsequently, you can use #n# to refer the same object in another place. Here, n is some integer. For example, here is how to make a list in which the first element recurs as the third element: (#1=(a) b #1#) This differs from ordinary syntax such as this ((a) b (a)) which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference: (prog1 nil (setq x ’(#1=(a) b #1#))) (eq (nth 0 x) (nth 2 x)) ⇒t (setq x ’((a) b (a))) (eq (nth 0 x) (nth 2 x)) ⇒ nil You can also use the same syntax to make a circular structure, which appears as an “element” within itself. Here is an example: #1=(a #1#) This makes a list whose second element is the list itself. Here’s how you can see that it really works: (prog1 nil (setq x ’#1=(a #1#))) (eq x (cadr x)) ⇒t The Lisp printer can produce this syntax to record circular and shared structure in a Lisp object, if you bind the variable print-circle to a non-nil value. See Section 18.6 [Output Variables], page 288. 2.6 Type Predicates The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use. All built-in functions do check the types of their actual arguments when appropriate, and signal a wrong-type-argument error if an argument is of the wrong type. For example, here is what happens if you pass an argument to + that it cannot handle: Chapter 2: Lisp Data Types 28 (+ 2 ’a) error Wrong type argument: number-or-marker-p, a If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a type predicate function. Emacs has a type predicate for each type, as well as some predicates for combinations of types. A type predicate function takes one argument; it returns t if the argument belongs to the appropriate type, and nil otherwise. Following a general Lisp convention for predicate functions, most type predicates’ names end with ‘p’. Here is an example which uses the predicates listp to check for a list and symbolp to check for a symbol. (defun add-on (x) (cond ((symbolp x) ;; If X is a symbol, put it on LIST. (setq list (cons x list))) ((listp x) ;; If X is a list, add its elements to LIST. (setq list (append x list))) (t ;; We handle only symbols and lists. (error "Invalid argument %s in add-on" x)))) Here is a table of predefined type predicates, in alphabetical order, with references to further information. atom See Section 5.2 [List-related Predicates], page 64. arrayp See Section 6.3 [Array Functions], page 91. bool-vector-p See Section 6.7 [Bool-Vectors], page 96. bufferp See Section 26.1 [Buffer Basics], page 523. byte-code-function-p See Section 2.3.16 [Byte-Code Type], page 23. case-table-p See Section 4.9 [Case Tables], page 61. char-or-string-p See Section 4.2 [Predicates for Strings], page 49. char-table-p See Section 6.6 [Char-Tables], page 94. commandp See Section 20.3 [Interactive Call], page 328. consp See Section 5.2 [List-related Predicates], page 64. custom-variable-p See Section 14.3 [Variable Definitions], page 209. Chapter 2: Lisp Data Types 29 display-table-p See Section 37.21.2 [Display Tables], page 916. floatp See Section 3.3 [Predicates on Numbers], page 35. fontp See Section 37.12.12 [Low-Level Font], page 878. frame-configuration-p See Section 28.12 [Frame Configurations], page 620. frame-live-p See Section 28.6 [Deleting Frames], page 615. framep See Chapter 28 [Frames], page 597. functionp See Chapter 12 [Functions], page 172. hash-table-p See Section 7.4 [Other Hash], page 104. integer-or-marker-p See Section 30.2 [Predicates on Markers], page 647. integerp See Section 3.3 [Predicates on Numbers], page 35. keymapp See Section 21.4 [Creating Keymaps], page 370. keywordp See Section 11.2 [Constant Variables], page 143. listp See Section 5.2 [List-related Predicates], page 64. markerp See Section 30.2 [Predicates on Markers], page 647. wholenump See Section 3.3 [Predicates on Numbers], page 35. nlistp See Section 5.2 [List-related Predicates], page 64. numberp See Section 3.3 [Predicates on Numbers], page 35. number-or-marker-p See Section 30.2 [Predicates on Markers], page 647. overlayp See Section 37.9 [Overlays], page 851. processp See Chapter 36 [Processes], page 792. sequencep See Section 6.1 [Sequence Functions], page 88. stringp See Section 4.2 [Predicates for Strings], page 49. subrp See Section 12.8 [Function Cells], page 184. symbolp See Chapter 8 [Symbols], page 105. syntax-table-p See Chapter 34 [Syntax Tables], page 769. vectorp See Section 6.4 [Vectors], page 92. Chapter 2: Lisp Data Types 30 window-configuration-p See Section 27.24 [Window Configurations], page 590. window-live-p See Section 27.6 [Deleting Windows], page 554. windowp See Section 27.1 [Basic Windows], page 540. booleanp See Section 1.3.2 [nil and t], page 2. string-or-null-p See Section 4.2 [Predicates for Strings], page 49. The most general way to check the type of an object is to call the function type-of. Recall that each object belongs to one and only one primitive type; type-of tells you which one (see Chapter 2 [Lisp Data Types], page 8). But type-of knows nothing about nonprimitive types. In most cases, it is more convenient to use type predicates than type-of. type-of object [Function] This function returns a symbol naming the primitive type of object. The value is one of the symbols bool-vector, buffer, char-table, compiled-function, cons, float, font-entity, font-object, font-spec, frame, hash-table, integer, marker, overlay, process, string, subr, symbol, vector, window, or window-configuration. (type-of 1) ⇒ integer (type-of ’nil) ⇒ symbol (type-of ’()) ⇒ symbol (type-of ’(x)) ⇒ cons ; () is nil. 2.7 Equality Predicates Here we describe functions that test for equality between two objects. Other functions test equality of contents between objects of specific types, e.g., strings. For these predicates, see the appropriate chapter describing the data type. eq object1 object2 [Function] This function returns t if object1 and object2 are the same object, and nil otherwise. If object1 and object2 are integers with the same value, they are considered to be the same object (i.e., eq returns t). If object1 and object2 are symbols with the same name, they are normally the same object—but see Section 8.3 [Creating Symbols], page 107 for exceptions. For other types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarily eq to each other: they are eq only if they are the same object, meaning that a change in the contents of one will be reflected by the same change in the contents of the other. (eq ’foo ’foo) ⇒t Chapter 2: Lisp Data Types 31 (eq 456 456) ⇒t (eq "asdf" "asdf") ⇒ nil (eq "" "") ⇒t ;; This exception occurs because Emacs Lisp ;; makes just one multibyte empty string, to save space. (eq ’(1 (2 (3))) ’(1 (2 (3)))) ⇒ nil (setq foo ’(1 (2 (3)))) ⇒ (1 (2 (3))) (eq foo foo) ⇒t (eq foo ’(1 (2 (3)))) ⇒ nil (eq [(1 2) 3] [(1 2) 3]) ⇒ nil (eq (point-marker) (point-marker)) ⇒ nil The make-symbol function returns an uninterned symbol, distinct from the symbol that is used if you write the name in a Lisp expression. Distinct symbols with the same name are not eq. See Section 8.3 [Creating Symbols], page 107. (eq (make-symbol "foo") ’foo) ⇒ nil equal object1 object2 [Function] This function returns t if object1 and object2 have equal components, and nil oth- erwise. Whereas eq tests if its arguments are the same object, equal looks inside nonidentical arguments to see if their elements or contents are the same. So, if two objects are eq, they are equal, but the converse is not always true. (equal ’foo ’foo) ⇒t (equal 456 456) ⇒t (equal "asdf" "asdf") ⇒t Chapter 2: Lisp Data Types 32 (eq "asdf" "asdf") ⇒ nil (equal ’(1 (2 (3))) ’(1 (2 (3)))) ⇒t (eq ’(1 (2 (3))) ’(1 (2 (3)))) ⇒ nil (equal [(1 2) 3] [(1 2) 3]) ⇒t (eq [(1 2) 3] [(1 2) 3]) ⇒ nil (equal (point-marker) (point-marker)) ⇒t (eq (point-marker) (point-marker)) ⇒ nil Comparison of strings is case-sensitive, but does not take account of text properties— it compares only the characters in the strings. See Section 31.19 [Text Properties], page 690. Use equal-including-properties to also compare text properties. For technical reasons, a unibyte string and a multibyte string are equal if and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (eight-bit-graphic). (see Section 32.1 [Text Representations], page 717). (equal "asdf" "ASDF") ⇒ nil However, two distinct buffers are never considered equal, even if their textual contents are the same. The test for equality is implemented recursively; for example, given two cons cells x and y, (equal x y) returns t if and only if both the expressions below return t: (equal (car x) (car y)) (equal (cdr x) (cdr y)) Because of this recursive method, circular lists may therefore cause infinite recursion (leading to an error). equal-including-properties object1 object2 [Function] This function behaves like equal in all cases but also requires that for two strings to be equal, they have the same text properties. (equal "asdf" (propertize "asdf" ’(asdf t))) ⇒t (equal-including-properties "asdf" (propertize "asdf" ’(asdf t))) ⇒ nil Chapter 3: Numbers 33 3 Numbers GNU Emacs supports two numeric data types: integers and floating-point numbers. Integers are whole numbers such as −3, 0, 7, 13, and 511. Floating-point numbers are numbers with fractional parts, such as −4.5, 0.0, and 2.71828. They can also be expressed in exponential notation: ‘1.5e2’ is the same as ‘150.0’; here, ‘e2’ stands for ten to the second power, and that is multiplied by 1.5. Integer computations are exact, though they may overflow. Floating-point computations often involve rounding errors, as the numbers have a fixed amount of precision. 3.1 Integer Basics The range of values for an integer depends on the machine. The minimum range is −536,870,912 to 536,870,911 (30 bits; i.e., −229 to 229 − 1), but many machines provide a wider range. Many examples in this chapter assume the minimum integer width of 30 bits. The Lisp reader reads an integer as a sequence of digits with optional initial sign and optional final period. An integer that is out of the Emacs range is treated as a floating-point number. 1 ; The integer 1. 1. ; The integer 1. +1 ; Also the integer 1. -1 ; The integer −1. 9000000000000000000 ; The floating-point number 9e18. 0 ; The integer 0. -0 ; The integer 0. The syntax for integers in bases other than 10 uses ‘#’ followed by a letter that specifies the radix: ‘b’ for binary, ‘o’ for octal, ‘x’ for hex, or ‘radixr’ to specify radix radix. Case is not significant for the letter that specifies the radix. Thus, ‘#binteger’ reads integer in binary, and ‘#radixrinteger’ reads integer in radix radix. Allowed values of radix run from 2 to 36. For example: #b101100 ⇒ 44 #o54 ⇒ 44 #x2c ⇒ 44 #24r1k ⇒ 44 To understand how various functions work on integers, especially the bitwise operators (see Section 3.8 [Bitwise Operations], page 42), it is often helpful to view the numbers in their binary form. In 30-bit binary, the decimal integer 5 looks like this: 0000...000101 (30 bits total) (The ‘...’ stands for enough bits to fill out a 30-bit word; in this case, ‘...’ stands for twenty 0 bits. Later examples also use the ‘...’ notation to make binary integers easier to read.) The integer −1 looks like this: Chapter 3: Numbers 34 1111...111111 (30 bits total) −1 is represented as 30 ones. (This is called two’s complement notation.) Subtracting 4 from −1 returns the negative integer −5. In binary, the decimal integer 4 is 100. Consequently, −5 looks like this: 1111...111011 (30 bits total) In this implementation, the largest 30-bit binary integer is 536,870,911 in decimal. In binary, it looks like this: 0111...111111 (30 bits total) Since the arithmetic functions do not check whether integers go outside their range, when you add 1 to 536,870,911, the value is the negative integer −536,870,912: (+ 1 536870911) ⇒ -536870912 ⇒ 1000...000000 (30 bits total) Many of the functions described in this chapter accept markers for arguments in place of numbers. (See Chapter 30 [Markers], page 646.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name number-or-marker. When the argument value is a marker, its position value is used and its buffer is ignored. most-positive-fixnum [Variable] The value of this variable is the largest integer that Emacs Lisp can handle. Typical values are 229 − 1 on 32-bit and 261 − 1 on 64-bit platforms. most-negative-fixnum [Variable] The value of this variable is the smallest integer that Emacs Lisp can handle. It is negative. Typical values are −229 on 32-bit and −261 on 64-bit platforms. In Emacs Lisp, text characters are represented by integers. Any integer between zero and the value of (max-char), inclusive, is considered to be valid as a character. See Section 32.5 [Character Codes], page 721. 3.2 Floating-Point Basics Floating-point numbers are useful for representing numbers that are not integral. The range of floating-point numbers is the same as the range of the C data type double on the machine you are using. On all computers currently supported by Emacs, this is double-precision IEEE floating point. The read syntax for floating-point numbers requires either a decimal point, an exponent, or both. Optional signs (‘+’ or ‘-’) precede the number and its exponent. For example, ‘1500.0’, ‘+15e2’, ‘15.0e+2’, ‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floatingpoint number whose value is 1500. They are all equivalent. Like Common Lisp, Emacs Lisp requires at least one digit after any decimal point in a floating-point number; ‘1500.’ is an integer, not a floating-point number. Emacs Lisp treats -0.0 as numerically equal to ordinary zero with respect to equal and =. This follows the IEEE floating-point standard, which says -0.0 and 0.0 are numerically equal even though other operations can distinguish them. Chapter 3: Numbers 35 The IEEE floating-point standard supports positive infinity and negative infinity as floating-point values. It also provides for a class of values called NaN or “not-a-number”; numerical functions return such values in cases where there is no correct answer. For example, (/ 0.0 0.0) returns a NaN. Although NaN values carry a sign, for practical purposes there is no other significant difference between different NaN values in Emacs Lisp. Here are read syntaxes for these special floating-point values: infinity ‘1.0e+INF’ and ‘-1.0e+INF’ not-a-number ‘0.0e+NaN’ and ‘-0.0e+NaN’ The following functions are specialized for handling floating-point numbers: isnan x [Function] This predicate returns t if its floating-point argument is a NaN, nil otherwise. frexp x [Function] This function returns a cons cell (s . e), where s and e are respectively the significand and exponent of the floating-point number x. If x is finite, then s is a floating-point number between 0.5 (inclusive) and 1.0 (exclusive), e is an integer, and x = s2e. If x is zero or infinity, then s is the same as x. If x is a NaN, then s is also a NaN. If x is zero, then e is 0. ldexp sig &optional exp [Function] This function returns a floating-point number corresponding to the significand sig and exponent exp. copysign x1 x2 [Function] This function copies the sign of x2 to the value of x1, and returns the result. x1 and x2 must be floating point. logb x [Function] This function returns the binary exponent of x. More precisely, the value is the logarithm base 2 of |x|, rounded down to an integer. (logb 10) ⇒3 (logb 10.0e20) ⇒ 69 3.3 Type Predicates for Numbers The functions in this section test for numbers, or for a specific type of number. The functions integerp and floatp can take any type of Lisp object as argument (they would not be of much use otherwise), but the zerop predicate requires a number as its argument. See also integer-or-marker-p and number-or-marker-p, in Section 30.2 [Predicates on Markers], page 647. floatp object [Function] This predicate tests whether its argument is floating point and returns t if so, nil otherwise. Chapter 3: Numbers 36 integerp object [Function] This predicate tests whether its argument is an integer, and returns t if so, nil otherwise. numberp object [Function] This predicate tests whether its argument is a number (either integer or floating point), and returns t if so, nil otherwise. natnump object [Function] This predicate (whose name comes from the phrase “natural number”) tests to see whether its argument is a nonnegative integer, and returns t if so, nil otherwise. 0 is considered non-negative. wholenump is a synonym for natnump. zerop number [Function] This predicate tests whether its argument is zero, and returns t if so, nil otherwise. The argument must be a number. (zerop x) is equivalent to (= x 0). 3.4 Comparison of Numbers To test numbers for numerical equality, you should normally use =, not eq. There can be many distinct floating-point objects with the same numeric value. If you use eq to compare them, then you test whether two values are the same object. By contrast, = compares only the numeric values of the objects. In Emacs Lisp, each integer is a unique Lisp object. Therefore, eq is equivalent to = where integers are concerned. It is sometimes convenient to use eq for comparing an unknown value with an integer, because eq does not report an error if the unknown value is not a number—it accepts arguments of any type. By contrast, = signals an error if the arguments are not numbers or markers. However, it is better programming practice to use = if you can, even for comparing integers. Sometimes it is useful to compare numbers with equal, which treats two numbers as equal if they have the same data type (both integers, or both floating point) and the same value. By contrast, = can treat an integer and a floating-point number as equal. See Section 2.7 [Equality Predicates], page 30. There is another wrinkle: because floating-point arithmetic is not exact, it is often a bad idea to check for equality of floating-point values. Usually it is better to test for approximate equality. Here’s a function to do this: (defvar fuzz-factor 1.0e-6) (defun approx-equal (x y) (or (= x y) (< (/ (abs (- x y)) (max (abs x) (abs y))) fuzz-factor))) Common Lisp note: Comparing numbers in Common Lisp always requires = because Common Lisp implements multi-word integers, and two distinct integer objects can have the same numeric value. Emacs Lisp can have just one integer object for any given value because it has a limited range of integers. Chapter 3: Numbers 37 = number-or-marker &rest number-or-markers [Function] This function tests whether all its arguments are numerically equal, and returns t if so, nil otherwise. eql value1 value2 [Function] This function acts like eq except when both arguments are numbers. It compares numbers by type and numeric value, so that (eql 1.0 1) returns nil, but (eql 1.0 1.0) and (eql 1 1) both return t. /= number-or-marker1 number-or-marker2 [Function] This function tests whether its arguments are numerically equal, and returns t if they are not, and nil if they are. < number-or-marker &rest number-or-markers [Function] This function tests whether each argument is strictly less than the following argument. It returns t if so, nil otherwise. <= number-or-marker &rest number-or-markers [Function] This function tests whether each argument is less than or equal to the following argument. It returns t if so, nil otherwise. > number-or-marker &rest number-or-markers [Function] This function tests whether each argument is strictly greater than the following ar- gument. It returns t if so, nil otherwise. >= number-or-marker &rest number-or-markers [Function] This function tests whether each argument is greater than or equal to the following argument. It returns t if so, nil otherwise. max number-or-marker &rest numbers-or-markers [Function] This function returns the largest of its arguments. If any of the arguments is floating point, the value is returned as floating point, even if it was given as an integer. (max 20) ⇒ 20 (max 1 2.5) ⇒ 2.5 (max 1 3 2.5) ⇒ 3.0 min number-or-marker &rest numbers-or-markers [Function] This function returns the smallest of its arguments. If any of the arguments is floating point, the value is returned as floating point, even if it was given as an integer. (min -4 1) ⇒ -4 abs number This function returns the absolute value of number. [Function] Chapter 3: Numbers 38 3.5 Numeric Conversions To convert an integer to floating point, use the function float. float number [Function] This returns number converted to floating point. If number is already floating point, float returns it unchanged. There are four functions to convert floating-point numbers to integers; they differ in how they round. All accept an argument number and an optional argument divisor. Both arguments may be integers or floating-point numbers. divisor may also be nil. If divisor is nil or omitted, these functions convert number to an integer, or return it unchanged if it already is an integer. If divisor is non-nil, they divide number by divisor and convert the result to an integer. If divisor is zero (whether integer or floating point), Emacs signals an arith-error error. truncate number &optional divisor [Function] This returns number, converted to an integer by rounding towards zero. (truncate 1.2) ⇒1 (truncate 1.7) ⇒1 (truncate -1.2) ⇒ -1 (truncate -1.7) ⇒ -1 floor number &optional divisor [Function] This returns number, converted to an integer by rounding downward (towards nega- tive infinity). If divisor is specified, this uses the kind of division operation that corresponds to mod, rounding downward. (floor 1.2) ⇒1 (floor 1.7) ⇒1 (floor -1.2) ⇒ -2 (floor -1.7) ⇒ -2 (floor 5.99 3) ⇒1 ceiling number &optional divisor [Function] This returns number, converted to an integer by rounding upward (towards positive infinity). (ceiling 1.2) ⇒2 Chapter 3: Numbers 39 (ceiling 1.7) ⇒2 (ceiling -1.2) ⇒ -1 (ceiling -1.7) ⇒ -1 round number &optional divisor [Function] This returns number, converted to an integer by rounding towards the nearest integer. Rounding a value equidistant between two integers returns the even integer. (round 1.2) ⇒1 (round 1.7) ⇒2 (round -1.2) ⇒ -1 (round -1.7) ⇒ -2 3.6 Arithmetic Operations Emacs Lisp provides the traditional four arithmetic operations (addition, subtraction, multiplication, and division), as well as remainder and modulus functions, and functions to add or subtract 1. Except for %, each of these functions accepts both integer and floating-point arguments, and returns a floating-point number if any argument is floating point. Emacs Lisp arithmetic functions do not check for integer overflow. Thus (1+ 536870911) may evaluate to −536870912, depending on your hardware. 1+ number-or-marker This function returns number-or-marker plus 1. For example, [Function] (setq foo 4) ⇒4 (1+ foo) ⇒5 This function is not analogous to the C operator ++—it does not increment a variable. It just computes a sum. Thus, if we continue, foo ⇒4 If you want to increment the variable, you must use setq, like this: (setq foo (1+ foo)) ⇒5 1- number-or-marker This function returns number-or-marker minus 1. [Function] + &rest numbers-or-markers [Function] This function adds its arguments together. When given no arguments, + returns 0. Chapter 3: Numbers 40 (+) ⇒0 (+ 1) ⇒1 (+ 1 2 3 4) ⇒ 10 - &optional number-or-marker &rest more-numbers-or-markers [Function] The - function serves two purposes: negation and subtraction. When - has a single argument, the value is the negative of the argument. When there are multiple ar- guments, - subtracts each of the more-numbers-or-markers from number-or-marker, cumulatively. If there are no arguments, the result is 0. (- 10 1 2 3 4) ⇒0 (- 10) ⇒ -10 (-) ⇒0 * &rest numbers-or-markers [Function] This function multiplies its arguments together, and returns the product. When given no arguments, * returns 1. (*) ⇒1 (* 1) ⇒1 (* 1 2 3 4) ⇒ 24 / dividend divisor &rest divisors [Function] This function divides dividend by divisor and returns the quotient. If there are additional arguments divisors, then it divides dividend by each divisor in turn. Each argument may be a number or a marker. If all the arguments are integers, the result is an integer, obtained by rounding the quotient towards zero after each division. (/ 6 2) ⇒3 (/ 5 2) ⇒2 (/ 5.0 2) ⇒ 2.5 (/ 5 2.0) ⇒ 2.5 (/ 5.0 2.0) ⇒ 2.5 (/ 25 3 2) ⇒4 Chapter 3: Numbers 41 (/ -17 6) ⇒ -2 If you divide an integer by the integer 0, Emacs signals an arith-error error (see Section 10.5.3 [Errors], page 134). Floating-point division of a nonzero number by zero yields either positive or negative infinity (see Section 3.2 [Float Basics], page 34). % dividend divisor [Function] This function returns the integer remainder after division of dividend by divisor. The arguments must be integers or markers. For any two integers dividend and divisor, (+ (% dividend divisor) (* (/ dividend divisor) divisor)) always equals dividend if divisor is nonzero. (% 9 4) ⇒1 (% -9 4) ⇒ -1 (% 9 -4) ⇒1 (% -9 -4) ⇒ -1 mod dividend divisor [Function] This function returns the value of dividend modulo divisor; in other words, the re- mainder after division of dividend by divisor, but with the same sign as divisor. The arguments must be numbers or markers. Unlike %, mod permits floating-point arguments; it rounds the quotient downward (towards minus infinity) to an integer, and uses that quotient to compute the remainder. If divisor is zero, mod signals an arith-error error if both arguments are integers, and returns a NaN otherwise. (mod 9 4) ⇒1 (mod -9 4) ⇒3 (mod 9 -4) ⇒ -3 (mod -9 -4) ⇒ -1 (mod 5.5 2.5) ⇒ .5 For any two numbers dividend and divisor, (+ (mod dividend divisor) (* (floor dividend divisor) divisor)) always equals dividend, subject to rounding error if either argument is floating point and to an arith-error if dividend is an integer and divisor is 0. For floor, see Section 3.5 [Numeric Conversions], page 38. Chapter 3: Numbers 42 3.7 Rounding Operations The functions ffloor, fceiling, fround, and ftruncate take a floating-point argument and return a floating-point result whose value is a nearby integer. ffloor returns the nearest integer below; fceiling, the nearest integer above; ftruncate, the nearest integer in the direction towards zero; fround, the nearest integer. ffloor float [Function] This function rounds float to the next lower integral value, and returns that value as a floating-point number. fceiling float [Function] This function rounds float to the next higher integral value, and returns that value as a floating-point number. ftruncate float [Function] This function rounds float towards zero to an integral value, and returns that value as a floating-point number. fround float [Function] This function rounds float to the nearest integral value, and returns that value as a floating-point number. Rounding a value equidistant between two integers returns the even integer. 3.8 Bitwise Operations on Integers In a computer, an integer is represented as a binary number, a sequence of bits (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, shifting moves the whole sequence left or right one or more places, reproducing the same pattern “moved over”. The bitwise operations in Emacs Lisp apply only to integers. lsh integer1 count [Function] lsh, which is an abbreviation for logical shift, shifts the bits in integer1 to the left count places, or to the right if count is negative, bringing zeros into the vacated bits. If count is negative, lsh shifts zeros into the leftmost (most-significant) bit, producing a positive result even if integer1 is negative. Contrast this with ash, below. Here are two examples of lsh, shifting a pattern of bits one place to the left. We show only the low-order eight bits of the binary pattern; the rest are all zero. (lsh 5 1) ⇒ 10 ;; Decimal 5 becomes decimal 10. 00000101 ⇒ 00001010 (lsh 7 1) ⇒ 14 ;; Decimal 7 becomes decimal 14. 00000111 ⇒ 00001110 Chapter 3: Numbers 43 As the examples illustrate, shifting the pattern of bits one place to the left produces a number that is twice the value of the previous number. Shifting a pattern of bits two places to the left produces results like this (with 8-bit binary numbers): (lsh 3 2) ⇒ 12 ;; Decimal 3 becomes decimal 12. 00000011 ⇒ 00001100 On the other hand, shifting one place to the right looks like this: (lsh 6 -1) ⇒3 ;; Decimal 6 becomes decimal 3. 00000110 ⇒ 00000011 (lsh 5 -1) ⇒2 ;; Decimal 5 becomes decimal 2. 00000101 ⇒ 00000010 As the example illustrates, shifting one place to the right divides the value of a positive integer by two, rounding downward. The function lsh, like all Emacs Lisp arithmetic functions, does not check for overflow, so shifting left can discard significant bits and change the sign of the number. For example, left shifting 536,870,911 produces −2 in the 30-bit implementation: (lsh 536870911 1) ⇒ -2 ; left shift In binary, the argument looks like this: ;; Decimal 536,870,911 0111...111111 (30 bits total) which becomes the following when left shifted: ;; Decimal −2 1111...111110 (30 bits total) ash integer1 count [Function] ash (arithmetic shift) shifts the bits in integer1 to the left count places, or to the right if count is negative. ash gives the same results as lsh except when integer1 and count are both negative. In that case, ash puts ones in the empty bit positions on the left, while lsh puts zeros in those bit positions. Thus, with ash, shifting the pattern of bits one place to the right looks like this: (ash -6 -1) ⇒ -3 ;; Decimal −6 becomes decimal −3. 1111...111010 (30 bits total) ⇒ 1111...111101 (30 bits total) Chapter 3: Numbers 44 In contrast, shifting the pattern of bits one place to the right with lsh looks like this: (lsh -6 -1) ⇒ 536870909 ;; Decimal −6 becomes decimal 536,870,909. 1111...111010 (30 bits total) ⇒ 0111...111101 (30 bits total) Here are other examples: ; 30-bit binary values (lsh 5 2) ; 5 = 0000...000101 ⇒ 20 ; = 0000...010100 (ash 5 2) ⇒ 20 (lsh -5 2) ; -5 = 1111...111011 ⇒ -20 ; = 1111...101100 (ash -5 2) ⇒ -20 (lsh 5 -2) ; 5 = 0000...000101 ⇒1 ; = 0000...000001 (ash 5 -2) ⇒1 (lsh -5 -2) ; -5 = 1111...111011 ⇒ 268435454 ; = 0011...111110 (ash -5 -2) ; -5 = 1111...111011 ⇒ -2 ; = 1111...111110 logand &rest ints-or-markers [Function] This function returns the “logical and” of the arguments: the nth bit is set in the result if, and only if, the nth bit is set in all the arguments. (“Set” means that the value of the bit is 1 rather than 0.) For example, using 4-bit binary numbers, the “logical and” of 13 and 12 is 12: 1101 combined with 1100 produces 1100. In both the binary numbers, the leftmost two bits are set (i.e., they are 1’s), so the leftmost two bits of the returned value are set. However, for the rightmost two bits, each is zero in at least one of the arguments, so the rightmost two bits of the returned value are 0’s. Therefore, (logand 13 12) ⇒ 12 If logand is not passed any argument, it returns a value of −1. This number is an identity element for logand because its binary representation consists entirely of ones. If logand is passed just one argument, it returns that argument. ; 30-bit binary values (logand 14 13) ⇒ 12 ; 14 = 0000...001110 ; 13 = 0000...001101 ; 12 = 0000...001100 (logand 14 13 4) ⇒4 ; 14 = 0000...001110 ; 13 = 0000...001101 ; 4 = 0000...000100 ; 4 = 0000...000100 Chapter 3: Numbers 45 (logand) ⇒ -1 ; -1 = 1111...111111 logior &rest ints-or-markers [Function] This function returns the “inclusive or” of its arguments: the nth bit is set in the result if, and only if, the nth bit is set in at least one of the arguments. If there are no arguments, the result is zero, which is an identity element for this operation. If logior is passed just one argument, it returns that argument. ; 30-bit binary values (logior 12 5) ⇒ 13 ; 12 = 0000...001100 ; 5 = 0000...000101 ; 13 = 0000...001101 (logior 12 5 7) ⇒ 15 ; 12 = 0000...001100 ; 5 = 0000...000101 ; 7 = 0000...000111 ; 15 = 0000...001111 logxor &rest ints-or-markers [Function] This function returns the “exclusive or” of its arguments: the nth bit is set in the result if, and only if, the nth bit is set in an odd number of the arguments. If there are no arguments, the result is 0, which is an identity element for this operation. If logxor is passed just one argument, it returns that argument. ; 30-bit binary values (logxor 12 5) ⇒9 ; 12 = 0000...001100 ; 5 = 0000...000101 ; 9 = 0000...001001 (logxor 12 5 7) ⇒ 14 ; 12 = 0000...001100 ; 5 = 0000...000101 ; 7 = 0000...000111 ; 14 = 0000...001110 lognot integer [Function] This function returns the logical complement of its argument: the nth bit is one in the result if, and only if, the nth bit is zero in integer, and vice-versa. (lognot 5) ⇒ -6 ;; 5 = 0000...000101 (30 bits total) ;; becomes ;; -6 = 1111...111010 (30 bits total) 3.9 Standard Mathematical Functions These mathematical functions allow integers as well as floating-point numbers as arguments. sin arg [Function] cos arg [Function] tan arg [Function] These are the basic trigonometric functions, with argument arg measured in radians. Chapter 3: Numbers 46 asin arg [Function] The value of (asin arg) is a number between −π/2 and π/2 (inclusive) whose sine is arg. If arg is out of range (outside [−1, 1]), asin returns a NaN. acos arg [Function] The value of (acos arg) is a number between 0 and π (inclusive) whose cosine is arg. If arg is out of range (outside [−1, 1]), acos returns a NaN. atan y &optional x [Function] The value of (atan y) is a number between −π/2 and π/2 (exclusive) whose tangent is y. If the optional second argument x is given, the value of (atan y x) is the angle in radians between the vector [x, y] and the X axis. exp arg This is the exponential function; it returns e to the power arg. [Function] log arg &optional base [Function] This function returns the logarithm of arg, with base base. If you don’t specify base, the natural base e is used. If arg or base is negative, log returns a NaN. expt x y [Function] This function returns x raised to power y. If both arguments are integers and y is positive, the result is an integer; in this case, overflow causes truncation, so watch out. If x is a finite negative number and y is a finite non-integer, expt returns a NaN. sqrt arg [Function] This returns the square root of arg. If arg is finite and less than zero, sqrt returns a NaN. In addition, Emacs defines the following common mathematical constants: float-e The mathematical constant e (2.71828. . . ). [Variable] float-pi The mathematical constant pi (3.14159. . . ). [Variable] 3.10 Random Numbers A deterministic computer program cannot generate true random numbers. For most purposes, pseudo-random numbers suffice. A series of pseudo-random numbers is generated in a deterministic fashion. The numbers are not truly random, but they have certain properties that mimic a random series. For example, all possible values occur equally often in a pseudo-random series. Pseudo-random numbers are generated from a “seed”. Starting from any given seed, the random function always generates the same sequence of numbers. By default, Emacs initializes the random seed at startup, in such a way that the sequence of values of random (with overwhelming likelihood) differs in each Emacs run. Sometimes you want the random number sequence to be repeatable. For example, when debugging a program whose behavior depends on the random number sequence, it is helpful Chapter 3: Numbers 47 to get the same behavior in each program run. To make the sequence repeat, execute (random ""). This sets the seed to a constant value for your particular Emacs executable (though it may differ for other Emacs builds). You can use other strings to choose various seed values. random &optional limit [Function] This function returns a pseudo-random integer. Repeated calls return a series of pseudo-random integers. If limit is a positive integer, the value is chosen to be nonnegative and less than limit. Otherwise, the value might be any integer representable in Lisp, i.e., an integer between most-negative-fixnum and most-positive-fixnum (see Section 3.1 [Integer Basics], page 33). If limit is t, it means to choose a new seed as if Emacs were restarting. If limit is a string, it means to choose a new seed based on the string’s contents. Chapter 4: Strings and Characters 48 4 Strings and Characters A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files; to send messages to users; to hold text being copied between buffers; and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters. See Section 20.7.15 [Strings of Events], page 348, for special considerations for strings of keyboard character events. 4.1 String and Character Basics A character is a Lisp object which represents a single character of text. In Emacs Lisp, characters are simply integers; whether an integer is a character or not is determined only by how it is used. See Section 32.5 [Character Codes], page 721, for details about character representation in Emacs. A string is a fixed sequence of characters. It is a type of sequence called a array, meaning that its length is fixed and cannot be altered once it is created (see Chapter 6 [Sequences Arrays Vectors], page 88). Unlike in C, Emacs Lisp strings are not terminated by a distinguished character code. Since strings are arrays, and therefore sequences as well, you can operate on them with the general array and sequence functions documented in Chapter 6 [Sequences Arrays Vectors], page 88. For example, you can access or change individual characters in a string using the functions aref and aset (see Section 6.3 [Array Functions], page 91). However, note that length should not be used for computing the width of a string on display; use string-width (see Section 37.10 [Size of Displayed Text], page 858) instead. There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. For most Lisp programming, you don’t need to be concerned with these two representations. See Section 32.1 [Text Representations], page 717, for details. Sometimes key sequences are represented as unibyte strings. When a unibyte string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are large integers) rather than character codes in the range 128 to 255. Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. See Section 2.3.3 [Character Type], page 10, for more information about keyboard input characters. Strings are useful for holding regular expressions. You can also match regular expressions against strings with string-match (see Section 33.4 [Regexp Search], page 757). The functions match-string (see Section 33.6.2 [Simple Match Data], page 761) and replace-match (see Section 33.6.1 [Replacing Match], page 760) are useful for decomposing and modifying strings after matching regular expressions against them. Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See Section 31.19 [Text Properties], page 690. All the Lisp Chapter 4: Strings and Characters 49 primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied. See Chapter 31 [Text], page 655, for information about functions that display strings or copy them into buffers. See Section 2.3.3 [Character Type], page 10, and Section 2.3.8 [String Type], page 18, for information about the syntax of characters and strings. See Chapter 32 [Non-ASCII Characters], page 717, for functions to convert between text representations and to encode and decode character codes. 4.2 Predicates for Strings For more information about general sequence and array predicates, see Chapter 6 [Sequences Arrays Vectors], page 88, and Section 6.2 [Arrays], page 90. stringp object This function returns t if object is a string, nil otherwise. [Function] string-or-null-p object [Function] This function returns t if object is a string or nil. It returns nil otherwise. char-or-string-p object [Function] This function returns t if object is a string or a character (i.e., an integer), nil otherwise. 4.3 Creating Strings The following functions create strings, either from scratch, or by putting strings together, or by taking them apart. make-string count character [Function] This function returns a string made up of count repetitions of character. If count is negative, an error is signaled. (make-string 5 ?x) ⇒ "xxxxx" (make-string 0 ?x) ⇒ "" Other functions to compare with this one include make-vector (see Section 6.4 [Vectors], page 92) and make-list (see Section 5.4 [Building Lists], page 68). string &rest characters This returns a string containing the characters characters. (string ?a ?b ?c) ⇒ "abc" [Function] substring string start &optional end [Function] This function returns a new string which consists of those characters from string in the range from (and including) the character at the index start up to (but excluding) the character at the index end. The first character is at index zero. Chapter 4: Strings and Characters 50 (substring "abcdefg" 0 3) ⇒ "abc" In the above example, the index for ‘a’ is 0, the index for ‘b’ is 1, and the index for ‘c’ is 2. The index 3—which is the fourth character in the string—marks the character position up to which the substring is copied. Thus, ‘abc’ is copied from the string "abcdefg". A negative number counts from the end of the string, so that −1 signifies the index of the last character of the string. For example: (substring "abcdefg" -3 -1) ⇒ "ef" In this example, the index for ‘e’ is −3, the index for ‘f’ is −2, and the index for ‘g’ is −1. Therefore, ‘e’ and ‘f’ are included, and ‘g’ is excluded. When nil is used for end, it stands for the length of the string. Thus, (substring "abcdefg" -3 nil) ⇒ "efg" Omitting the argument end is equivalent to specifying nil. It follows that (substring string 0) returns a copy of all of string. (substring "abcdefg" 0) ⇒ "abcdefg" But we recommend copy-sequence for this purpose (see Section 6.1 [Sequence Functions], page 88). If the characters copied from string have text properties, the properties are copied into the new string also. See Section 31.19 [Text Properties], page 690. substring also accepts a vector for the first argument. For example: (substring [a b (c) "d"] 1 3) ⇒ [b (c)] A wrong-type-argument error is signaled if start is not an integer or if end is neither an integer nor nil. An args-out-of-range error is signaled if start indicates a character following end, or if either integer is out of range for string. Contrast this function with buffer-substring (see Section 31.2 [Buffer Contents], page 656), which returns a string containing a portion of the text in the current buffer. The beginning of a string is at index 0, but the beginning of a buffer is at index 1. substring-no-properties string &optional start end [Function] This works like substring but discards all text properties from the value. Also, start may be omitted or nil, which is equivalent to 0. Thus, (substring-no-properties string) returns a copy of string, with all text properties removed. concat &rest sequences [Function] This function returns a new string consisting of the characters in the arguments passed to it (along with their text properties, if any). The arguments may be strings, lists of numbers, or vectors of numbers; they are not themselves changed. If concat receives no arguments, it returns an empty string. Chapter 4: Strings and Characters 51 (concat "abc" "-def") ⇒ "abc-def" (concat "abc" (list 120 121) [122]) ⇒ "abcxyz" ;; nil is an empty sequence. (concat "abc" nil "-def") ⇒ "abc-def" (concat "The " "quick brown " "fox.") ⇒ "The quick brown fox." (concat) ⇒ "" This function always constructs a new string that is not eq to any existing string, except when the result is the empty string (to save space, Emacs makes only one empty multibyte string). For information about other concatenation functions, see the description of mapconcat in Section 12.6 [Mapping Functions], page 181, vconcat in Section 6.5 [Vector Functions], page 93, and append in Section 5.4 [Building Lists], page 68. For concatenating individual command-line arguments into a string to be used as a shell command, see Section 36.2 [Shell Arguments], page 793. split-string string &optional separators omit-nulls trim [Function] This function splits string into substrings based on the regular expression separators (see Section 33.3 [Regular Expressions], page 747). Each match for separators defines a splitting point; the substrings between splitting points are made into a list, which is returned. If omit-nulls is nil (or omitted), the result contains null strings whenever there are two consecutive matches for separators, or a match is adjacent to the beginning or end of string. If omit-nulls is t, these null strings are omitted from the result. If separators is nil (or omitted), the default is the value of split-string-defaultseparators. As a special case, when separators is nil (or omitted), null strings are always omitted from the result. Thus: (split-string " two words ") ⇒ ("two" "words") The result is not ("" "two" "words" ""), which would rarely be useful. If you need such a result, use an explicit value for separators: (split-string " two words " split-string-default-separators) ⇒ ("" "two" "words" "") More examples: (split-string "Soup is good food" "o") ⇒ ("S" "up is g" "" "d f" "" "d") (split-string "Soup is good food" "o" t) ⇒ ("S" "up is g" "d f" "d") (split-string "Soup is good food" "o+") Chapter 4: Strings and Characters 52 ⇒ ("S" "up is g" "d f" "d") Empty matches do count, except that split-string will not look for a final empty match when it already reached the end of the string using a non-empty match or when string is empty: (split-string "aooob" "o*") ⇒ ("" "a" "" "b" "") (split-string "ooaboo" "o*") ⇒ ("" "" "a" "b" "") (split-string "" "") ⇒ ("") However, when separators can match the empty string, omit-nulls is usually t, so that the subtleties in the three previous examples are rarely relevant: (split-string "Soup is good food" "o*" t) ⇒ ("S" "u" "p" " " "i" "s" " " "g" "d" " " "f" "d") (split-string "Nice doggy!" "" t) ⇒ ("N" "i" "c" "e" " " "d" "o" "g" "g" "y" "!") (split-string "" "" t) ⇒ nil Somewhat odd, but predictable, behavior can occur for certain “non-greedy” values of separators that can prefer empty matches over non-empty matches. Again, such values rarely occur in practice: (split-string "ooo" "o*" t) ⇒ nil (split-string "ooo" "\\|o+" t) ⇒ ("o" "o" "o") If the optional argument trim is non-nil, it should be a regular expression to match text to trim from the beginning and end of each substring. If trimming makes the substring empty, it is treated as null. If you need to split a string into a list of individual command-line arguments suitable for call-process or start-process, see Section 36.2 [Shell Arguments], page 793. split-string-default-separators The default value of separators for split-string. "[ \f\t\n\r\v]+". [Variable] Its usual value is 4.4 Modifying Strings The most basic way to alter the contents of an existing string is with aset (see Section 6.3 [Array Functions], page 91). (aset string idx char) stores char into string at index idx. Each character occupies one or more bytes, and if char needs a different number of bytes from the character already present at that index, aset signals an error. A more powerful function is store-substring: store-substring string idx obj [Function] This function alters part of the contents of the string string, by storing obj starting at index idx. The argument obj may be either a character or a (smaller) string. Chapter 4: Strings and Characters 53 Since it is impossible to change the length of an existing string, it is an error if obj doesn’t fit within string’s actual length, or if any new character requires a different number of bytes from the character currently present at that point in string. To clear out a string that contained a password, use clear-string: clear-string string [Function] This makes string a unibyte string and clears its contents to zeros. It may also change string’s length. 4.5 Comparison of Characters and Strings char-equal character1 character2 [Function] This function returns t if the arguments represent the same character, nil otherwise. This function ignores differences in case if case-fold-search is non-nil. (char-equal ?x ?x) ⇒t (let ((case-fold-search nil)) (char-equal ?x ?X)) ⇒ nil string= string1 string2 [Function] This function returns t if the characters of the two strings match exactly. Symbols are also allowed as arguments, in which case the symbol names are used. Case is always significant, regardless of case-fold-search. This function is equivalent to equal for comparing two strings (see Section 2.7 [Equality Predicates], page 30). In particular, the text properties of the two strings are ignored; use equal-including-properties if you need to distinguish between strings that differ only in their text properties. However, unlike equal, if either argument is not a string or symbol, string= signals an error. (string= "abc" "abc") ⇒t (string= "abc" "ABC") ⇒ nil (string= "ab" "ABC") ⇒ nil For technical reasons, a unibyte and a multibyte string are equal if and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (eight-bit-graphic). However, when a unibyte string is converted to a multibyte string, all characters with codes in the range 160 through 255 are converted to characters with higher codes, whereas ASCII characters remain unchanged. Thus, a unibyte string and its conversion to multibyte are only equal if the string is all ASCII. Character codes 160 through 255 are not entirely proper in multibyte text, even though they can occur. As a consequence, the situation where a unibyte and a multibyte string are equal without both being all ASCII is a technical oddity that very few Emacs Lisp programmers ever get confronted with. See Section 32.1 [Text Representations], page 717. Chapter 4: Strings and Characters 54 string-equal string1 string2 string-equal is another name for string=. [Function] string< string1 string2 [Function] This function compares two strings a character at a time. It scans both the strings at the same time to find the first pair of corresponding characters that do not match. If the lesser character of these two is the character from string1, then string1 is less, and this function returns t. If the lesser character is the one from string2, then string1 is greater, and this function returns nil. If the two strings match entirely, the value is nil. Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (see Section 32.1 [Text Representations], page 717). (string< "abc" "abd") ⇒t (string< "abd" "abc") ⇒ nil (string< "123" "abc") ⇒t When the strings have different lengths, and they match up to the length of string1, then the result is t. If they match up to the length of string2, the result is nil. A string of no characters is less than any other string. (string< "" "abc") ⇒t (string< "ab" "abc") ⇒t (string< "abc" "") ⇒ nil (string< "abc" "ab") ⇒ nil (string< "" "") ⇒ nil Symbols are also allowed as arguments, in which case their print names are used. string-lessp string1 string2 string-lessp is another name for string<. [Function] string-prefix-p string1 string2 &optional ignore-case [Function] This function returns non-nil if string1 is a prefix of string2; i.e., if string2 starts with string1. If the optional argument ignore-case is non-nil, the comparison ignores case differences. Chapter 4: Strings and Characters 55 string-suffix-p suffix string &optional ignore-case [Function] This function returns non-nil if suffix is a suffix of string; i.e., if string ends with suffix. If the optional argument ignore-case is non-nil, the comparison ignores case differences. compare-strings string1 start1 end1 string2 start2 end2 &optional [Function] ignore-case This function compares a specified part of string1 with a specified part of string2. The specified part of string1 runs from index start1 (inclusive) up to index end1 (exclusive); nil for start1 means the start of the string, while nil for end1 means the length of the string. Likewise, the specified part of string2 runs from index start2 up to index end2. The strings are compared by the numeric values of their characters. For instance, str1 is considered “smaller than” str2 if its first differing character has a smaller numeric value. If ignore-case is non-nil, characters are converted to lower-case before comparing them. Unibyte strings are converted to multibyte for comparison (see Section 32.1 [Text Representations], page 717), so that a unibyte string and its conversion to multibyte are always regarded as equal. If the specified portions of the two strings match, the value is t. Otherwise, the value is an integer which indicates how many leading characters agree, and which string is less. Its absolute value is one plus the number of characters that agree at the beginning of the two strings. The sign is negative if string1 (or its specified portion) is less. assoc-string key alist &optional case-fold [Function] This function works like assoc, except that key must be a string or symbol, and comparison is done using compare-strings. Symbols are converted to strings before testing. If case-fold is non-nil, it ignores case differences. Unlike assoc, this function can also match elements of the alist that are strings or symbols rather than conses. In particular, alist can be a list of strings or symbols rather than an actual alist. See Section 5.8 [Association Lists], page 82. See also the function compare-buffer-substrings in Section 31.3 [Comparing Text], page 658, for a way to compare text in buffers. The function string-match, which matches a regular expression against a string, can be used for a kind of string comparison; see Section 33.4 [Regexp Search], page 757. 4.6 Conversion of Characters and Strings This section describes functions for converting between characters, strings and integers. format (see Section 4.7 [Formatting Strings], page 57) and prin1-to-string (see Section 18.5 [Output Functions], page 285) can also convert Lisp objects into strings. read-from-string (see Section 18.3 [Input Functions], page 282) can “convert” a string representation of a Lisp object into an object. The functions string-to-multibyte and string-to-unibyte convert the text representation of a string (see Section 32.3 [Converting Representations], page 719). Chapter 4: Strings and Characters 56 See Chapter 23 [Documentation], page 461, for functions that produce textual descriptions of text characters and general input events (single-key-description and text-char-description). These are used primarily for making help messages. number-to-string number [Function] This function returns a string consisting of the printed base-ten representation of number. The returned value starts with a minus sign if the argument is negative. (number-to-string 256) ⇒ "256" (number-to-string -23) ⇒ "-23" (number-to-string -23.5) ⇒ "-23.5" int-to-string is a semi-obsolete alias for this function. See also the function format in Section 4.7 [Formatting Strings], page 57. string-to-number string &optional base [Function] This function returns the numeric value of the characters in string. If base is non-nil, it must be an integer between 2 and 16 (inclusive), and integers are converted in that base. If base is nil, then base ten is used. Floating-point conversion only works in base ten; we have not implemented other radices for floating-point numbers, because that would be much more work and does not seem useful. If string looks like an integer but its value is too large to fit into a Lisp integer, string-to-number returns a floating-point result. The parsing skips spaces and tabs at the beginning of string, then reads as much of string as it can interpret as a number in the given base. (On some systems it ignores other whitespace at the beginning, not just spaces and tabs.) If string cannot be interpreted as a number, this function returns 0. (string-to-number "256") ⇒ 256 (string-to-number "25 is a perfect square.") ⇒ 25 (string-to-number "X256") ⇒0 (string-to-number "-4.5") ⇒ -4.5 (string-to-number "1e5") ⇒ 100000.0 string-to-int is an obsolete alias for this function. char-to-string character [Function] This function returns a new string containing one character, character. This func- tion is semi-obsolete because the function string is more general. See Section 4.3 [Creating Strings], page 49. string-to-char string [Function] This function returns the first character in string. This mostly identical to (aref string 0), except that it returns 0 if the string is empty. (The value is also 0 when Chapter 4: Strings and Characters 57 the first character of string is the null character, ASCII code 0.) This function may be eliminated in the future if it does not seem useful enough to retain. Here are some other functions that can convert to or from a string: concat This function converts a vector or a list into a string. See Section 4.3 [Creating Strings], page 49. vconcat This function converts a string into a vector. See Section 6.5 [Vector Functions], page 93. append This function converts a string into a list. See Section 5.4 [Building Lists], page 68. byte-to-string This function converts a byte of character data into a unibyte string. See Section 32.3 [Converting Representations], page 719. 4.7 Formatting Strings Formatting means constructing a string by substituting computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a format string. Formatting is often useful for computing messages to be displayed. In fact, the functions message and error provide the same formatting feature described here; they differ from format only in how they use the result of formatting. format string &rest objects [Function] This function returns a new string that is made by copying string and then replacing any format specification in the copy with encodings of the corresponding objects. The arguments objects are the computed values to be formatted. The characters in string, other than the format specifications, are copied directly into the output, including their text properties, if any. A format specification is a sequence of characters beginning with a ‘%’. Thus, if there is a ‘%d’ in string, the format function replaces it with the printed representation of one of the values to be formatted (one of the arguments objects). For example: (format "The value of fill-column is %d." fill-column) ⇒ "The value of fill-column is 72." Since format interprets ‘%’ characters as format specifications, you should never pass an arbitrary string as the first argument. This is particularly true when the string is generated by some Lisp code. Unless the string is known to never include any ‘%’ characters, pass "%s", described below, as the first argument, and the string as the second, like this: (format "%s" arbitrary-string) If string contains more than one format specification, the format specifications correspond to successive values from objects. Thus, the first format specification in string uses the first such value, the second format specification uses the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause an error. Any extra values to be formatted are ignored. Chapter 4: Strings and Characters 58 Certain format specifications require values of particular types. If you supply a value that doesn’t fit the requirements, an error is signaled. Here is a table of valid format specifications: ‘%s’ Replace the specification with the printed representation of the object, made without quoting (that is, using princ, not prin1—see Section 18.5 [Output Functions], page 285). Thus, strings are represented by their contents alone, with no ‘"’ characters, and symbols appear without ‘\’ characters. If the object is a string, its text properties are copied into the output. The text properties of the ‘%s’ itself are also copied, but those of the object take priority. ‘%S’ Replace the specification with the printed representation of the object, made with quoting (that is, using prin1—see Section 18.5 [Output Functions], page 285). Thus, strings are enclosed in ‘"’ characters, and ‘\’ characters appear where necessary before special characters. ‘%o’ Replace the specification with the base-eight representation of an integer. ‘%d’ Replace the specification with the base-ten representation of an integer. ‘%x’ ‘%X’ Replace the specification with the base-sixteen representation of an integer. ‘%x’ uses lower case and ‘%X’ uses upper case. ‘%c’ Replace the specification with the character which is the value given. ‘%e’ Replace the specification with the exponential notation for a floating-point num- ber. ‘%f’ Replace the specification with the decimal-point notation for a floating-point number. ‘%g’ Replace the specification with notation for a floating-point number, using either exponential notation or decimal-point notation, whichever is shorter. ‘%%’ Replace the specification with a single ‘%’. This format specification is unusual in that it does not use a value. For example, (format "%% %d" 30) returns "% 30". Any other format character results in an ‘Invalid format operation’ error. Here are several examples: (format "The name of this buffer is %s." (buffer-name)) ⇒ "The name of this buffer is strings.texi." (format "The buffer object prints as %s." (current-buffer)) ⇒ "The buffer object prints as strings.texi." (format "The octal value of %d is %o, and the hex value is %x." 18 18 18) ⇒ "The octal value of 18 is 22, and the hex value is 12." A specification can have a width, which is a decimal number between the ‘%’ and the specification character. If the printed representation of the object contains fewer characters Chapter 4: Strings and Characters 59 than this width, format extends it with padding. The width specifier is ignored for the ‘%%’ specification. Any padding introduced by the width specifier normally consists of spaces inserted on the left: (format "%5d is padded on the left with spaces" 123) ⇒ " 123 is padded on the left with spaces" If the width is too small, format does not truncate the object’s printed representation. Thus, you can use a width to specify a minimum spacing between columns with no risk of losing information. In the following three examples, ‘%7s’ specifies a minimum width of 7. In the first case, the string inserted in place of ‘%7s’ has only 3 letters, and needs 4 blank spaces as padding. In the second case, the string "specification" is 13 letters wide but is not truncated. (format "The word ‘%7s’ has %d letters in it." "foo" (length "foo")) ⇒ "The word ‘ foo’ has 3 letters in it." (format "The word ‘%7s’ has %d letters in it." "specification" (length "specification")) ⇒ "The word ‘specification’ has 13 letters in it." Immediately after the ‘%’ and before the optional width specifier, you can also put certain flag characters. The flag ‘+’ inserts a plus sign before a positive number, so that it always has a sign. A space character as flag inserts a space before a positive number. (Otherwise, positive numbers start with the first digit.) These flags are useful for ensuring that positive numbers and negative numbers use the same number of columns. They are ignored except for ‘%d’, ‘%e’, ‘%f’, ‘%g’, and if both flags are used, ‘+’ takes precedence. The flag ‘#’ specifies an “alternate form” which depends on the format in use. For ‘%o’, it ensures that the result begins with a ‘0’. For ‘%x’ and ‘%X’, it prefixes the result with ‘0x’ or ‘0X’. For ‘%e’, ‘%f’, and ‘%g’, the ‘#’ flag means include a decimal point even if the precision is zero. The flag ‘0’ ensures that the padding consists of ‘0’ characters instead of spaces. This flag is ignored for non-numerical specification characters like ‘%s’, ‘%S’ and ‘%c’. These specification characters accept the ‘0’ flag, but still pad with spaces. The flag ‘-’ causes the padding inserted by the width specifier, if any, to be inserted on the right rather than the left. If both ‘-’ and ‘0’ are present, the ‘0’ flag is ignored. (format "%06d is padded on the left with zeros" 123) ⇒ "000123 is padded on the left with zeros" (format "%-6d is padded on the right" 123) ⇒ "123 is padded on the right" (format "The word ‘%-7s’ actually has %d letters in it." "foo" (length "foo")) ⇒ "The word ‘foo ’ actually has 3 letters in it." All the specification characters allow an optional precision before the character (after the width, if present). The precision is a decimal-point ‘.’ followed by a digit-string. For the floating-point specifications (‘%e’, ‘%f’, ‘%g’), the precision specifies how many decimal places Chapter 4: Strings and Characters 60 to show; if zero, the decimal-point itself is also omitted. For ‘%s’ and ‘%S’, the precision truncates the string to the given width, so ‘%.3s’ shows only the first three characters of the representation for object. Precision has no effect for other specification characters. 4.8 Case Conversion in Lisp The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters ‘A’ through ‘Z’ and ‘a’ through ‘z’, as well as non-ASCII letters); other characters are not altered. You can specify a different case conversion mapping by specifying a case table (see Section 4.9 [Case Tables], page 61). These functions do not modify the strings that are passed to them as arguments. The examples below use the characters ‘X’ and ‘x’ which have ASCII codes 88 and 120 respectively. downcase string-or-char [Function] This function converts string-or-char, which should be either a character or a string, to lower case. When string-or-char is a string, this function returns a new string in which each letter in the argument that is upper case is converted to lower case. When string-or-char is a character, this function returns the corresponding lower case character (an integer); if the original character is lower case, or is not a letter, the return value is equal to the original character. (downcase "The cat in the hat") ⇒ "the cat in the hat" (downcase ?X) ⇒ 120 upcase string-or-char [Function] This function converts string-or-char, which should be either a character or a string, to upper case. When string-or-char is a string, this function returns a new string in which each letter in the argument that is lower case is converted to upper case. When string-or-char is a character, this function returns the corresponding upper case character (an integer); if the original character is upper case, or is not a letter, the return value is equal to the original character. (upcase "The cat in the hat") ⇒ "THE CAT IN THE HAT" (upcase ?x) ⇒ 88 capitalize string-or-char [Function] This function capitalizes strings or characters. If string-or-char is a string, the func- tion returns a new string whose contents are a copy of string-or-char in which each Chapter 4: Strings and Characters 61 word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case. The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Section 34.2.1 [Syntax Class Table], page 770). When string-or-char is a character, this function does the same thing as upcase. (capitalize "The cat in the hat") ⇒ "The Cat In The Hat" (capitalize "THE 77TH-HATTED CAT") ⇒ "The 77th-Hatted Cat" (capitalize ?x) ⇒ 88 upcase-initials string-or-char [Function] If string-or-char is a string, this function capitalizes the initials of the words in string- or-char, without altering any letters other than the initials. It returns a new string whose contents are a copy of string-or-char, in which each word has had its initial letter converted to upper case. The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Section 34.2.1 [Syntax Class Table], page 770). When the argument to upcase-initials is a character, upcase-initials has the same result as upcase. (upcase-initials "The CAT in the hAt") ⇒ "The CAT In The HAt" See Section 4.5 [Text Comparison], page 53, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences. 4.9 The Case Table You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see Section 31.18 [Case Changes], page 689). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers. A case table is a char-table (see Section 6.6 [Char-Tables], page 94) whose subtype is case-table. This char-table maps each character into the corresponding lower case character. It has three extra slots, which hold related tables: upcase The upcase table maps each character into the corresponding upper case character. canonicalize The canonicalize table maps all of a set of case-related characters into a particular member of that set. Chapter 4: Strings and Characters 62 equivalences The equivalences table maps each one of a set of case-related characters into the next character in that set. In simple cases, all you need to specify is the mapping to lower-case; the three related tables will be calculated automatically from that one. For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both lower case and upper case. The extra table canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. For example, since ‘a’ and ‘A’ are related by case-conversion, they should have the same canonical equivalent character (which should be either ‘a’ for both of them, or ‘A’ for both of them). The extra table equivalences is a map that cyclically permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map ‘a’ into ‘A’ and ‘A’ into ‘a’, and likewise for each set of equivalent characters.) When constructing a case table, you can provide nil for canonicalize; then Emacs fills in this slot from the lower case and upper case mappings. You can also provide nil for equivalences; then Emacs fills in this slot from canonicalize. In a case table that is actually in use, those components are non-nil. Do not try to specify equivalences without also specifying canonicalize. Here are the functions for working with case tables: case-table-p object This predicate returns non-nil if object is a valid case table. [Function] set-standard-case-table table [Function] This function makes table the standard case table, so that it will be used in any buffers created subsequently. standard-case-table This returns the standard case table. [Function] current-case-table This function returns the current buffer’s case table. [Function] set-case-table table This sets the current buffer’s case table to table. [Function] with-case-table table body. . . [Macro] The with-case-table macro saves the current case table, makes table the current case table, evaluates the body forms, and finally restores the case table. The return value is the value of the last form in body. The case table is restored even in case of an abnormal exit via throw or error (see Section 10.5 [Nonlocal Exits], page 131). Some language environments modify the case conversions of ASCII characters; for example, in the Turkish language environment, the ASCII character ‘I’ is downcased into a Chapter 4: Strings and Characters 63 Turkish “dotless i”. This can interfere with code that requires ordinary ASCII case conversion, such as implementations of ASCII-based network protocols. In that case, use the with-case-table macro with the variable ascii-case-table, which stores the unmodified case table for the ASCII character set. ascii-case-table [Variable] The case table for the ASCII character set. This should not be modified by any language environment settings. The following three functions are convenient subroutines for packages that define nonASCII character sets. They modify the specified case table case-table; they also modify the standard syntax table. See Chapter 34 [Syntax Tables], page 769. Normally you would use these functions to change the standard case table. set-case-syntax-pair uc lc case-table [Function] This function specifies a pair of corresponding letters, one upper case and one lower case. set-case-syntax-delims l r case-table [Function] This function makes characters l and r a matching pair of case-invariant delimiters. set-case-syntax char syntax case-table This function makes char case-invariant, with syntax syntax. [Function] describe-buffer-case-table [Command] This command displays a description of the contents of the current buffer’s case table. Chapter 5: Lists 64 5 Lists A list represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list. 5.1 Lists and Cons Cells Lists in Lisp are not a primitive data type; they are built up from cons cells (see Section 2.3.6 [Cons Cell Type], page 14). A cons cell is a data object that represents an ordered pair. That is, it has two slots, and each slot holds, or refers to, some Lisp object. One slot is known as the car, and the other is known as the cdr. (These names are traditional; see Section 2.3.6 [Cons Cell Type], page 14.) cdr is pronounced “could-er”. We say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr. A list is a series of cons cells “chained together”, so that each cell refers to the next one. There is one cons cell for each element of the list. By convention, the cars of the cons cells hold the elements of the list, and the cdrs are used to chain the list (this asymmetry between car and cdr is entirely a matter of convention; at the level of cons cells, the car and cdr slots have similar properties). Hence, the cdr slot of each cons cell in a list refers to the following cons cell. Also by convention, the cdr of the last cons cell in a list is nil. We call such a nilterminated structure a true list. In Emacs Lisp, the symbol nil is both a symbol and a list with no elements. For convenience, the symbol nil is considered to have nil as its cdr (and also as its car). Hence, the cdr of a true list is always a true list. The cdr of a nonempty true list is a true list containing all the elements except the first. If the cdr of a list’s last cons cell is some value other than nil, we call the structure a dotted list, since its printed representation would use dotted pair notation (see Section 2.3.6.2 [Dotted Pair Notation], page 16). There is one other possibility: some cons cell’s cdr could point to one of the previous cons cells in the list. We call that structure a circular list. For some purposes, it does not matter whether a list is true, circular or dotted. If a program doesn’t look far enough down the list to see the cdr of the final cons cell, it won’t care. However, some functions that operate on lists demand true lists and signal errors if given a dotted list. Most functions that try to find the end of a list enter infinite loops if given a circular list. Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a list structure. 5.2 Predicates on Lists The following predicates test whether a Lisp object is an atom, whether it is a cons cell or is a list, or whether it is the distinguished object nil. (Many of these predicates can be defined in terms of the others, but they are used so often that it is worth having them.) Chapter 5: Lists 65 consp object [Function] This function returns t if object is a cons cell, nil otherwise. nil is not a cons cell, although it is a list. atom object [Function] This function returns t if object is an atom, nil otherwise. All objects except cons cells are atoms. The symbol nil is an atom and is also a list; it is the only Lisp object that is both. (atom object) ≡ (not (consp object)) listp object [Function] This function returns t if object is a cons cell or nil. Otherwise, it returns nil. (listp ’(1)) ⇒t (listp ’()) ⇒t nlistp object [Function] This function is the opposite of listp: it returns t if object is not a list. Otherwise, it returns nil. (listp object) ≡ (not (nlistp object)) null object [Function] This function returns t if object is nil, and returns nil otherwise. This function is identical to not, but as a matter of clarity we use null when object is considered a list and not when it is considered a truth value (see not in Section 10.3 [Combining Conditions], page 129). (null ’(1)) ⇒ nil (null ’()) ⇒t 5.3 Accessing Elements of Lists car cons-cell [Function] This function returns the value referred to by the first slot of the cons cell cons-cell. In other words, it returns the car of cons-cell. As a special case, if cons-cell is nil, this function returns nil. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or nil. (car ’(a b c)) ⇒a (car ’()) ⇒ nil cdr cons-cell [Function] This function returns the value referred to by the second slot of the cons cell cons-cell. In other words, it returns the cdr of cons-cell. Chapter 5: Lists 66 As a special case, if cons-cell is nil, this function returns nil; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or nil. (cdr ’(a b c)) ⇒ (b c) (cdr ’()) ⇒ nil car-safe object [Function] This function lets you take the car of a cons cell while avoiding errors for other data types. It returns the car of object if object is a cons cell, nil otherwise. This is in contrast to car, which signals an error if object is not a list. (car-safe object) ≡ (let ((x object)) (if (consp x) (car x) nil)) cdr-safe object [Function] This function lets you take the cdr of a cons cell while avoiding errors for other data types. It returns the cdr of object if object is a cons cell, nil otherwise. This is in contrast to cdr, which signals an error if object is not a list. (cdr-safe object) ≡ (let ((x object)) (if (consp x) (cdr x) nil)) pop listname [Macro] This macro provides a convenient way to examine the car of a list, and take it off the list, all at once. It operates on the list stored in listname. It removes the first element from the list, saves the cdr into listname, then returns the removed element. In the simplest case, listname is an unquoted symbol naming a list; in that case, this macro is equivalent to (prog1 (car listname) (setq listname (cdr listname))). x ⇒ (a b c) (pop x) ⇒a x ⇒ (b c) More generally, listname can be a generalized variable. In that case, this macro saves into listname using setf. See Section 11.15 [Generalized Variables], page 169. For the push macro, which adds an element to a list, See Section 5.5 [List Variables], page 71. Chapter 5: Lists 67 nth n list [Function] This function returns the nth element of list. Elements are numbered starting with zero, so the car of list is element number zero. If the length of list is n or less, the value is nil. (nth 2 ’(1 2 3 4)) ⇒3 (nth 10 ’(1 2 3 4)) ⇒ nil (nth n x) ≡ (car (nthcdr n x)) The function elt is similar, but applies to any kind of sequence. For historical reasons, it takes its arguments in the opposite order. See Section 6.1 [Sequence Functions], page 88. nthcdr n list [Function] This function returns the nth cdr of list. In other words, it skips past the first n links of list and returns what follows. If n is zero, nthcdr returns all of list. If the length of list is n or less, nthcdr returns nil. (nthcdr 1 ’(1 2 3 4)) ⇒ (2 3 4) (nthcdr 10 ’(1 2 3 4)) ⇒ nil (nthcdr 0 ’(1 2 3 4)) ⇒ (1 2 3 4) last list &optional n [Function] This function returns the last link of list. The car of this link is the list’s last element. If list is null, nil is returned. If n is non-nil, the nth-to-last link is returned instead, or the whole of list if n is bigger than list’s length. safe-length list [Function] This function returns the length of list, with no risk of either an error or an infinite loop. It generally returns the number of distinct cons cells in the list. However, for circular lists, the value is just an upper bound; it is often too large. If list is not nil or a cons cell, safe-length returns 0. The most common way to compute the length of a list, when you are not worried that it may be circular, is with length. See Section 6.1 [Sequence Functions], page 88. caar cons-cell This is the same as (car (car cons-cell)). [Function] cadr cons-cell This is the same as (car (cdr cons-cell)) or (nth 1 cons-cell). [Function] cdar cons-cell This is the same as (cdr (car cons-cell)). [Function] Chapter 5: Lists 68 cddr cons-cell [Function] This is the same as (cdr (cdr cons-cell)) or (nthcdr 2 cons-cell). butlast x &optional n [Function] This function returns the list x with the last element, or the last n elements, removed. If n is greater than zero it makes a copy of the list so as not to damage the original list. In general, (append (butlast x n) (last x n)) will return a list equal to x. nbutlast x &optional n [Function] This is a version of butlast that works by destructively modifying the cdr of the appropriate element, rather than making a copy of the list. 5.4 Building Cons Cells and Lists Many functions build lists, as lists reside at the very heart of Lisp. cons is the fundamental list-building function; however, it is interesting to note that list is used more times in the source code for Emacs than cons. cons object1 object2 [Function] This function is the most basic function for building new list structure. It creates a new cons cell, making object1 the car, and object2 the cdr. It then returns the new cons cell. The arguments object1 and object2 may be any Lisp objects, but most often object2 is a list. (cons 1 ’(2)) ⇒ (1 2) (cons 1 ’()) ⇒ (1) (cons 1 2) ⇒ (1 . 2) cons is often used to add a single element to the front of a list. This is called consing the element onto the list.1 For example: (setq list (cons newelt list)) Note that there is no conflict between the variable named list used in this example and the function named list described below; any symbol can serve both purposes. list &rest objects [Function] This function creates a list with objects as its elements. The resulting list is always nil-terminated. If no objects are given, the empty list is returned. (list 1 2 3 4 5) ⇒ (1 2 3 4 5) (list 1 2 ’(3 4 5) ’foo) ⇒ (1 2 (3 4 5) foo) (list) ⇒ nil 1 There is no strictly equivalent way to add an element to the end of a list. You can use (append listname (list newelt)), which creates a whole new list by copying listname and adding newelt to its end. Or you can use (nconc listname (list newelt)), which modifies listname by following all the cdrs and then replacing the terminating nil. Compare this to adding an element to the beginning of a list with cons, which neither copies nor modifies the list. Chapter 5: Lists 69 make-list length object [Function] This function creates a list of length elements, in which each element is object. Com- pare make-list with make-string (see Section 4.3 [Creating Strings], page 49). (make-list 3 ’pigs) ⇒ (pigs pigs pigs) (make-list 0 ’pigs) ⇒ nil (setq l (make-list 3 ’(a b))) ⇒ ((a b) (a b) (a b)) (eq (car l) (cadr l)) ⇒t append &rest sequences [Function] This function returns a list containing all the elements of sequences. The sequences may be lists, vectors, bool-vectors, or strings, but the last one should usually be a list. All arguments except the last one are copied, so none of the arguments is altered. (See nconc in Section 5.6.3 [Rearrangement], page 76, for a way to join lists with no copying.) More generally, the final argument to append may be any Lisp object. The final argument is not copied or converted; it becomes the cdr of the last cons cell in the new list. If the final argument is itself a list, then its elements become in effect elements of the result list. If the final element is not a list, the result is a dotted list since its final cdr is not nil as required in a true list. Here is an example of using append: (setq trees ’(pine oak)) ⇒ (pine oak) (setq more-trees (append ’(maple birch) trees)) ⇒ (maple birch pine oak) trees ⇒ (pine oak) more-trees ⇒ (maple birch pine oak) (eq trees (cdr (cdr more-trees))) ⇒t You can see how append works by looking at a box diagram. The variable trees is set to the list (pine oak) and then the variable more-trees is set to the list (maple birch pine oak). However, the variable trees continues to refer to the original list: more-trees trees | | | --- --- --- --- -> --- --- --- --- --> | | |--> | | |--> | | |--> | | |--> nil --- --- --- --- --- --- --- --- | | | | | | | | --> maple -->birch --> pine --> oak Chapter 5: Lists 70 An empty sequence contributes nothing to the value returned by append. As a consequence of this, a final nil argument forces a copy of the previous argument: trees ⇒ (pine oak) (setq wood (append trees nil)) ⇒ (pine oak) wood ⇒ (pine oak) (eq wood trees) ⇒ nil This once was the usual way to copy a list, before the function copy-sequence was invented. See Chapter 6 [Sequences Arrays Vectors], page 88. Here we show the use of vectors and strings as arguments to append: (append [a b] "cd" nil) ⇒ (a b 99 100) With the help of apply (see Section 12.5 [Calling Functions], page 179), we can append all the lists in a list of lists: (apply ’append ’((a b c) nil (x y z) nil)) ⇒ (a b c x y z) If no sequences are given, nil is returned: (append) ⇒ nil Here are some examples where the final argument is not a list: (append ’(x y) ’z) ⇒ (x y . z) (append ’(x y) [z]) ⇒ (x y . [z]) The second example shows that when the final argument is a sequence but not a list, the sequence’s elements do not become elements of the resulting list. Instead, the sequence becomes the final cdr, like any other non-list final argument. reverse list [Function] This function creates a new list whose elements are the elements of list, but in reverse order. The original argument list is not altered. (setq x ’(1 2 3 4)) ⇒ (1 2 3 4) (reverse x) ⇒ (4 3 2 1) x ⇒ (1 2 3 4) copy-tree tree &optional vecp [Function] This function returns a copy of the tree tree. If tree is a cons cell, this makes a new cons cell with the same car and cdr, then recursively copies the car and cdr in the same way. Chapter 5: Lists 71 Normally, when tree is anything other than a cons cell, copy-tree simply returns tree. However, if vecp is non-nil, it copies vectors too (and operates recursively on their elements). number-sequence from &optional to separation [Function] This returns a list of numbers starting with from and incrementing by separation, and ending at or just before to. separation can be positive or negative and defaults to 1. If to is nil or numerically equal to from, the value is the one-element list (from). If to is less than from with a positive separation, or greater than from with a negative separation, the value is nil because those arguments specify an empty sequence. If separation is 0 and to is neither nil nor numerically equal to from, number-sequence signals an error, since those arguments specify an infinite sequence. All arguments are numbers. Floating-point arguments can be tricky, because floatingpoint arithmetic is inexact. For instance, depending on the machine, it may quite well happen that (number-sequence 0.4 0.6 0.2) returns the one element list (0.4), whereas (number-sequence 0.4 0.8 0.2) returns a list with three elements. The nth element of the list is computed by the exact formula (+ from (* n separation)). Thus, if one wants to make sure that to is included in the list, one can pass an expression of this exact type for to. Alternatively, one can replace to with a slightly larger value (or a slightly more negative value if separation is negative). Some examples: (number-sequence 4 9) ⇒ (4 5 6 7 8 9) (number-sequence 9 4 -1) ⇒ (9 8 7 6 5 4) (number-sequence 9 4 -2) ⇒ (9 7 5) (number-sequence 8) ⇒ (8) (number-sequence 8 5) ⇒ nil (number-sequence 5 8 -1) ⇒ nil (number-sequence 1.5 6 2) ⇒ (1.5 3.5 5.5) 5.5 Modifying List Variables These functions, and one macro, provide convenient ways to modify a list which is stored in a variable. push element listname [Macro] This macro creates a new list whose car is element and whose cdr is the list specified by listname, and saves that list in listname. In the simplest case, listname is an unquoted symbol naming a list, and this macro is equivalent to (setq listname (cons element listname)). Chapter 5: Lists 72 (setq l ’(a b)) ⇒ (a b) (push ’c l) ⇒ (c a b) l ⇒ (c a b) More generally, listname can be a generalized variable. In that case, this macro does the equivalent of (setf listname (cons element listname)). See Section 11.15 [Generalized Variables], page 169. For the pop macro, which removes the first element from a list, See Section 5.3 [List Elements], page 65. Two functions modify lists that are the values of variables. add-to-list symbol element &optional append compare-fn [Function] This function sets the variable symbol by consing element onto the old value, if element is not already a member of that value. It returns the resulting list, whether updated or not. The value of symbol had better be a list already before the call. add-to-list uses compare-fn to compare element against existing list members; if compare-fn is nil, it uses equal. Normally, if element is added, it is added to the front of symbol, but if the optional argument append is non-nil, it is added at the end. The argument symbol is not implicitly quoted; add-to-list is an ordinary function, like set and unlike setq. Quote the argument yourself if that is what you want. Here’s a scenario showing how to use add-to-list: (setq foo ’(a b)) ⇒ (a b) (add-to-list ’foo ’c) ⇒ (c a b) ;; Add c. (add-to-list ’foo ’b) ⇒ (c a b) ;; No effect. foo ⇒ (c a b) ;; foo was changed. An equivalent expression for (add-to-list ’var value) is this: (or (member value var) (setq var (cons value var))) add-to-ordered-list symbol element &optional order [Function] This function sets the variable symbol by inserting element into the old value, which must be a list, at the position specified by order. If element is already a member of the list, its position in the list is adjusted according to order. Membership is tested using eq. This function returns the resulting list, whether updated or not. Chapter 5: Lists 73 The order is typically a number (integer or float), and the elements of the list are sorted in non-decreasing numerical order. order may also be omitted or nil. Then the numeric order of element stays unchanged if it already has one; otherwise, element has no numeric order. Elements without a numeric list order are placed at the end of the list, in no particular order. Any other value for order removes the numeric order of element if it already has one; otherwise, it is equivalent to nil. The argument symbol is not implicitly quoted; add-to-ordered-list is an ordinary function, like set and unlike setq. Quote the argument yourself if necessary. The ordering information is stored in a hash table on symbol’s list-order property. Here’s a scenario showing how to use add-to-ordered-list: (setq foo ’()) ⇒ nil (add-to-ordered-list ’foo ’a 1) ⇒ (a) ;; Add a. (add-to-ordered-list ’foo ’c 3) ⇒ (a c) ;; Add c. (add-to-ordered-list ’foo ’b 2) ⇒ (a b c) ;; Add b. (add-to-ordered-list ’foo ’b 4) ⇒ (a c b) ;; Move b. (add-to-ordered-list ’foo ’d) ⇒ (a c b d) ;; Append d. (add-to-ordered-list ’foo ’e) ⇒ (a c b e d) ;; Add e. foo ⇒ (a c b e d) ;; foo was changed. 5.6 Modifying Existing List Structure You can modify the car and cdr contents of a cons cell with the primitives setcar and setcdr. We call these “destructive” operations because they change existing list structure. Common Lisp note: Common Lisp uses functions rplaca and rplacd to alter list structure; they change structure the same way as setcar and setcdr, but the Common Lisp functions return the cons cell while setcar and setcdr return the new car or cdr. Chapter 5: Lists 74 5.6.1 Altering List Elements with setcar Changing the car of a cons cell is done with setcar. When used on a list, setcar replaces one element of a list with a different element. setcar cons object [Function] This function stores object as the new car of cons, replacing its previous car. In other words, it changes the car slot of cons to refer to object. It returns the value object. For example: (setq x ’(1 2)) ⇒ (1 2) (setcar x 4) ⇒4 x ⇒ (4 2) When a cons cell is part of the shared structure of several lists, storing a new car into the cons changes one element of each of these lists. Here is an example: ;; Create two lists that are partly shared. (setq x1 ’(a b c)) ⇒ (a b c) (setq x2 (cons ’z (cdr x1))) ⇒ (z b c) ;; Replace the car of a shared link. (setcar (cdr x1) ’foo) ⇒ foo x1 ; Both lists are changed. ⇒ (a foo c) x2 ⇒ (z foo c) ;; Replace the car of a link that is not shared. (setcar x1 ’baz) ⇒ baz x1 ; Only one list is changed. ⇒ (baz foo c) x2 ⇒ (z foo c) Here is a graphical depiction of the shared structure of the two lists in the variables x1 and x2, showing why replacing b changes them both: Chapter 5: Lists 75 --- --- --- --- --- --- x1---> | | |----> | | |--> | | |--> nil --- --- --- --- --- --- | --> | | | | | | --> a | --> b --> c | --- --- | x2--> | | |-- --- --- | | --> z Here is an alternative form of box diagram, showing the same relationship: x1: -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | a | o------->| b | o------->| c | nil | | | | -->| | || | | -------------- | -------------- -------------- | x2: | -------------- | | car | cdr | | | z | o---- | | | -------------- 5.6.2 Altering the CDR of a List The lowest-level primitive for modifying a cdr is setcdr: setcdr cons object [Function] This function stores object as the new cdr of cons, replacing its previous cdr. In other words, it changes the cdr slot of cons to refer to object. It returns the value object. Here is an example of replacing the cdr of a list with a different list. All but the first element of the list are removed in favor of a different sequence of elements. The first element is unchanged, because it resides in the car of the list, and is not reached via the cdr. (setq x ’(1 2 3)) ⇒ (1 2 3) (setcdr x ’(4)) ⇒ (4) x ⇒ (1 4) Chapter 5: Lists 76 You can delete elements from the middle of a list by altering the cdrs of the cons cells in the list. For example, here we delete the second element, b, from the list (a b c), by changing the cdr of the first cons cell: (setq x1 ’(a b c)) ⇒ (a b c) (setcdr x1 (cdr (cdr x1))) ⇒ (c) x1 ⇒ (a c) Here is the result in box notation: -------------------- | | -------------- | -------------- | -------------- | car | cdr | | | car | cdr | -->| car | cdr | | a | o----- | b | o-------->| c | nil | | | | | | | | | | -------------- -------------- -------------- The second cons cell, which previously held the element b, still exists and its car is still b, but it no longer forms part of this list. It is equally easy to insert a new element by changing cdrs: (setq x1 ’(a b c)) ⇒ (a b c) (setcdr x1 (cons ’d (cdr x1))) ⇒ (d b c) x1 ⇒ (a d b c) Here is this result in box notation: -------------- ------------- ------------- | car | cdr | | car | cdr | | car | cdr | | a | o | -->| b | o------->| c | nil | | | | || | | || | | --------- | -- | ------------- ------------- | | ----- -------- | | | --------------- | | | car | cdr | | -->| d | o------ | | | --------------- 5.6.3 Functions that Rearrange Lists Here are some functions that rearrange lists “destructively” by modifying the cdrs of their component cons cells. We call these functions “destructive” because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value. The function delq in the following section is another example of destructive list manipulation. Chapter 5: Lists 77 nconc &rest lists [Function] This function returns a list containing all the elements of lists. Unlike append (see Section 5.4 [Building Lists], page 68), the lists are not copied. Instead, the last cdr of each of the lists is changed to refer to the following list. The last of the lists is not altered. For example: (setq x ’(1 2 3)) ⇒ (1 2 3) (nconc x ’(4 5)) ⇒ (1 2 3 4 5) x ⇒ (1 2 3 4 5) Since the last argument of nconc is not itself modified, it is reasonable to use a constant list, such as ’(4 5), as in the above example. For the same reason, the last argument need not be a list: (setq x ’(1 2 3)) ⇒ (1 2 3) (nconc x ’z) ⇒ (1 2 3 . z) x ⇒ (1 2 3 . z) However, the other arguments (all but the last) must be lists. A common pitfall is to use a quoted constant list as a non-last argument to nconc. If you do this, your program will change each time you run it! Here is what happens: (defun add-foo (x) (nconc ’(foo) x)) ; We want this function to add ; foo to the front of its arg. (symbol-function ’add-foo) ⇒ (lambda (x) (nconc (quote (foo)) x)) (setq xx (add-foo ’(1 2))) ⇒ (foo 1 2) (setq xy (add-foo ’(3 4))) ⇒ (foo 1 2 3 4) (eq xx xy) ⇒t ; It seems to work. ; What happened? (symbol-function ’add-foo) ⇒ (lambda (x) (nconc (quote (foo 1 2 3 4) x))) nreverse list [Function] This function reverses the order of the elements of list. Unlike reverse, nreverse alters its argument by reversing the cdrs in the cons cells forming the list. The cons cell that used to be the last one in list becomes the first cons cell of the value. For example: (setq x ’(a b c)) ⇒ (a b c) Chapter 5: Lists 78 x ⇒ (a b c) (nreverse x) ⇒ (c b a) ;; The cons cell that was first is now last. x ⇒ (a) To avoid confusion, we usually store the result of nreverse back in the same variable which held the original list: (setq x (nreverse x)) Here is the nreverse of our favorite example, (a b c), presented graphically: Original list head: Reversed list: ------------- ------------- ------------ | car | cdr | | car | cdr | | car | cdr | | a | nil |<-- | b | o |<-- | c | o | | | | || | || || | || ------------- | --------- | - | -------- | - | | | | ------------- ------------ sort list predicate [Function] This function sorts list stably, though destructively, and returns the sorted list. It compares elements using predicate. A stable sort is one in which elements with equal sort keys maintain their relative order before and after the sort. Stability is important when successive sorts are used to order elements according to different criteria. The argument predicate must be a function that accepts two arguments. It is called with two elements of list. To get an increasing order sort, the predicate should return non-nil if the first element is “less than” the second, or nil if not. The comparison function predicate must give reliable results for any given pair of arguments, at least within a single call to sort. It must be antisymmetric; that is, if a is less than b, b must not be less than a. It must be transitive—that is, if a is less than b, and b is less than c, then a must be less than c. If you use a comparison function which does not meet these requirements, the result of sort is unpredictable. The destructive aspect of sort is that it rearranges the cons cells forming list by changing cdrs. A nondestructive sort function would create new cons cells to store the elements in their sorted order. If you wish to make a sorted copy without destroying the original, copy it first with copy-sequence and then sort. Sorting does not change the cars of the cons cells in list; the cons cell that originally contained the element a in list still has a in its car after sorting, but it now appears in a different position in the list due to the change of cdrs. For example: (setq nums ’(1 3 2 6 5 4 0)) ⇒ (1 3 2 6 5 4 0) (sort nums ’<) ⇒ (0 1 2 3 4 5 6) nums ⇒ (1 2 3 4 5 6) Chapter 5: Lists 79 Warning: Note that the list in nums no longer contains 0; this is the same cons cell that it was before, but it is no longer the first one in the list. Don’t assume a variable that formerly held the argument now holds the entire sorted list! Instead, save the result of sort and use that. Most often we store the result back into the variable that held the original list: (setq nums (sort nums ’<)) See Section 31.15 [Sorting], page 680, for more functions that perform sorting. See documentation in Section 23.2 [Accessing Documentation], page 462, for a useful example of sort. 5.7 Using Lists as Sets A list can represent an unordered mathematical set—simply consider a value an element of a set if it appears in the list, and ignore the order of the list. To form the union of two sets, use append (as long as you don’t mind having duplicate elements). You can remove equal duplicates using delete-dups. Other useful functions for sets include memq and delq, and their equal versions, member and delete. Common Lisp note: Common Lisp has functions union (which avoids duplicate elements) and intersection for set operations. Although standard GNU Emacs Lisp does not have them, the cl-lib library provides versions. See Section “Lists as Sets” in Common Lisp Extensions. memq object list [Function] This function tests to see whether object is a member of list. If it is, memq returns a list starting with the first occurrence of object. Otherwise, it returns nil. The letter ‘q’ in memq says that it uses eq to compare object against the elements of the list. For example: (memq ’b ’(a b c b a)) ⇒ (b c b a) (memq ’(2) ’((1) (2))) ⇒ nil ; (2) and (2) are not eq. delq object list [Function] This function destructively removes all elements eq to object from list, and returns the resulting list. The letter ‘q’ in delq says that it uses eq to compare object against the elements of the list, like memq and remq. Typically, when you invoke delq, you should use the return value by assigning it to the variable which held the original list. The reason for this is explained below. The delq function deletes elements from the front of the list by simply advancing down the list, and returning a sublist that starts after those elements. For example: (delq ’a ’(a b c)) ≡ (cdr ’(a b c)) When an element to be deleted appears in the middle of the list, removing it involves changing the cdrs (see Section 5.6.2 [Setcdr], page 75). (setq sample-list ’(a b c (4))) ⇒ (a b c (4)) Chapter 5: Lists 80 (delq ’a sample-list) ⇒ (b c (4)) sample-list ⇒ (a b c (4)) (delq ’c sample-list) ⇒ (a b (4)) sample-list ⇒ (a b (4)) Note that (delq ’c sample-list) modifies sample-list to splice out the third element, but (delq ’a sample-list) does not splice anything—it just returns a shorter list. Don’t assume that a variable which formerly held the argument list now has fewer elements, or that it still holds the original list! Instead, save the result of delq and use that. Most often we store the result back into the variable that held the original list: (setq flowers (delq ’rose flowers)) In the following example, the (4) that delq attempts to match and the (4) in the sample-list are not eq: (delq ’(4) sample-list) ⇒ (a c (4)) If you want to delete elements that are equal to a given value, use delete (see below). remq object list [Function] This function returns a copy of list, with all elements removed which are eq to object. The letter ‘q’ in remq says that it uses eq to compare object against the elements of list. (setq sample-list ’(a b c a b c)) ⇒ (a b c a b c) (remq ’a sample-list) ⇒ (b c b c) sample-list ⇒ (a b c a b c) memql object list [Function] The function memql tests to see whether object is a member of list, comparing mem- bers with object using eql, so floating-point elements are compared by value. If object is a member, memql returns a list starting with its first occurrence in list. Otherwise, it returns nil. Compare this with memq: (memql 1.2 ’(1.1 1.2 1.3)) ; 1.2 and 1.2 are eql. ⇒ (1.2 1.3) (memq 1.2 ’(1.1 1.2 1.3)) ; 1.2 and 1.2 are not eq. ⇒ nil The following three functions are like memq, delq and remq, but use equal rather than eq to compare elements. See Section 2.7 [Equality Predicates], page 30. Chapter 5: Lists 81 member object list [Function] The function member tests to see whether object is a member of list, comparing members with object using equal. If object is a member, member returns a list starting with its first occurrence in list. Otherwise, it returns nil. Compare this with memq: (member ’(2) ’((1) (2))) ; (2) and (2) are equal. ⇒ ((2)) (memq ’(2) ’((1) (2))) ; (2) and (2) are not eq. ⇒ nil ;; Two strings with the same contents are equal. (member "foo" ’("foo" "bar")) ⇒ ("foo" "bar") delete object sequence [Function] This function removes all elements equal to object from sequence, and returns the resulting sequence. If sequence is a list, delete is to delq as member is to memq: it uses equal to compare elements with object, like member; when it finds an element that matches, it cuts the element out just as delq would. As with delq, you should typically use the return value by assigning it to the variable which held the original list. If sequence is a vector or string, delete returns a copy of sequence with all elements equal to object removed. For example: (setq l ’((2) (1) (2))) (delete ’(2) l) ⇒ ((1)) l ⇒ ((2) (1)) ;; If you want to change l reliably, ;; write (setq l (delete ’(2) l)). (setq l ’((2) (1) (2))) (delete ’(1) l) ⇒ ((2) (2)) l ⇒ ((2) (2)) ;; In this case, it makes no difference whether you set l, ;; but you should do so for the sake of the other case. (delete ’(2) [(2) (1) (2)]) ⇒ [(1)] remove object sequence [Function] This function is the non-destructive counterpart of delete. It returns a copy of sequence, a list, vector, or string, with elements equal to object removed. For example: (remove ’(2) ’((2) (1) (2))) ⇒ ((1)) Chapter 5: Lists 82 (remove ’(2) [(2) (1) (2)]) ⇒ [(1)] Common Lisp note: The functions member, delete and remove in GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not use equal to compare elements. member-ignore-case object list [Function] This function is like member, except that object should be a string and that it ignores differences in letter-case and text representation: upper-case and lower-case letters are treated as equal, and unibyte strings are converted to multibyte prior to comparison. delete-dups list [Function] This function destructively removes all equal duplicates from list, stores the result in list and returns it. Of several equal occurrences of an element in list, delete-dups keeps the first one. See also the function add-to-list, in Section 5.5 [List Variables], page 71, for a way to add an element to a list stored in a variable and used as a set. 5.8 Association Lists An association list, or alist for short, records a mapping from keys to values. It is a list of cons cells called associations: the car of each cons cell is the key, and the cdr is the associated value.2 Here is an example of an alist. The key pine is associated with the value cones; the key oak is associated with acorns; and the key maple is associated with seeds. ((pine . cones) (oak . acorns) (maple . seeds)) Both the values and the keys in an alist may be any Lisp objects. For example, in the following alist, the symbol a is associated with the number 1, and the string "b" is associated with the list (2 3), which is the cdr of the alist element: ((a . 1) ("b" 2 3)) Sometimes it is better to design an alist to store the associated value in the car of the cdr of the element. Here is an example of such an alist: ((rose red) (lily white) (buttercup yellow)) Here we regard red as the value associated with rose. One advantage of this kind of alist is that you can store other related information—even a list of other items—in the cdr of the cdr. One disadvantage is that you cannot use rassq (see below) to find the element containing a given value. When neither of these considerations is important, the choice is a matter of taste, as long as you are consistent about it for any given alist. The same alist shown above could be regarded as having the associated value in the cdr of the element; the value associated with rose would be the list (red). 2 This usage of “key” is not related to the term “key sequence”; it means a value used to look up an item in a table. In this case, the table is the alist, and the alist associations are the items. Chapter 5: Lists 83 Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one. In Emacs Lisp, it is not an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases. Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. See Section 5.9 [Property Lists], page 86, for a comparison of property lists and association lists. assoc key alist [Function] This function returns the first association for key in alist, comparing key against the alist elements using equal (see Section 2.7 [Equality Predicates], page 30). It returns nil if no association in alist has a car equal to key. For example: (setq trees ’((pine . cones) (oak . acorns) (maple . seeds))) ⇒ ((pine . cones) (oak . acorns) (maple . seeds)) (assoc ’oak trees) ⇒ (oak . acorns) (cdr (assoc ’oak trees)) ⇒ acorns (assoc ’birch trees) ⇒ nil Here is another example, in which the keys and values are not symbols: (setq needles-per-cluster ’((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine"))) (cdr (assoc 3 needles-per-cluster)) ⇒ ("Pitch Pine") (cdr (assoc 2 needles-per-cluster)) ⇒ ("Austrian Pine" "Red Pine") The function assoc-string is much like assoc except that it ignores certain differences between strings. See Section 4.5 [Text Comparison], page 53. rassoc value alist [Function] This function returns the first association with value value in alist. It returns nil if no association in alist has a cdr equal to value. rassoc is like assoc except that it compares the cdr of each alist association instead of the car. You can think of this as “reverse assoc”, finding the key for a given value. assq key alist [Function] This function is like assoc in that it returns the first association for key in alist, but it makes the comparison using eq instead of equal. assq returns nil if no association in alist has a car eq to key. This function is used more often than assoc, since eq is faster than equal and most alists use symbols as keys. See Section 2.7 [Equality Predicates], page 30. Chapter 5: Lists 84 (setq trees ’((pine . cones) (oak . acorns) (maple . seeds))) ⇒ ((pine . cones) (oak . acorns) (maple . seeds)) (assq ’pine trees) ⇒ (pine . cones) On the other hand, assq is not usually useful in alists where the keys may not be symbols: (setq leaves ’(("simple leaves" . oak) ("compound leaves" . horsechestnut))) (assq "simple leaves" leaves) ⇒ nil (assoc "simple leaves" leaves) ⇒ ("simple leaves" . oak) rassq value alist [Function] This function returns the first association with value value in alist. It returns nil if no association in alist has a cdr eq to value. rassq is like assq except that it compares the cdr of each alist association instead of the car. You can think of this as “reverse assq”, finding the key for a given value. For example: (setq trees ’((pine . cones) (oak . acorns) (maple . seeds))) (rassq ’acorns trees) ⇒ (oak . acorns) (rassq ’spores trees) ⇒ nil rassq cannot search for a value stored in the car of the cdr of an element: (setq colors ’((rose red) (lily white) (buttercup yellow))) (rassq ’white colors) ⇒ nil In this case, the cdr of the association (lily white) is not the symbol white, but rather the list (white). This becomes clearer if the association is written in dotted pair notation: (lily white) ≡ (lily . (white)) assoc-default key alist &optional test default [Function] This function searches alist for a match for key. For each element of alist, it compares the element (if it is an atom) or the element’s car (if it is a cons) against key, by calling test with two arguments: the element or its car, and key. The arguments are passed in that order so that you can get useful results using string-match with an alist that contains regular expressions (see Section 33.4 [Regexp Search], page 757). If test is omitted or nil, equal is used for comparison. If an alist element matches key by this criterion, then assoc-default returns a value based on this element. If the element is a cons, then the value is the element’s cdr. Otherwise, the return value is default. If no alist element matches key, assoc-default returns nil. Chapter 5: Lists 85 copy-alist alist [Function] This function returns a two-level deep copy of alist: it creates a new copy of each association, so that you can alter the associations of the new alist without changing the old one. (setq needles-per-cluster ’((2 . ("Austrian Pine" "Red Pine")) (3 . ("Pitch Pine")) (5 . ("White Pine")))) ⇒ ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (setq copy (copy-alist needles-per-cluster)) ⇒ ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (eq needles-per-cluster copy) ⇒ nil (equal needles-per-cluster copy) ⇒t (eq (car needles-per-cluster) (car copy)) ⇒ nil (cdr (car (cdr needles-per-cluster))) ⇒ ("Pitch Pine") (eq (cdr (car (cdr needles-per-cluster))) (cdr (car (cdr copy)))) ⇒t This example shows how copy-alist makes it possible to change the associations of one copy without affecting the other: (setcdr (assq 3 copy) ’("Martian Vacuum Pine")) (cdr (assq 3 needles-per-cluster)) ⇒ ("Pitch Pine") assq-delete-all key alist [Function] This function deletes from alist all the elements whose car is eq to key, much as if you used delq to delete each such element one by one. It returns the shortened alist, and often modifies the original list structure of alist. For correct results, use the return value of assq-delete-all rather than looking at the saved value of alist. (setq alist ’((foo 1) (bar 2) (foo 3) (lose 4))) ⇒ ((foo 1) (bar 2) (foo 3) (lose 4)) (assq-delete-all ’foo alist) ⇒ ((bar 2) (lose 4)) alist ⇒ ((foo 1) (bar 2) (lose 4)) rassq-delete-all value alist [Function] This function deletes from alist all the elements whose cdr is eq to value. It returns the shortened alist, and often modifies the original list structure of alist. rassq-delete-all is like assq-delete-all except that it compares the cdr of each alist association instead of the car. Chapter 5: Lists 86 5.9 Property Lists A property list (plist for short) is a list of paired elements. Each of the pairs associates a property name (usually a symbol) with a property or value. Here is an example of a property list: (pine cones numbers (1 2 3) color "blue") This property list associates pine with cones, numbers with (1 2 3), and color with "blue". The property names and values can be any Lisp objects, but the names are usually symbols (as they are in this example). Property lists are used in several contexts. For instance, the function put-textproperty takes an argument which is a property list, specifying text properties and associated values which are to be applied to text in a string or buffer. See Section 31.19 [Text Properties], page 690. Another prominent use of property lists is for storing symbol properties. Every symbol possesses a list of properties, used to record miscellaneous information about the symbol; these properties are stored in the form of a property list. See Section 8.4 [Symbol Properties], page 109. 5.9.1 Property Lists and Association Lists Association lists (see Section 5.8 [Association Lists], page 82) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant, since the property names must be distinct. Property lists are better than association lists for attaching information to various Lisp function names or variables. If your program keeps all such information in one association list, it will typically need to search that entire list each time it checks for an association for a particular Lisp function name or variable, which could be slow. By contrast, if you keep the same information in the property lists of the function names or variables themselves, each search will scan only the length of one property list, which is usually short. This is why the documentation for a variable is recorded in a property named variable-documentation. The byte compiler likewise uses properties to record those functions needing special treatment. However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by beginning the property name with the program’s usual name-prefix for variables and functions.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list. 5.9.2 Property Lists Outside Symbols The following functions can be used to manipulate property lists. They all compare property names using eq. Chapter 5: Lists 87 plist-get plist property [Function] This returns the value of the property property stored in the property list plist. It accepts a malformed plist argument. If property is not found in the plist, it returns nil. For example, (plist-get ’(foo 4) ’foo) ⇒4 (plist-get ’(foo 4 bad) ’foo) ⇒4 (plist-get ’(foo 4 bad) ’bad) ⇒ nil (plist-get ’(foo 4 bad) ’bar) ⇒ nil plist-put plist property value [Function] This stores value as the value of the property property in the property list plist. It may modify plist destructively, or it may construct a new list structure without altering the old. The function returns the modified property list, so you can store that back in the place where you got plist. For example, (setq my-plist ’(bar t foo 4)) ⇒ (bar t foo 4) (setq my-plist (plist-put my-plist ’foo 69)) ⇒ (bar t foo 69) (setq my-plist (plist-put my-plist ’quux ’(a))) ⇒ (bar t foo 69 quux (a)) lax-plist-get plist property [Function] Like plist-get except that it compares properties using equal instead of eq. lax-plist-put plist property value [Function] Like plist-put except that it compares properties using equal instead of eq. plist-member plist property [Function] This returns non-nil if plist contains the given property. Unlike plist-get, this allows you to distinguish between a missing property and a property with the value nil. The value is actually the tail of plist whose car is property. Chapter 6: Sequences, Arrays, and Vectors 88 6 Sequences, Arrays, and Vectors The sequence type is the union of two other Lisp types: lists and arrays. In other words, any list is a sequence, and any array is a sequence. The common property that all sequences have is that each is an ordered collection of elements. An array is a fixed-length object with a slot for each of its elements. All the elements are accessible in constant time. The four types of arrays are strings, vectors, char-tables and bool-vectors. A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. Finding the nth element requires looking through n cons cells, so elements farther from the beginning of the list take longer to access. But it is possible to add elements to the list, or remove elements. The following diagram shows the relationship between these types: _____________________________________________ | | | Sequence | | ______ ________________________________ | || || || | | List | | Array || || | | ________ ________ || | |______| | | | | | || | | | Vector | | String | | | | | |________| |________| | | | | ____________ _____________ | | | || || ||| | | | Char-table | | Bool-vector | | | | | |____________| |_____________| | | | |________________________________| | |_____________________________________________| 6.1 Sequences This section describes functions that accept any kind of sequence. sequencep object [Function] This function returns t if object is a list, vector, string, bool-vector, or char-table, nil otherwise. length sequence [Function] This function returns the number of elements in sequence. If sequence is a dotted list, a wrong-type-argument error is signaled. Circular lists may cause an infinite loop. For a char-table, the value returned is always one more than the maximum Emacs character code. See [Definition of safe-length], page 67, for the related function safe-length. (length ’(1 2 3)) ⇒3 Chapter 6: Sequences, Arrays, and Vectors 89 (length ()) ⇒0 (length "foobar") ⇒6 (length [1 2 3]) ⇒3 (length (make-bool-vector 5 nil)) ⇒5 See also string-bytes, in Section 32.1 [Text Representations], page 717. If you need to compute the width of a string on display, you should use string-width (see Section 37.10 [Size of Displayed Text], page 858), not length, since length only counts the number of characters, but does not account for the display width of each character. elt sequence index [Function] This function returns the element of sequence indexed by index. Legitimate values of index are integers ranging from 0 up to one less than the length of sequence. If sequence is a list, out-of-range values behave as for nth. See [Definition of nth], page 67. Otherwise, out-of-range values trigger an args-out-of-range error. (elt [1 2 3 4] 2) ⇒3 (elt ’(1 2 3 4) 2) ⇒3 ;; We use string to show clearly which character elt returns. (string (elt "1234" 2)) ⇒ "3" (elt [1 2 3 4] 4) error Args out of range: [1 2 3 4], 4 (elt [1 2 3 4] -1) error Args out of range: [1 2 3 4], -1 This function generalizes aref (see Section 6.3 [Array Functions], page 91) and nth (see [Definition of nth], page 67). copy-sequence sequence [Function] This function returns a copy of sequence. The copy is the same type of object as the original sequence, and it has the same elements in the same order. Storing a new element into the copy does not affect the original sequence, and vice versa. However, the elements of the new sequence are not copies; they are identical (eq) to the elements of the original. Therefore, changes made within these elements, as found via the copied sequence, are also visible in the original sequence. If the sequence is a string with text properties, the property list in the copy is itself a copy, not shared with the original’s property list. However, the actual values of the properties are shared. See Section 31.19 [Text Properties], page 690. This function does not work for dotted lists. Trying to copy a circular list may cause an infinite loop. Chapter 6: Sequences, Arrays, and Vectors 90 See also append in Section 5.4 [Building Lists], page 68, concat in Section 4.3 [Creating Strings], page 49, and vconcat in Section 6.5 [Vector Functions], page 93, for other ways to copy sequences. (setq bar ’(1 2)) ⇒ (1 2) (setq x (vector ’foo bar)) ⇒ [foo (1 2)] (setq y (copy-sequence x)) ⇒ [foo (1 2)] (eq x y) ⇒ nil (equal x y) ⇒t (eq (elt x 1) (elt y 1)) ⇒t ;; Replacing an element of one sequence. (aset x 0 ’quux) x ⇒ [quux (1 2)] y ⇒ [foo (1 2)] ;; Modifying the inside of a shared element. (setcar (aref x 1) 69) x ⇒ [quux (69 2)] y ⇒ [foo (69 2)] 6.2 Arrays An array object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, the time to access an element of a list is proportional to the position of that element in the list. Emacs defines four types of array, all one-dimensional: strings (see Section 2.3.8 [String Type], page 18), vectors (see Section 2.3.9 [Vector Type], page 20), bool-vectors (see Section 2.3.11 [Bool-Vector Type], page 21), and char-tables (see Section 2.3.10 [CharTable Type], page 21). Vectors and char-tables can hold elements of any type, but strings can only hold characters, and bool-vectors can only hold t and nil. All four kinds of array share these characteristics: • The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3. • The length of the array is fixed once you create it; you cannot change the length of an existing array. • For purposes of evaluation, the array is a constant—i.e., it evaluates to itself. • The elements of an array may be referenced or changed with the functions aref and aset, respectively (see Section 6.3 [Array Functions], page 91). Chapter 6: Sequences, Arrays, and Vectors 91 When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes. In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons: • They occupy one-fourth the space of a vector of the same elements. • Strings are printed in a way that shows the contents more clearly as text. • Strings can hold text properties. See Section 31.19 [Text Properties], page 690. • Many of the specialized editing and I/O facilities of Emacs accept only strings. For example, you cannot insert a vector of characters into a buffer the way you can insert a string. See Chapter 4 [Strings and Characters], page 48. By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are outside the range that will fit in a string. See Section 20.8.1 [Key Sequence Input], page 349. 6.3 Functions that Operate on Arrays In this section, we describe the functions that accept all types of arrays. arrayp object [Function] This function returns t if object is an array (i.e., a vector, a string, a bool-vector or a char-table). (arrayp [a]) ⇒t (arrayp "asdf") ⇒t (arrayp (syntax-table)) ⇒t ;; A char-table. aref array index [Function] This function returns the indexth element of array. The first element is at index zero. (setq primes [2 3 5 7 11 13]) ⇒ [2 3 5 7 11 13] (aref primes 4) ⇒ 11 (aref "abcdefg" 1) ⇒ 98 ; ‘b’ is ASCII code 98. See also the function elt, in Section 6.1 [Sequence Functions], page 88. aset array index object [Function] This function sets the indexth element of array to be object. It returns object. (setq w [foo bar baz]) ⇒ [foo bar baz] (aset w 0 ’fu) ⇒ fu w ⇒ [fu bar baz] Chapter 6: Sequences, Arrays, and Vectors 92 (setq x "asdfasfd") ⇒ "asdfasfd" (aset x 3 ?Z) ⇒ 90 x ⇒ "asdZasfd" If array is a string and object is not a character, a wrong-type-argument error results. The function converts a unibyte string to multibyte if necessary to insert a character. fillarray array object [Function] This function fills the array array with object, so that each element of array is object. It returns array. (setq a [a b c d e f g]) ⇒ [a b c d e f g] (fillarray a 0) ⇒ [0 0 0 0 0 0 0] a ⇒ [0 0 0 0 0 0 0] (setq s "When in the course") ⇒ "When in the course" (fillarray s ?-) ⇒ "------------------" If array is a string and object is not a character, a wrong-type-argument error results. The general sequence functions copy-sequence and length are often useful for objects known to be arrays. See Section 6.1 [Sequence Functions], page 88. 6.4 Vectors A vector is a general-purpose array whose elements can be any Lisp objects. (By contrast, the elements of a string can only be characters. See Chapter 4 [Strings and Characters], page 48.) Vectors are used in Emacs for many purposes: as key sequences (see Section 21.1 [Key Sequences], page 367), as symbol-lookup tables (see Section 8.3 [Creating Symbols], page 107), as part of the representation of a byte-compiled function (see Chapter 16 [Byte Compilation], page 239), and more. Like other arrays, vectors use zero-origin indexing: the first element has index 0. Vectors are printed with square brackets surrounding the elements. Thus, a vector whose elements are the symbols a, b and a is printed as [a b a]. You can write vectors in the same way in Lisp input. A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. See Section 9.1.1 [Self-Evaluating Forms], page 114. Here are examples illustrating these principles: Chapter 6: Sequences, Arrays, and Vectors 93 (setq avector [1 two ’(three) "four" [five]]) ⇒ [1 two (quote (three)) "four" [five]] (eval avector) ⇒ [1 two (quote (three)) "four" [five]] (eq avector (eval avector)) ⇒t 6.5 Functions for Vectors Here are some functions that relate to vectors: vectorp object This function returns t if object is a vector. (vectorp [a]) ⇒t (vectorp "asdf") ⇒ nil [Function] vector &rest objects [Function] This function creates and returns a vector whose elements are the arguments, objects. (vector ’foo 23 [bar baz] "rats") ⇒ [foo 23 [bar baz] "rats"] (vector) ⇒ [] make-vector length object [Function] This function returns a new vector consisting of length elements, each initialized to object. (setq sleepy (make-vector 9 ’Z)) ⇒ [Z Z Z Z Z Z Z Z Z] vconcat &rest sequences [Function] This function returns a new vector containing all the elements of sequences. The ar- guments sequences may be true lists, vectors, strings or bool-vectors. If no sequences are given, the empty vector is returned. The value is either the empty vector, or is a newly constructed nonempty vector that is not eq to any existing vector. (setq a (vconcat ’(A B C) ’(D E F))) ⇒ [A B C D E F] (eq a (vconcat a)) ⇒ nil (vconcat) ⇒ [] (vconcat [A B C] "aa" ’(foo (6 7))) ⇒ [A B C 97 97 foo (6 7)] The vconcat function also allows byte-code function objects as arguments. This is a special feature to make it easy to access the entire contents of a byte-code function object. See Section 16.7 [Byte-Code Objects], page 245. Chapter 6: Sequences, Arrays, and Vectors 94 For other concatenation functions, see mapconcat in Section 12.6 [Mapping Functions], page 181, concat in Section 4.3 [Creating Strings], page 49, and append in Section 5.4 [Building Lists], page 68. The append function also provides a way to convert a vector into a list with the same elements: (setq avector [1 two (quote (three)) "four" [five]]) ⇒ [1 two (quote (three)) "four" [five]] (append avector nil) ⇒ (1 two (quote (three)) "four" [five]) 6.6 Char-Tables A char-table is much like a vector, except that it is indexed by character codes. Any valid character code, without modifiers, can be used as an index in a char-table. You can access a char-table’s elements with aref and aset, as with any array. In addition, a char-table can have extra slots to hold additional data not associated with particular character codes. Like vectors, char-tables are constants when evaluated, and can hold elements of any type. Each char-table has a subtype, a symbol, which serves two purposes: • The subtype provides an easy way to tell what the char-table is for. For instance, display tables are char-tables with display-table as the subtype, and syntax tables are char-tables with syntax-table as the subtype. The subtype can be queried using the function char-table-subtype, described below. • The subtype controls the number of extra slots in the char-table. This number is specified by the subtype’s char-table-extra-slots symbol property (see Section 8.4 [Symbol Properties], page 109), whose value should be an integer between 0 and 10. If the subtype has no such symbol property, the char-table has no extra slots. A char-table can have a parent, which is another char-table. If it does, then whenever the char-table specifies nil for a particular character c, it inherits the value specified in the parent. In other words, (aref char-table c) returns the value from the parent of char-table if char-table itself specifies nil. A char-table can also have a default value. If so, then (aref char-table c) returns the default value whenever the char-table does not specify any other non-nil value. make-char-table subtype &optional init [Function] Return a newly-created char-table, with subtype subtype (a symbol). Each element is initialized to init, which defaults to nil. You cannot alter the subtype of a char-table after the char-table is created. There is no argument to specify the length of the char-table, because all char-tables have room for any valid character code as an index. If subtype has the char-table-extra-slots symbol property, that specifies the number of extra slots in the char-table. This should be an integer between 0 and 10; otherwise, make-char-table raises an error. If subtype has no char-table-extraslots symbol property (see Section 5.9 [Property Lists], page 86), the char-table has no extra slots. Chapter 6: Sequences, Arrays, and Vectors 95 char-table-p object This function returns t if object is a char-table, and nil otherwise. [Function] char-table-subtype char-table This function returns the subtype symbol of char-table. [Function] There is no special function to access default values in a char-table. To do that, use char-table-range (see below). char-table-parent char-table [Function] This function returns the parent of char-table. The parent is always either nil or another char-table. set-char-table-parent char-table new-parent This function sets the parent of char-table to new-parent. [Function] char-table-extra-slot char-table n [Function] This function returns the contents of extra slot n of char-table. The number of extra slots in a char-table is determined by its subtype. set-char-table-extra-slot char-table n value This function stores value in extra slot n of char-table. [Function] A char-table can specify an element value for a single character code; it can also specify a value for an entire character set. char-table-range char-table range [Function] This returns the value specified in char-table for a range of characters range. Here are the possibilities for range: nil Refers to the default value. char Refers to the element for character char (supposing char is a valid char- acter code). (from . to) A cons cell refers to all the characters in the inclusive range ‘[from..to]’. set-char-table-range char-table range value [Function] This function sets the value in char-table for a range of characters range. Here are the possibilities for range: nil Refers to the default value. t Refers to the whole range of character codes. char Refers to the element for character char (supposing char is a valid char- acter code). (from . to) A cons cell refers to all the characters in the inclusive range ‘[from..to]’. Chapter 6: Sequences, Arrays, and Vectors 96 map-char-table function char-table [Function] This function calls its argument function for each element of char-table that has a non-nil value. The call to function is with two arguments, a key and a value. The key is a possible range argument for char-table-range—either a valid character or a cons cell (from . to), specifying a range of characters that share the same value. The value is what (char-table-range char-table key) returns. Overall, the key-value pairs passed to function describe all the values stored in chartable. The return value is always nil; to make calls to map-char-table useful, function should have side effects. For example, here is how to examine the elements of the syntax table: (let (accumulator) (map-char-table #’(lambda (key value) (setq accumulator (cons (list (if (consp key) (list (car key) (cdr key)) key) value) accumulator))) (syntax-table)) accumulator) ⇒ (((2597602 4194303) (2)) ((2597523 2597601) (3)) ... (65379 (5 . 65378)) (65378 (4 . 65379)) (65377 (1)) ... (12 (0)) (11 (3)) (10 (12)) (9 (0)) ((0 8) (3))) 6.7 Bool-vectors A bool-vector is much like a vector, except that it stores only the values t and nil. If you try to store any non-nil value into an element of the bool-vector, the effect is to store t there. As with all arrays, bool-vector indices start from 0, and the length cannot be changed once the bool-vector is created. Bool-vectors are constants when evaluated. There are two special functions for working with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays. make-bool-vector length initial [Function] Return a new bool-vector of length elements, each one initialized to initial. bool-vector-p object This returns t if object is a bool-vector, and nil otherwise. [Function] There are also some bool-vector set operation functions, described below: bool-vector-exclusive-or a b &optional c [Function] Return bitwise exclusive or of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length. Chapter 6: Sequences, Arrays, and Vectors 97 bool-vector-union a b &optional c [Function] Return bitwise or of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length. bool-vector-intersection a b &optional c [Function] Return bitwise and of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length. bool-vector-set-difference a b &optional c [Function] Return set difference of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length. bool-vector-not a &optional b [Function] Return set complement of bool vector a. If optional argument b is given, the result of this operation is stored into b. All arguments should be bool vectors of the same length. bool-vector-subsetp a b [Function] Return t if every t value in a is also t in b, nil otherwise. All arguments should be bool vectors of the same length. bool-vector-count-consecutive a b i [Function] Return the number of consecutive elements in a equal b starting at i. a is a bool vector, b is t or nil, and i is an index into a. bool-vector-count-population a Return the number of elements that are t in bool vector a. [Function] Here is an example of creating, examining, and updating a bool-vector. Note that the printed form represents up to 8 boolean values as a single character. (setq bv (make-bool-vector 5 t)) ⇒ #&5"^_" (aref bv 1) ⇒t (aset bv 3 nil) ⇒ nil bv ⇒ #&5"^W" These results make sense because the binary codes for control- and control-W are 11111 and 10111, respectively. 6.8 Managing a Fixed-Size Ring of Objects A ring is a fixed-size data structure that supports insertion, deletion, rotation, and moduloindexed reference and traversal. An efficient ring data structure is implemented by the ring package. It provides the functions listed in this section. Chapter 6: Sequences, Arrays, and Vectors 98 Note that several “rings” in Emacs, like the kill ring and the mark ring, are actually implemented as simple lists, not using the ring package; thus the following functions won’t work on them. make-ring size [Function] This returns a new ring capable of holding size objects. size should be an integer. ring-p object This returns t if object is a ring, nil otherwise. [Function] ring-size ring This returns the maximum capacity of the ring. [Function] ring-length ring [Function] This returns the number of objects that ring currently contains. The value will never exceed that returned by ring-size. ring-elements ring This returns a list of the objects in ring, in order, newest first. [Function] ring-copy ring [Function] This returns a new ring which is a copy of ring. The new ring contains the same (eq) objects as ring. ring-empty-p ring This returns t if ring is empty, nil otherwise. [Function] The newest element in the ring always has index 0. Higher indices correspond to older elements. Indices are computed modulo the ring length. Index −1 corresponds to the oldest element, −2 to the next-oldest, and so forth. ring-ref ring index [Function] This returns the object in ring found at index index. index may be negative or greater than the ring length. If ring is empty, ring-ref signals an error. ring-insert ring object [Function] This inserts object into ring, making it the newest element, and returns object. If the ring is full, insertion removes the oldest element to make room for the new element. ring-remove ring &optional index [Function] Remove an object from ring, and return that object. The argument index specifies which item to remove; if it is nil, that means to remove the oldest item. If ring is empty, ring-remove signals an error. ring-insert-at-beginning ring object [Function] This inserts object into ring, treating it as the oldest element. The return value is not significant. If the ring is full, this function removes the newest element to make room for the inserted element. Chapter 6: Sequences, Arrays, and Vectors 99 If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example: (let ((fifo (make-ring 5))) (mapc (lambda (obj) (ring-insert fifo obj)) ’(0 one "two")) (list (ring-remove fifo) t (ring-remove fifo) t (ring-remove fifo))) ⇒ (0 t one t "two") Chapter 7: Hash Tables 100 7 Hash Tables A hash table is a very fast kind of lookup table, somewhat like an alist (see Section 5.8 [Association Lists], page 82) in that it maps keys to corresponding values. It differs from an alist in these ways: • Lookup in a hash table is extremely fast for large tables—in fact, the time required is essentially independent of how many elements are stored in the table. For smaller tables (a few tens of elements) alists may still be faster because hash tables have a more-or-less constant overhead. • The correspondences in a hash table are in no particular order. • There is no way to share structure between two hash tables, the way two alists can share a common tail. Emacs Lisp provides a general-purpose hash table data type, along with a series of functions for operating on them. Hash tables have a special printed representation, which consists of ‘#s’ followed by a list specifying the hash table properties and contents. See Section 7.1 [Creating Hash], page 100. (Note that the term “hash notation”, which refers to the initial ‘#’ character used in the printed representations of objects with no read representation, has nothing to do with the term “hash table”. See Section 2.1 [Printed Representation], page 8.) Obarrays are also a kind of hash table, but they are a different type of object and are used only for recording interned symbols (see Section 8.3 [Creating Symbols], page 107). 7.1 Creating Hash Tables The principal function for creating a hash table is make-hash-table. make-hash-table &rest keyword-args [Function] This function creates a new hash table according to the specified arguments. The arguments should consist of alternating keywords (particular symbols recognized spe- cially) and values corresponding to them. Several keywords make sense in make-hash-table, but the only two that you really need to know about are :test and :weakness. :test test This specifies the method of key lookup for this hash table. The default is eql; eq and equal are other alternatives: eql Keys which are numbers are “the same” if they are equal, that is, if they are equal in value and either both are integers or both are floating point; otherwise, two distinct objects are never “the same”. eq Any two distinct Lisp objects are “different” as keys. equal Two Lisp objects are “the same”, as keys, if they are equal according to equal. You can use define-hash-table-test (see Section 7.3 [Defining Hash], page 103) to define additional possibilities for test. Chapter 7: Hash Tables 101 :weakness weak The weakness of a hash table specifies whether the presence of a key or value in the hash table preserves it from garbage collection. The value, weak, must be one of nil, key, value, key-or-value, key-and-value, or t which is an alias for key-and-value. If weak is key then the hash table does not prevent its keys from being collected as garbage (if they are not referenced anywhere else); if a particular key does get collected, the corresponding association is removed from the hash table. If weak is value, then the hash table does not prevent values from being collected as garbage (if they are not referenced anywhere else); if a particular value does get collected, the corresponding association is removed from the hash table. If weak is key-and-value or t, both the key and the value must be live in order to preserve the association. Thus, the hash table does not protect either keys or values from garbage collection; if either one is collected as garbage, that removes the association. If weak is key-or-value, either the key or the value can preserve the association. Thus, associations are removed from the hash table when both their key and value would be collected as garbage (if not for references from weak hash tables). The default for weak is nil, so that all keys and values referenced in the hash table are preserved from garbage collection. :size size This specifies a hint for how many associations you plan to store in the hash table. If you know the approximate number, you can make things a little more efficient by specifying it this way. If you specify too small a size, the hash table will grow automatically when necessary, but doing that takes some extra time. The default size is 65. :rehash-size rehash-size When you add an association to a hash table and the table is “full”, it grows automatically. This value specifies how to make the hash table larger, at that time. If rehash-size is an integer, it should be positive, and the hash table grows by adding that much to the nominal size. If rehash-size is floating point, it had better be greater than 1, and the hash table grows by multiplying the old size by that number. The default value is 1.5. :rehash-threshold threshold This specifies the criterion for when the hash table is “full” (so it should be made larger). The value, threshold, should be a positive floating-point number, no greater than 1. The hash table is “full” whenever the actual Chapter 7: Hash Tables 102 number of entries exceeds this fraction of the nominal size. The default for threshold is 0.8. makehash &optional test [Function] This is equivalent to make-hash-table, but with a different style argument list. The argument test specifies the method of key lookup. This function is obsolete. Use make-hash-table instead. You can also create a new hash table using the printed representation for hash tables. The Lisp reader can read this printed representation, provided each element in the specified hash table has a valid read syntax (see Section 2.1 [Printed Representation], page 8). For instance, the following specifies a new hash table containing the keys key1 and key2 (both symbols) associated with val1 (a symbol) and 300 (a number) respectively. #s(hash-table size 30 data (key1 val1 key2 300)) The printed representation for a hash table consists of ‘#s’ followed by a list beginning with ‘hash-table’. The rest of the list should consist of zero or more property-value pairs specifying the hash table’s properties and initial contents. The properties and values are read literally. Valid property names are size, test, weakness, rehash-size, rehash-threshold, and data. The data property should be a list of key-value pairs for the initial contents; the other properties have the same meanings as the matching make-hash-table keywords (:size, :test, etc.), described above. Note that you cannot specify a hash table whose initial contents include objects that have no read syntax, such as buffers and frames. Such objects may be added to the hash table after it is created. 7.2 Hash Table Access This section describes the functions for accessing and storing associations in a hash table. In general, any Lisp object can be used as a hash key, unless the comparison method imposes limits. Any Lisp object can also be used as the value. gethash key table &optional default [Function] This function looks up key in table, and returns its associated value—or default, if key has no association in table. puthash key value table [Function] This function enters an association for key in table, with value value. If key already has an association in table, value replaces the old associated value. remhash key table [Function] This function removes the association for key from table, if there is one. If key has no association, remhash does nothing. Common Lisp note: In Common Lisp, remhash returns non-nil if it actually removed an association and nil otherwise. In Emacs Lisp, remhash always returns nil. clrhash table [Function] This function removes all the associations from hash table table, so that it becomes empty. This is also called clearing the hash table. Chapter 7: Hash Tables 103 Common Lisp note: In Common Lisp, clrhash returns the empty table. In Emacs Lisp, it returns nil. maphash function table [Function] This function calls function once for each of the associations in table. The function function should accept two arguments—a key listed in table, and its associated value. maphash returns nil. 7.3 Defining Hash Comparisons You can define new methods of key lookup by means of define-hash-table-test. In order to use this feature, you need to understand how hash tables work, and what a hash code means. You can think of a hash table conceptually as a large array of many slots, each capable of holding one association. To look up a key, gethash first computes an integer, the hash code, from the key. It reduces this integer modulo the length of the array, to produce an index in the array. Then it looks in that slot, and if necessary in other nearby slots, to see if it has found the key being sought. Thus, to define a new method of key lookup, you need to specify both a function to compute the hash code from a key, and a function to compare two keys directly. define-hash-table-test name test-fn hash-fn This function defines a new hash table test, named name. [Function] After defining name in this way, you can use it as the test argument in make-hashtable. When you do that, the hash table will use test-fn to compare key values, and hash-fn to compute a “hash code” from a key value. The function test-fn should accept two arguments, two keys, and return non-nil if they are considered “the same”. The function hash-fn should accept one argument, a key, and return an integer that is the “hash code” of that key. For good results, the function should use the whole range of integers for hash codes, including negative integers. The specified functions are stored in the property list of name under the property hash-table-test; the property value’s form is (test-fn hash-fn). sxhash obj [Function] This function returns a hash code for Lisp object obj. This is an integer which reflects the contents of obj and the other Lisp objects it points to. If two objects obj1 and obj2 are equal, then (sxhash obj1) and (sxhash obj2) are the same integer. If the two objects are not equal, the values returned by sxhash are usually different, but not always; once in a rare while, by luck, you will encounter two distinct-looking objects that give the same result from sxhash. This example creates a hash table whose keys are strings that are compared caseinsensitively. Chapter 7: Hash Tables 104 (defun case-fold-string= (a b) (eq t (compare-strings a nil nil b nil nil t))) (defun case-fold-string-hash (a) (sxhash (upcase a))) (define-hash-table-test ’case-fold ’case-fold-string= ’case-fold-string-hash) (make-hash-table :test ’case-fold) Here is how you could define a hash table test equivalent to the predefined test value equal. The keys can be any Lisp object, and equal-looking objects are considered the same key. (define-hash-table-test ’contents-hash ’equal ’sxhash) (make-hash-table :test ’contents-hash) 7.4 Other Hash Table Functions Here are some other functions for working with hash tables. hash-table-p table This returns non-nil if table is a hash table object. [Function] copy-hash-table table [Function] This function creates and returns a copy of table. Only the table itself is copied—the keys and values are shared. hash-table-count table This function returns the actual number of entries in table. [Function] hash-table-test table [Function] This returns the test value that was given when table was created, to specify how to hash and compare keys. See make-hash-table (see Section 7.1 [Creating Hash], page 100). hash-table-weakness table [Function] This function returns the weak value that was specified for hash table table. hash-table-rehash-size table This returns the rehash size of table. [Function] hash-table-rehash-threshold table This returns the rehash threshold of table. [Function] hash-table-size table This returns the current nominal size of table. [Function] Chapter 8: Symbols 105 8 Symbols A symbol is an object with a unique name. This chapter describes symbols, their components, their property lists, and how they are created and interned. Separate chapters describe the use of symbols as variables and as function names; see Chapter 11 [Variables], page 143, and Chapter 12 [Functions], page 172. For the precise read syntax for symbols, see Section 2.3.4 [Symbol Type], page 13. You can test whether an arbitrary Lisp object is a symbol with symbolp: symbolp object This function returns t if object is a symbol, nil otherwise. [Function] 8.1 Symbol Components Each symbol has four components (or “cells”), each of which references another object: Print name The symbol’s name. Value The symbol’s current value as a variable. Function The symbol’s function definition. It can also hold a symbol, a keymap, or a keyboard macro. Property list The symbol’s property list. The print name cell always holds a string, and cannot be changed. Each of the other three cells can be set to any Lisp object. The print name cell holds the string that is the name of a symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. To get a symbol’s name, use the function symbol-name (see Section 8.3 [Creating Symbols], page 107). The value cell holds a symbol’s value as a variable, which is what you get if the symbol itself is evaluated as a Lisp expression. See Chapter 11 [Variables], page 143, for details about how values are set and retrieved, including complications such as local bindings and scoping rules. Most symbols can have any Lisp object as a value, but certain special symbols have values that cannot be changed; these include nil and t, and any symbol whose name starts with ‘:’ (those are called keywords). See Section 11.2 [Constant Variables], page 143. The function cell holds a symbol’s function definition. Often, we refer to “the function foo” when we really mean the function stored in the function cell of foo; we make the distinction explicit only when necessary. Typically, the function cell is used to hold a function (see Chapter 12 [Functions], page 172) or a macro (see Chapter 13 [Macros], page 198). However, it can also be used to hold a symbol (see Section 9.1.4 [Function Indirection], page 115), keyboard macro (see Section 20.16 [Keyboard Macros], page 365), keymap (see Chapter 21 [Keymaps], page 367), or autoload object (see Section 9.1.8 [Autoloading], page 119). To get the contents of a symbol’s function cell, use the function symbol-function (see Section 12.8 [Function Cells], page 184). Chapter 8: Symbols 106 The property list cell normally should hold a correctly formatted property list. To get a symbol’s property list, use the function symbol-plist. See Section 8.4 [Symbol Properties], page 109. The function cell or the value cell may be void, which means that the cell does not reference any object. (This is not the same thing as holding the symbol void, nor the same as holding the symbol nil.) Examining a function or value cell that is void results in an error, such as ‘Symbol’s value as variable is void’. Because each symbol has separate value and function cells, variables names and function names do not conflict. For example, the symbol buffer-file-name has a value (the name of the file being visited in the current buffer) as well as a function definition (a primitive function that returns the name of the file): buffer-file-name ⇒ "/gnu/elisp/symbols.texi" (symbol-function ’buffer-file-name) ⇒ # 8.2 Defining Symbols A definition is a special kind of Lisp expression that announces your intention to use a symbol in a particular way. It typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable. defvar and defconst are special forms that define a symbol as a global variable—a variable that can be accessed at any point in a Lisp program. See Chapter 11 [Variables], page 143, for details about variables. To define a customizable variable, use the defcustom macro, which also calls defvar as a subroutine (see Chapter 14 [Customization], page 206). In principle, you can assign a variable value to any symbol with setq, whether not it has first been defined as a variable. However, you ought to write a variable definition for each global variable that you want to use; otherwise, your Lisp program may not act correctly if it is evaluated with lexical scoping enabled (see Section 11.9 [Variable Scoping], page 152). defun defines a symbol as a function, creating a lambda expression and storing it in the function cell of the symbol. This lambda expression thus becomes the function definition of the symbol. (The term “function definition”, meaning the contents of the function cell, is derived from the idea that defun gives the symbol its definition as a function.) defsubst and defalias are two other ways of defining a function. See Chapter 12 [Functions], page 172. defmacro defines a symbol as a macro. It creates a macro object and stores it in the function cell of the symbol. Note that a given symbol can be a macro or a function, but not both at once, because both macro and function definitions are kept in the function cell, and that cell can hold only one Lisp object at any given time. See Chapter 13 [Macros], page 198. As previously noted, Emacs Lisp allows the same symbol to be defined both as a variable (e.g., with defvar) and as a function or macro (e.g., with defun). Such definitions do not conflict. Chapter 8: Symbols 107 These definition also act as guides for programming tools. For example, the C-h f and C-h v commands create help buffers containing links to the relevant variable, function, or macro definitions. See Section “Name Help” in The GNU Emacs Manual. 8.3 Creating and Interning Symbols To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion. When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it “hashes” those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J’s and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name’s hash code. (The same idea is used for general Emacs hash tables, but they are a different data type; see Chapter 7 [Hash Tables], page 100.) If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called interning it, and the symbol is then called an interned symbol. Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray. Interning usually happens automatically in the reader, but sometimes other programs need to do it. For example, after the M-x command obtains the command name as a string using the minibuffer, it then interns the string, to get the interned symbol with that name. No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called uninterned symbols. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable. Creating an uninterned symbol is useful in generating Lisp code, because an uninterned symbol used as a variable in the code you generate cannot clash with any variables used in other Lisp programs. In Emacs Lisp, an obarray is actually a vector. Each element of the vector is a bucket; its value is either an interned symbol whose name hashes to that bucket, or 0 if the bucket is empty. Each interned symbol has an internal link (invisible to the user) to the next symbol in the bucket. Because these links are invisible, there is no way to find all the symbols in an obarray except using mapatoms (below). The order of symbols in a bucket is not significant. In an empty obarray, every element is 0, so you can create an obarray with (make-vector length 0). This is the only valid way to create an obarray. Prime numbers as lengths tend to result in good hashing; lengths one less than a power of two are also good. Do not try to put symbols in an obarray yourself. This does not work—only intern can enter a symbol in an obarray properly. Chapter 8: Symbols 108 Common Lisp note: Unlike Common Lisp, Emacs Lisp does not provide for interning a single symbol in several obarrays. Most of the functions below take a name and sometimes an obarray as arguments. A wrong-type-argument error is signaled if the name is not a string, or if the obarray is not a vector. symbol-name symbol This function returns the string that is symbol’s name. For example: [Function] (symbol-name ’foo) ⇒ "foo" Warning: Changing the string by substituting characters does change the name of the symbol, but fails to update the obarray, so don’t do it! make-symbol name [Function] This function returns a newly-allocated, uninterned symbol whose name is name (which must be a string). Its value and function definition are void, and its property list is nil. In the example below, the value of sym is not eq to foo because it is a distinct uninterned symbol whose name is also ‘foo’. (setq sym (make-symbol "foo")) ⇒ foo (eq sym ’foo) ⇒ nil intern name &optional obarray [Function] This function returns the interned symbol whose name is name. If there is no such symbol in the obarray obarray, intern creates a new one, adds it to the obarray, and returns it. If obarray is omitted, the value of the global variable obarray is used. (setq sym (intern "foo")) ⇒ foo (eq sym ’foo) ⇒t (setq sym1 (intern "foo" other-obarray)) ⇒ foo (eq sym1 ’foo) ⇒ nil Common Lisp note: In Common Lisp, you can intern an existing symbol in an obarray. In Emacs Lisp, you cannot do this, because the argument to intern must be a string, not a symbol. intern-soft name &optional obarray [Function] This function returns the symbol in obarray whose name is name, or nil if obarray has no symbol with that name. Therefore, you can use intern-soft to test whether a symbol with a given name is already interned. If obarray is omitted, the value of the global variable obarray is used. The argument name may also be a symbol; in that case, the function returns name if name is interned in the specified obarray, and otherwise nil. Chapter 8: Symbols 109 (intern-soft "frazzle") ⇒ nil (make-symbol "frazzle") ⇒ frazzle (intern-soft "frazzle") ⇒ nil (setq sym (intern "frazzle")) ⇒ frazzle (intern-soft "frazzle") ⇒ frazzle (eq sym ’frazzle) ⇒t ; No such symbol exists. ; Create an uninterned one. ; That one cannot be found. ; Create an interned one. ; That one can be found! ; And it is the same one. obarray This variable is the standard obarray for use by intern and read. [Variable] mapatoms function &optional obarray [Function] This function calls function once with each symbol in the obarray obarray. Then it returns nil. If obarray is omitted, it defaults to the value of obarray, the standard obarray for ordinary symbols. (setq count 0) ⇒0 (defun count-syms (s) (setq count (1+ count))) ⇒ count-syms (mapatoms ’count-syms) ⇒ nil count ⇒ 1871 See documentation in Section 23.2 [Accessing Documentation], page 462, for another example using mapatoms. unintern symbol obarray [Function] This function deletes symbol from the obarray obarray. If symbol is not actually in the obarray, unintern does nothing. If obarray is nil, the current obarray is used. If you provide a string instead of a symbol as symbol, it stands for a symbol name. Then unintern deletes the symbol (if any) in the obarray which has that name. If there is no such symbol, unintern does nothing. If unintern does delete a symbol, it returns t. Otherwise it returns nil. 8.4 Symbol Properties A symbol may possess any number of symbol properties, which can be used to record miscellaneous information about the symbol. For example, when a symbol has a risky-localvariable property with a non-nil value, that means the variable which the symbol names is a risky file-local variable (see Section 11.11 [File Local Variables], page 163). Chapter 8: Symbols 110 Each symbol’s properties and property values are stored in the symbol’s property list cell (see Section 8.1 [Symbol Components], page 105), in the form of a property list (see Section 5.9 [Property Lists], page 86). 8.4.1 Accessing Symbol Properties The following functions can be used to access symbol properties. get symbol property [Function] This function returns the value of the property named property in symbol’s property list. If there is no such property, it returns nil. Thus, there is no distinction between a value of nil and the absence of the property. The name property is compared with the existing property names using eq, so any object is a legitimate property. See put for an example. put symbol property value [Function] This function puts value onto symbol’s property list under the property name property, replacing any previous property value. The put function returns value. (put ’fly ’verb ’transitive) ⇒’transitive (put ’fly ’noun ’(a buzzing little bug)) ⇒ (a buzzing little bug) (get ’fly ’verb) ⇒ transitive (symbol-plist ’fly) ⇒ (verb transitive noun (a buzzing little bug)) symbol-plist symbol This function returns the property list of symbol. [Function] setplist symbol plist [Function] This function sets symbol’s property list to plist. Normally, plist should be a well- formed property list, but this is not enforced. The return value is plist. (setplist ’foo ’(a 1 b (2 3) c nil)) ⇒ (a 1 b (2 3) c nil) (symbol-plist ’foo) ⇒ (a 1 b (2 3) c nil) For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (see Chapter 35 [Abbrevs], page 785). You could define put in terms of setplist and plist-put, as follows: (defun put (symbol prop value) (setplist symbol (plist-put (symbol-plist symbol) prop value))) Chapter 8: Symbols 111 function-get symbol property [Function] This function is identical to get, except that if symbol is the name of a function alias, it looks in the property list of the symbol naming the actual function. See Section 12.4 [Defining Functions], page 178. 8.4.2 Standard Symbol Properties Here, we list the symbol properties which are used for special purposes in Emacs. In the following table, whenever we say “the named function”, that means the function whose name is the relevant symbol; similarly for “the named variable” etc. :advertised-binding This property value specifies the preferred key binding, when showing documentation, for the named function. See Section 23.3 [Keys in Documentation], page 464. char-table-extra-slots The value, if non-nil, specifies the number of extra slots in the named chartable type. See Section 6.6 [Char-Tables], page 94. customized-face face-defface-spec saved-face theme-face These properties are used to record a face’s standard, saved, customized, and themed face specs. Do not set them directly; they are managed by defface and related functions. See Section 37.12.2 [Defining Faces], page 865. customized-value saved-value standard-value theme-value These properties are used to record a customizable variable’s standard value, saved value, customized-but-unsaved value, and themed values. Do not set them directly; they are managed by defcustom and related functions. See Section 14.3 [Variable Definitions], page 209. disabled If the value is non-nil, the named function is disabled as a command. See Section 20.14 [Disabling Commands], page 363. face-documentation The value stores the documentation string of the named face. This is set automatically by defface. See Section 37.12.2 [Defining Faces], page 865. history-length The value, if non-nil, specifies the maximum minibuffer history length for the named history list variable. See Section 19.4 [Minibuffer History], page 295. interactive-form The value is an interactive form for the named function. Normally, you should not set this directly; use the interactive special form instead. See Section 20.3 [Interactive Call], page 328. Chapter 8: Symbols 112 menu-enable The value is an expression for determining whether the named menu item should be enabled in menus. See Section 21.17.1.1 [Simple Menu Items], page 392. mode-class If the value is special, the named major mode is “special”. See Section 22.2.1 [Major Mode Conventions], page 409. permanent-local If the value is non-nil, the named variable is a buffer-local variable whose value should not be reset when changing major modes. See Section 11.10.2 [Creating Buffer-Local], page 158. permanent-local-hook If the value is non-nil, the named function should not be deleted from the local value of a hook variable when changing major modes. See Section 22.1.2 [Setting Hooks], page 407. pure If the value is non-nil, the named function is considered to be side-effect free. Calls with constant arguments can be evaluated at compile time. This may shift run time errors to compile time. risky-local-variable If the value is non-nil, the named variable is considered risky as a file-local variable. See Section 11.11 [File Local Variables], page 163. safe-function If the value is non-nil, the named function is considered generally safe for evaluation. See Section 12.15 [Function Safety], page 196. safe-local-eval-function If the value is non-nil, the named function is safe to call in file-local evaluation forms. See Section 11.11 [File Local Variables], page 163. safe-local-variable The value specifies a function for determining safe file-local values for the named variable. See Section 11.11 [File Local Variables], page 163. side-effect-free A non-nil value indicates that the named function is free of side-effects, for determining function safety (see Section 12.15 [Function Safety], page 196) as well as for byte compiler optimizations. Do not set it. variable-documentation If non-nil, this specifies the named variable’s documentation string. This is set automatically by defvar and related functions. See Section 37.12.2 [Defining Faces], page 865. Chapter 9: Evaluation 113 9 Evaluation The evaluation of expressions in Emacs Lisp is performed by the Lisp interpreter—a program that receives a Lisp object as input and computes its value as an expression. How it does this depends on the data type of the object, according to rules described in this chapter. The interpreter runs automatically to evaluate portions of your program, but can also be called explicitly via the Lisp primitive function eval. A Lisp object that is intended for evaluation is called a form or expression1. The fact that forms are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often. In subsequent sections, we will describe the details of what evaluation means for each kind of form. It is very common to read a Lisp form and then evaluate the form, but reading and evaluation are separate activities, and either can be performed alone. Reading per se does not evaluate anything; it converts the printed representation of a Lisp object to the object itself. It is up to the caller of read to specify whether this object is a form to be evaluated, or serves some entirely different purpose. See Section 18.3 [Input Functions], page 282. Evaluation is a recursive process, and evaluating a form often involves evaluating parts within that form. For instance, when you evaluate a function call form such as (car x), Emacs first evaluates the argument (the subform x). After evaluating the argument, Emacs executes the function (car), and if the function is written in Lisp, execution works by evaluating the body of the function (in this example, however, car is not a Lisp function; it is a primitive function implemented in C). See Chapter 12 [Functions], page 172, for more information about functions and function calls. Evaluation takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables (see Chapter 11 [Variables], page 143).2 Whenever a form refers to a variable without creating a new binding for it, the variable evaluates to the value given by the current environment. Evaluating a form may also temporarily alter the environment by binding variables (see Section 11.3 [Local Variables], page 144). Evaluating a form may also make changes that persist; these changes are called side effects. An example of a form that produces a side effect is (setq foo 1). Do not confuse evaluation with command key interpretation. The editor command loop translates keyboard input into a command (an interactively callable function) using the active keymaps, and then uses call-interactively to execute that command. Executing the command usually involves evaluation, if the command is written in Lisp; however, this step is not considered a part of command key interpretation. See Chapter 20 [Command Loop], page 321. 1 It is sometimes also referred to as an S-expression or sexp, but we generally do not use this terminology in this manual. 2 This definition of “environment” is specifically not intended to include all the data that can affect the result of a program. Chapter 9: Evaluation 114 9.1 Kinds of Forms A Lisp object that is intended to be evaluated is called a form (or an expression). How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and “all other types”. This section describes all three kinds, one by one, starting with the “all other types” which are self-evaluating forms. 9.1.1 Self-Evaluating Forms A self-evaluating form is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string "foo" evaluates to the string "foo". Likewise, evaluating a vector does not cause evaluation of the elements of the vector—it returns the same vector with its contents unchanged. ’123 ; A number, shown without evaluation. ⇒ 123 123 ; Evaluated as usual—result is the same. ⇒ 123 (eval ’123) ; Evaluated ‘‘by hand”—result is the same. ⇒ 123 (eval (eval ’123)) ; Evaluating twice changes nothing. ⇒ 123 It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there’s no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example: ;; Build an expression containing a buffer object. (setq print-exp (list ’print (current-buffer))) ⇒ (print #) ;; Evaluate it. (eval print-exp) # ⇒ # 9.1.2 Symbol Forms When a symbol is evaluated, it is treated as a variable. The result is the variable’s value, if it has one. If the symbol has no value as a variable, the Lisp interpreter signals an error. For more information on the use of variables, see Chapter 11 [Variables], page 143. In the following example, we set the value of a symbol with setq. Then we evaluate the symbol, and get back the value that setq stored. (setq a 123) ⇒ 123 (eval ’a) ⇒ 123 a ⇒ 123 Chapter 9: Evaluation 115 The symbols nil and t are treated specially, so that the value of nil is always nil, and the value of t is always t; you cannot set or bind them to any other values. Thus, these two symbols act like self-evaluating forms, even though eval treats them like any other symbol. A symbol whose name starts with ‘:’ also self-evaluates in the same way; likewise, its value ordinarily cannot be changed. See Section 11.2 [Constant Variables], page 143. 9.1.3 Classification of List Forms A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The remaining list elements constitute the arguments for the function, macro, or special form. The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is not evaluated, as it would be in some Lisp dialects such as Scheme. 9.1.4 Symbol Function Indirection If the first element of the list is a symbol then evaluation examines the symbol’s function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called symbol function indirection, is repeated until it obtains a non-symbol. See Section 12.3 [Function Names], page 177, for more information about symbol function indirection. One possible consequence of this process is an infinite loop, in the event that a symbol’s function cell refers to the same symbol. Otherwise, we eventually obtain a non-symbol, which ought to be a function or other suitable object. More precisely, we should now have a Lisp function (a lambda expression), a byte-code function, a primitive function, a Lisp macro, a special form, or an autoload object. Each of these types is a case described in one of the following sections. If the object is not one of these types, Emacs signals an invalid-function error. The following example illustrates the symbol indirection process. We use fset to set the function cell of a symbol and symbol-function to get the function cell contents (see Section 12.8 [Function Cells], page 184). Specifically, we store the symbol car into the function cell of first, and the symbol first into the function cell of erste. ;; Build this function cell linkage: ;; ------------- ----- ------- ------- ;; | # | <-- | car | <-- | first | <-- | erste | ;; ------------- ----- ------- ------- (symbol-function ’car) ⇒ # (fset ’first ’car) ⇒ car (fset ’erste ’first) ⇒ first (erste ’(1 2 3)) ; Call the function referenced by erste. ⇒1 By contrast, the following example calls a function without any symbol function indi- rection, because the first element is an anonymous Lisp function, not a symbol. Chapter 9: Evaluation 116 ((lambda (arg) (erste arg)) ’(1 2 3)) ⇒1 Executing the function itself evaluates its body; this does involve symbol function indirection when calling erste. This form is rarely used and is now deprecated. Instead, you should write it as: (funcall (lambda (arg) (erste arg)) ’(1 2 3)) or just (let ((arg ’(1 2 3))) (erste arg)) The built-in function indirect-function provides an easy way to perform symbol function indirection explicitly. indirect-function function &optional noerror [Function] This function returns the meaning of function as a function. If function is a symbol, then it finds function’s function definition and starts over with that value. If function is not a symbol, then it returns function itself. This function signals a void-function error if the final symbol is unbound and optional argument noerror is nil or omitted. Otherwise, if noerror is non-nil, it returns nil if the final symbol is unbound. It signals a cyclic-function-indirection error if there is a loop in the chain of symbols. Here is how you could define indirect-function in Lisp: (defun indirect-function (function) (if (symbolp function) (indirect-function (symbol-function function)) function)) 9.1.5 Evaluation of Function Forms If the first element of a list being evaluated is a Lisp function object, byte-code object or primitive function object, then that list is a function call. For example, here is a call to the function +: (+ 1 x) The first step in evaluating a function call is to evaluate the remaining elements of the list from left to right. The results are the actual argument values, one value for each list element. The next step is to call the function with this list of arguments, effectively using the function apply (see Section 12.5 [Calling Functions], page 179). If the function is written in Lisp, the arguments are used to bind the argument variables of the function (see Section 12.2 [Lambda Expressions], page 174); then the forms in the function body are evaluated in order, and the value of the last body form becomes the value of the function call. Chapter 9: Evaluation 117 9.1.6 Lisp Macro Evaluation If the first element of a list being evaluated is a macro object, then the list is a macro call. When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the expansion of the macro, to be evaluated in place of the original form. The expansion may be any sort of form: a selfevaluating constant, a symbol, or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results. Ordinary evaluation of a macro call finishes by evaluating the expansion. However, the macro expansion is not necessarily evaluated right away, or at all, because other programs also expand macro calls, and they may or may not evaluate the expansions. Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are computed when the expansion is evaluated. For example, given a macro defined as follows: (defmacro cadr (x) (list ’car (list ’cdr x))) an expression such as (cadr (assq ’handler list)) is a macro call, and its expansion is: (car (cdr (assq ’handler list))) Note that the argument (assq ’handler list) appears in the expansion. See Chapter 13 [Macros], page 198, for a complete description of Emacs Lisp macros. 9.1.7 Special Forms A special form is a primitive function specially marked so that its arguments are not all evaluated. Most special forms define control structures or perform variable bindings—things which functions cannot do. Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments. If an expression’s first symbol is that of a special form, the expression should follow the rules of that special form; otherwise, Emacs’s behavior is not well-defined (though it will not crash). For example, ((lambda (x) x . 3) 4) contains a subexpression that begins with lambda but is not a well-formed lambda expression, so Emacs may signal an error, or may return 3 or 4 or nil, or may behave in other ways. special-form-p object [Function] This predicate tests whether its argument is a special form, and returns t if so, nil otherwise. Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described. and see Section 10.3 [Combining Conditions], page 129 catch see Section 10.5.1 [Catch and Throw], page 131 cond see Section 10.2 [Conditionals], page 125 Chapter 9: Evaluation 118 condition-case see Section 10.5.3.3 [Handling Errors], page 136 defconst see Section 11.5 [Defining Variables], page 147 defvar see Section 11.5 [Defining Variables], page 147 function see Section 12.7 [Anonymous Functions], page 182 if see Section 10.2 [Conditionals], page 125 interactive see Section 20.3 [Interactive Call], page 328 lambda see Section 12.2 [Lambda Expressions], page 174 let let* see Section 11.3 [Local Variables], page 144 or see Section 10.3 [Combining Conditions], page 129 prog1 prog2 progn see Section 10.1 [Sequencing], page 124 quote see Section 9.2 [Quoting], page 119 save-current-buffer see Section 26.2 [Current Buffer], page 523 save-excursion see Section 29.3 [Excursions], page 642 save-restriction see Section 29.4 [Narrowing], page 643 setq see Section 11.8 [Setting Variables], page 151 setq-default see Section 11.10.2 [Creating Buffer-Local], page 158 track-mouse see Section 28.13 [Mouse Tracking], page 621 unwind-protect see Section 10.5 [Nonlocal Exits], page 131 while see Section 10.4 [Iteration], page 130 Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp. setq, if, and catch are special forms in both Emacs Lisp and Common Lisp. save-excursion is a special form in Emacs Lisp, but doesn’t exist in Common Lisp. throw is a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn’t have multiple values). Chapter 9: Evaluation 119 9.1.8 Autoloading The autoload feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs. It specifies which file contains the definition. When an autoload object appears as a symbol’s function definition, calling that symbol as a function automatically loads the specified file; then it calls the real definition loaded from that file. The way to arrange for an autoload object to appear as a symbol’s function definition is described in Section 15.5 [Autoload], page 230. 9.2 Quoting The special form quote returns its single argument, as written, without evaluating it. This provides a way to include constant symbols and lists, which are not self-evaluating objects, in a program. (It is not necessary to quote self-evaluating objects such as numbers, strings, and vectors.) quote object This special form returns object, without evaluating it. [Special Form] Because quote is used so often in programs, Lisp provides a convenient read syntax for it. An apostrophe character (‘’’) followed by a Lisp object (in read syntax) expands to a list whose first element is quote, and whose second element is the object. Thus, the read syntax ’x is an abbreviation for (quote x). Here are some examples of expressions that use quote: (quote (+ 1 2)) ⇒ (+ 1 2) (quote foo) ⇒ foo ’foo ⇒ foo ’’foo ⇒ (quote foo) ’(quote foo) ⇒ (quote foo) [’foo] ⇒ [(quote foo)] Other quoting constructs include function (see Section 12.7 [Anonymous Functions], page 182), which causes an anonymous lambda expression written in Lisp to be compiled, and ‘‘’ (see Section 9.3 [Backquote], page 119), which is used to quote only part of a list, while computing and substituting other parts. 9.3 Backquote Backquote constructs allow you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form quote For example, these two forms yield identical results: ‘(a list of (+ 2 3) elements) ⇒ (a list of (+ 2 3) elements) Chapter 9: Evaluation 120 ’(a list of (+ 2 3) elements) ⇒ (a list of (+ 2 3) elements) The special marker ‘,’ inside of the argument to backquote indicates a value that isn’t constant. The Emacs Lisp evaluator evaluates the argument of ‘,’, and puts the value in the list structure: ‘(a list of ,(+ 2 3) elements) ⇒ (a list of 5 elements) Substitution with ‘,’ is allowed at deeper levels of the list structure also. For example: ‘(1 2 (3 ,(+ 4 5))) ⇒ (1 2 (3 9)) You can also splice an evaluated value into the resulting list, using the special marker ‘,@’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘‘’ is often unreadable. Here are some examples: (setq some-list ’(2 3)) ⇒ (2 3) (cons 1 (append some-list ’(4) some-list)) ⇒ (1 2 3 4 2 3) ‘(1 ,@some-list 4 ,@some-list) ⇒ (1 2 3 4 2 3) (setq list ’(hack foo bar)) ⇒ (hack foo bar) (cons ’use (cons ’the (cons ’words (append (cdr list) ’(as elements))))) ⇒ (use the words foo bar as elements) ‘(use the words ,@(cdr list) as elements) ⇒ (use the words foo bar as elements) 9.4 Eval Most often, forms are evaluated automatically, by virtue of their occurrence in a program being run. On rare occasions, you may need to write code that evaluates a form that is computed at run time, such as after reading a form from text being edited or getting one from a property list. On these occasions, use the eval function. Often eval is not needed and something else should be used instead. For example, to get the value of a variable, while eval works, symbol-value is preferable; or rather than store expressions in a property list that then need to go through eval, it is better to store functions instead that are then passed to funcall. The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (see Chapter 15 [Loading], page 225). It is generally cleaner and more flexible to store a function in a data structure, and call it with funcall or apply, than to store an expression in the data structure and evaluate it. Using functions provides the ability to pass information to them as arguments. Chapter 9: Evaluation 121 eval form &optional lexical [Function] This is the basic function for evaluating an expression. It evaluates form in the current environment, and returns the result. The type of the form object determines how it is evaluated. See Section 9.1 [Forms], page 114. The argument lexical specifies the scoping rule for local variables (see Section 11.9 [Variable Scoping], page 152). If it is omitted or nil, that means to evaluate form using the default dynamic scoping rule. If it is t, that means to use the lexical scoping rule. The value of lexical can also be a non-empty alist specifying a particular lexical environment for lexical bindings; however, this feature is only useful for specialized purposes, such as in Emacs Lisp debuggers. See Section 11.9.3 [Lexical Binding], page 154. Since eval is a function, the argument expression that appears in a call to eval is evaluated twice: once as preparation before eval is called, and again by the eval function itself. Here is an example: (setq foo ’bar) ⇒ bar (setq bar ’baz) ⇒ baz ;; Here eval receives argument foo (eval ’foo) ⇒ bar ;; Here eval receives argument bar, which is the value of foo (eval foo) ⇒ baz The number of currently active calls to eval is limited to max-lisp-eval-depth (see below). eval-region start end &optional stream read-function [Command] This function evaluates the forms in the current buffer in the region defined by the positions start and end. It reads forms from the region and calls eval on them until the end of the region is reached, or until an error is signaled and not handled. By default, eval-region does not produce any output. However, if stream is nonnil, any output produced by output functions (see Section 18.5 [Output Functions], page 285), as well as the values that result from evaluating the expressions in the region are printed using stream. See Section 18.4 [Output Streams], page 283. If read-function is non-nil, it should be a function, which is used instead of read to read expressions one by one. This function is called with one argument, the stream for reading input. You can also use the variable load-read-function (see [How Programs Do Loading], page 227) to specify this function, but it is more robust to use the read-function argument. eval-region does not move point. It always returns nil. eval-buffer &optional buffer-or-name stream filename unibyte print [Command] This is similar to eval-region, but the arguments provide different optional fea- tures. eval-buffer operates on the entire accessible portion of buffer buffer-or-name. buffer-or-name can be a buffer, a buffer name (a string), or nil (or omitted), which Chapter 9: Evaluation 122 means to use the current buffer. stream is used as in eval-region, unless stream is nil and print non-nil. In that case, values that result from evaluating the expressions are still discarded, but the output of the output functions is printed in the echo area. filename is the file name to use for load-history (see Section 15.9 [Unloading], page 237), and defaults to buffer-file-name (see Section 26.4 [Buffer File Name], page 527). If unibyte is non-nil, read converts strings to unibyte whenever possible. eval-current-buffer is an alias for this command. max-lisp-eval-depth [User Option] This variable defines the maximum depth allowed in calls to eval, apply, and funcall before an error is signaled (with error message "Lisp nesting exceeds max-lisp- eval-depth"). This limit, with the associated error when it is exceeded, is one way Emacs Lisp avoids infinite recursion on an ill-defined function. If you increase the value of max-lispeval-depth too much, such code can cause stack overflow instead. The depth limit counts internal uses of eval, apply, and funcall, such as for calling the functions mentioned in Lisp expressions, and recursive evaluation of function call arguments and function body forms, as well as explicit calls in Lisp code. The default value of this variable is 400. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute. max-specpdl-size provides another limit on nesting. See [Local Variables], page 145. values [Variable] The value of this variable is a list of the values returned by all the expressions that were read, evaluated, and printed from buffers (including the minibuffer) by the standard Emacs commands which do this. (Note that this does not include evaluation in *ielm* buffers, nor evaluation using C-j in lisp-interaction-mode.) The elements are ordered most recent first. (setq x 1) ⇒1 (list ’A (1+ 2) auto-save-default) ⇒ (A 3 t) values ⇒ ((A 3 t) 1 ...) This variable is useful for referring back to values of forms recently evaluated. It is generally a bad idea to print the value of values itself, since this may be very long. Instead, examine particular elements, like this: ;; Refer to the most recent evaluation result. (nth 0 values) ⇒ (A 3 t) ;; That put a new element on, ;; so all elements move back one. (nth 1 values) ⇒ (A 3 t) Chapter 9: Evaluation 123 ;; This gets the element that was next-to-most-recent ;; before this example. (nth 3 values) ⇒1 Chapter 10: Control Structures 124 10 Control Structures A Lisp program consists of a set of expressions, or forms (see Section 9.1 [Forms], page 114). We control the order of execution of these forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain. The simplest order of execution is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code—the forms are executed in the order written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b. The result of evaluating b becomes the value of the function. Explicit control structures make possible an order of execution other than sequential. Emacs Lisp provides several kinds of control structure, including other varieties of sequencing, conditionals, iteration, and (controlled) jumps—all discussed below. The built-in control structures are special forms since their subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see Chapter 13 [Macros], page 198). 10.1 Sequencing Evaluating forms in the order they appear is the most common way control passes from one form to another. In some contexts, such as in a function body, this happens automatically. Elsewhere you must use a control structure construct to do this: progn, the simplest control construct of Lisp. A progn special form looks like this: (progn a b c ...) and it says to execute the forms a, b, c, and so on, in that order. These forms are called the body of the progn form. The value of the last form in the body becomes the value of the entire progn. (progn) returns nil. In the early days of Lisp, progn was the only way to execute two or more forms in succession and use the value of the last of them. But programmers found they often needed to use a progn in the body of a function, where (at that time) only one form was allowed. So the body of a function was made into an “implicit progn”: several forms are allowed just as in the body of an actual progn. Many other control structures likewise contain an implicit progn. As a result, progn is not used as much as it was many years ago. It is needed now most often inside an unwind-protect, and, or, or in the then-part of an if. progn forms. . . [Special Form] This special form evaluates all of the forms, in textual order, returning the result of the final form. Chapter 10: Control Structures 125 (progn (print "The first form") (print "The second form") (print "The third form")) "The first form" "The second form" "The third form" ⇒ "The third form" Two other constructs likewise evaluate a series of forms but return different values: prog1 form1 forms. . . [Special Form] This special form evaluates form1 and all of the forms, in textual order, returning the result of form1. (prog1 (print "The first form") (print "The second form") (print "The third form")) "The first form" "The second form" "The third form" ⇒ "The first form" Here is a way to remove the first element from a list in the variable x, then return the value of that former element: (prog1 (car x) (setq x (cdr x))) prog2 form1 form2 forms. . . [Special Form] This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2. (prog2 (print "The first form") (print "The second form") (print "The third form")) "The first form" "The second form" "The third form" ⇒ "The second form" 10.2 Conditionals Conditional control structures choose among alternatives. Emacs Lisp has four conditional forms: if, which is much the same as in other languages; when and unless, which are variants of if; and cond, which is a generalized case statement. if condition then-form else-forms. . . [Special Form] if chooses between the then-form and the else-forms based on the value of condition. If the evaluated condition is non-nil, then-form is evaluated and the result returned. Otherwise, the else-forms are evaluated in textual order, and the value of the last one is returned. (The else part of if is an example of an implicit progn. See Section 10.1 [Sequencing], page 124.) If condition has the value nil, and no else-forms are given, if returns nil. Chapter 10: Control Structures 126 if is a special form because the branch that is not selected is never evaluated—it is ignored. Thus, in this example, true is not printed because print is never called: (if nil (print ’true) ’very-false) ⇒ very-false when condition then-forms. . . [Macro] This is a variant of if where there are no else-forms, and possibly several then-forms. In particular, (when condition a b c) is entirely equivalent to (if condition (progn a b c) nil) unless condition forms. . . This is a variant of if where there is no then-form: (unless condition a b c) is entirely equivalent to (if condition nil a b c) [Macro] cond clause. . . [Special Form] cond chooses among an arbitrary number of alternatives. Each clause in the cond must be a list. The car of this list is the condition; the remaining elements, if any, the body-forms. Thus, a clause looks like this: (condition body-forms...) cond tries the clauses in textual order, by evaluating the condition of each clause. If the value of condition is non-nil, the clause “succeeds”; then cond evaluates its body-forms, and returns the value of the last of body-forms. Any remaining clauses are ignored. If the value of condition is nil, the clause “fails”, so the cond moves on to the following clause, trying its condition. A clause may also look like this: (condition) Then, if condition is non-nil when tested, the cond form returns the value of condition. If every condition evaluates to nil, so that every clause fails, cond returns nil. The following example has four clauses, which test for the cases where the value of x is a number, string, buffer and symbol, respectively: (cond ((numberp x) x) ((stringp x) x) ((bufferp x) (setq temporary-hack x) ; multiple body-forms (buffer-name x)) ; in one clause ((symbolp x) (symbol-value x))) Chapter 10: Control Structures 127 Often we want to execute the last clause whenever none of the previous clauses was successful. To do this, we use t as the condition of the last clause, like this: (t body-forms). The form t evaluates to t, which is never nil, so this clause never fails, provided the cond gets to it at all. For example: (setq a 5) (cond ((eq a ’hack) ’foo) (t "default")) ⇒ "default" This cond expression returns foo if the value of a is hack, and returns the string "default" otherwise. Any conditional construct can be expressed with cond or with if. Therefore, the choice between them is a matter of style. For example: (if a b c) ≡ (cond (a b) (t c)) 10.2.1 Pattern matching case statement To compare a particular value against various possible cases, the macro pcase can come handy. It takes the following form: (pcase exp branch1 branch2 branch3 ...) where each branch takes the form (upattern body-forms...). It will first evaluate exp and then compare the value against each upattern to see which branch to use, after which it will run the corresponding body-forms. A common use case is to distinguish between a few different constant values: (pcase (get-return-code x) (‘success (message "Done!")) (‘would-block (message "Sorry, can’t do it now")) (‘read-only (message "The shmliblick is read-only")) (‘access-denied (message "You do not have the needed rights")) (code (message "Unknown return code %S" code))) In the last clause, code is a variable that gets bound to the value that was returned by (get-return-code x). To give a more complex example, a simple interpreter for a little expression language could look like (note that this example requires lexical binding): (defun evaluate (exp env) (pcase exp (‘(add ,x ,y) (+ (evaluate x env) (evaluate y env))) (‘(call ,fun ,arg) (funcall (evaluate fun env) (evaluate arg env))) (‘(fn ,arg ,body) (lambda (val) (evaluate body (cons (cons arg val) env)))) ((pred numberp) exp) ((pred symbolp) (cdr (assq exp env))) (_ (error "Unknown expression %S" exp)))) Chapter 10: Control Structures 128 Where ‘(add ,x ,y) is a pattern that checks that exp is a three element list starting with the symbol add, then extracts the second and third elements and binds them to the variables x and y. (pred numberp) is a pattern that simply checks that exp is a number, and _ is the catch-all pattern that matches anything. Here are some sample programs including their evaluation results: (evaluate ’(add 1 2) nil) ;=> 3 (evaluate ’(add x y) ’((x . 1) (y . 2))) ;=> 3 (evaluate ’(call (fn x (add 1 x)) 2) nil) ;=> 3 (evaluate ’(sub 1 2) nil) ;=> error There are two kinds of patterns involved in pcase, called U-patterns and Q-patterns. The upattern mentioned above are U-patterns and can take the following forms: ‘qpattern This is one of the most common form of patterns. The intention is to mimic the backquote macro: this pattern matches those values that could have been built by such a backquote expression. Since we’re pattern matching rather than building a value, the unquote does not indicate where to plug an expression, but instead it lets one specify a U-pattern that should match the value at that location. More specifically, a Q-pattern can take the following forms: (qpattern1 . qpattern2) This pattern matches any cons cell whose car matches QPATTERN1 and whose cdr matches PATTERN2. atom This pattern matches any atom equal to atom. ,upattern This pattern matches any object that matches the upattern. symbol A mere symbol in a U-pattern matches anything, and additionally let-binds this symbol to the value that it matched, so that you can later refer to it, either in the body-forms or also later in the pattern. _ This so-called don’t care pattern matches anything, like the previous one, but unlike symbol patterns it does not bind any variable. (pred pred) This pattern matches if the function pred returns non-nil when called with the object being matched. (or upattern1 upattern2...) This pattern matches as soon as one of the argument patterns succeeds. All argument patterns should let-bind the same variables. (and upattern1 upattern2...) This pattern matches only if all the argument patterns succeed. (guard exp) This pattern ignores the object being examined and simply succeeds if exp evaluates to non-nil and fails otherwise. It is typically used inside an and pattern. For example, (and x (guard (< x 10))) is a pattern which matches any number smaller than 10 and let-binds it to the variable x. Chapter 10: Control Structures 129 10.3 Constructs for Combining Conditions This section describes three constructs that are often used together with if and cond to express complicated conditions. The constructs and and or can also be used individually as kinds of multiple conditional constructs. not condition [Function] This function tests for the falsehood of condition. It returns t if condition is nil, and nil otherwise. The function not is identical to null, and we recommend using the name null if you are testing for an empty list. and conditions. . . [Special Form] The and special form tests whether all the conditions are true. It works by evaluating the conditions one by one in the order written. If any of the conditions evaluates to nil, then the result of the and must be nil regardless of the remaining conditions; so and returns nil right away, ignoring the remaining conditions. If all the conditions turn out non-nil, then the value of the last of them becomes the value of the and form. Just (and), with no conditions, returns t, appropriate because all the conditions turned out non-nil. (Think about it; which one did not?) Here is an example. The first condition returns the integer 1, which is not nil. Similarly, the second condition returns the integer 2, which is not nil. The third condition is nil, so the remaining condition is never evaluated. (and (print 1) (print 2) nil (print 3)) 1 2 ⇒ nil Here is a more realistic example of using and: (if (and (consp foo) (eq (car foo) ’x)) (message "foo is a list starting with x")) Note that (car foo) is not executed if (consp foo) returns nil, thus avoiding an error. and expressions can also be written using either if or cond. Here’s how: (and arg1 arg2 arg3) ≡ (if arg1 (if arg2 arg3)) ≡ (cond (arg1 (cond (arg2 arg3)))) or conditions. . . [Special Form] The or special form tests whether at least one of the conditions is true. It works by evaluating all the conditions one by one in the order written. If any of the conditions evaluates to a non-nil value, then the result of the or must be non-nil; so or returns right away, ignoring the remaining conditions. The value it returns is the non-nil value of the condition just evaluated. Chapter 10: Control Structures 130 If all the conditions turn out nil, then the or expression returns nil. Just (or), with no conditions, returns nil, appropriate because all the conditions turned out nil. (Think about it; which one did not?) For example, this expression tests whether x is either nil or the integer zero: (or (eq x nil) (eq x 0)) Like the and construct, or can be written in terms of cond. For example: (or arg1 arg2 arg3) ≡ (cond (arg1) (arg2) (arg3)) You could almost write or in terms of if, but not quite: (if arg1 arg1 (if arg2 arg2 arg3)) This is not completely equivalent because it can evaluate arg1 or arg2 twice. By contrast, (or arg1 arg2 arg3) never evaluates any argument more than once. 10.4 Iteration Iteration means executing part of a program repetitively. For example, you might want to repeat some computation once for each element of a list, or once for each integer from 0 to n. You can do this in Emacs Lisp with the special form while: while condition forms. . . [Special Form] while first evaluates condition. If the result is non-nil, it evaluates forms in textual order. Then it reevaluates condition, and if the result is non-nil, it evaluates forms again. This process repeats until condition evaluates to nil. There is no limit on the number of iterations that may occur. The loop will continue until either condition evaluates to nil or until an error or throw jumps out of it (see Section 10.5 [Nonlocal Exits], page 131). The value of a while form is always nil. (setq num 0) ⇒0 (while (< num 4) (princ (format "Iteration %d." num)) (setq num (1+ num))) Iteration 0. Iteration 1. Iteration 2. Iteration 3. ⇒ nil To write a “repeat...until” loop, which will execute something on each iteration and then do the end-test, put the body followed by the end-test in a progn as the first argument of while, as shown here: Chapter 10: Control Structures 131 (while (progn (forward-line 1) (not (looking-at "^$")))) This moves forward one line and continues moving by lines until it reaches an empty line. It is peculiar in that the while has no body, just the end test (which also does the real work of moving point). The dolist and dotimes macros provide convenient ways to write two common kinds of loops. dolist (var list [result]) body. . . [Macro] This construct executes body once for each element of list, binding the variable var locally to hold the current element. Then it returns the value of evaluating result, or nil if result is omitted. For example, here is how you could use dolist to define the reverse function: (defun reverse (list) (let (value) (dolist (elt list value) (setq value (cons elt value))))) dotimes (var count [result]) body. . . [Macro] This construct executes body once for each integer from 0 (inclusive) to count (exclu- sive), binding the variable var to the integer for the current iteration. Then it returns the value of evaluating result, or nil if result is omitted. Here is an example of using dotimes to do something 100 times: (dotimes (i 100) (insert "I will not obey absurd orders\n")) 10.5 Nonlocal Exits A nonlocal exit is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited. 10.5.1 Explicit Nonlocal Exits: catch and throw Most control constructs affect only the flow of control within the construct itself. The function throw is the exception to this rule of normal program execution: it performs a nonlocal exit on request. (There are other exceptions, but they are for error handling only.) throw is used inside a catch, and jumps back to that catch. For example: Chapter 10: Control Structures 132 (defun foo-outer () (catch ’foo (foo-inner))) (defun foo-inner () ... (if x (throw ’foo t)) ...) The throw form, if executed, transfers control straight back to the corresponding catch, which returns immediately. The code following the throw is not executed. The second argument of throw is used as the return value of the catch. The function throw finds the matching catch based on the first argument: it searches for a catch whose first argument is eq to the one specified in the throw. If there is more than one applicable catch, the innermost one takes precedence. Thus, in the above example, the throw specifies foo, and the catch in foo-outer specifies the same symbol, so that catch is the applicable one (assuming there is no other matching catch in between). Executing throw exits all Lisp constructs up to the matching catch, including function calls. When binding constructs such as let or function calls are exited in this way, the bindings are unbound, just as they are when these constructs exit normally (see Section 11.3 [Local Variables], page 144). Likewise, throw restores the buffer and position saved by save-excursion (see Section 29.3 [Excursions], page 642), and the narrowing status saved by save-restriction. It also runs any cleanups established with the unwind-protect special form when it exits that form (see Section 10.5.4 [Cleanups], page 141). The throw need not appear lexically within the catch that it jumps to. It can equally well be called from another function called within the catch. As long as the throw takes place chronologically after entry to the catch, and chronologically before exit from it, it has access to that catch. This is why throw can be used in commands such as exit-recursiveedit that throw back to the editor command loop (see Section 20.13 [Recursive Editing], page 362). Common Lisp note: Most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially: return, return-from, and go, for example. Emacs Lisp has only throw. The cl-lib library provides versions of some of these. See Section “Blocks and Exits” in Common Lisp Extensions. catch tag body. . . [Special Form] catch establishes a return point for the throw function. The return point is distin- guished from other such return points by tag, which may be any Lisp object except nil. The argument tag is evaluated normally before the return point is established. With the return point in effect, catch evaluates the forms of the body in textual order. If the forms execute normally (without error or nonlocal exit) the value of the last body form is returned from the catch. If a throw is executed during the execution of body, specifying the same value tag, the catch form exits immediately; the value it returns is whatever was specified as the second argument of throw. Chapter 10: Control Structures 133 throw tag value [Function] The purpose of throw is to return from a return point previously established with catch. The argument tag is used to choose among the various existing return points; it must be eq to the value specified in the catch. If multiple return points match tag, the innermost one is used. The argument value is used as the value to return from that catch. If no return point is in effect with tag tag, then a no-catch error is signaled with data (tag value). 10.5.2 Examples of catch and throw One way to use catch and throw is to exit from a doubly nested loop. (In most languages, this would be done with a “goto”.) Here we compute (foo i j) for i and j varying from 0 to 9: (defun search-foo () (catch ’loop (let ((i 0)) (while (< i 10) (let ((j 0)) (while (< j 10) (if (foo i j) (throw ’loop (list i j))) (setq j (1+ j)))) (setq i (1+ i)))))) If foo ever returns non-nil, we stop immediately and return a list of i and j. If foo always returns nil, the catch returns normally, and the value is nil, since that is the result of the while. Here are two tricky examples, slightly different, showing two return points at once. First, two return points with the same tag, hack: (defun catch2 (tag) (catch tag (throw ’hack ’yes))) ⇒ catch2 (catch ’hack (print (catch2 ’hack)) ’no) yes ⇒ no Since both return points have tags that match the throw, it goes to the inner one, the one established in catch2. Therefore, catch2 returns normally with value yes, and this value is printed. Finally the second body form in the outer catch, which is ’no, is evaluated and returned from the outer catch. Now let’s change the argument given to catch2: Chapter 10: Control Structures 134 (catch ’hack (print (catch2 ’quux)) ’no) ⇒ yes We still have two return points, but this time only the outer one has the tag hack; the inner one has the tag quux instead. Therefore, throw makes the outer catch return the value yes. The function print is never called, and the body-form ’no is never evaluated. 10.5.3 Errors When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error. When an error is signaled, Emacs’s default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer. In complicated programs, simple termination may not be what you want. For example, the program may have made temporary changes in data structures, or created temporary buffers that should be deleted before the program is finished. In such cases, you would use unwind-protect to establish cleanup expressions to be evaluated in case of error. (See Section 10.5.4 [Cleanups], page 141.) Occasionally, you may wish the program to continue execution despite an error in a subroutine. In these cases, you would use condition-case to establish error handlers to recover control in case of error. Resist the temptation to use error handling to transfer control from one part of the program to another; use catch and throw instead. See Section 10.5.1 [Catch and Throw], page 131. 10.5.3.1 How to Signal an Error Signaling an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (see Section 10.5.3.2 [Processing of Errors], page 136). Here we describe how to signal an error. Most errors are signaled “automatically” within Lisp primitives which you call for other purposes, such as if you try to take the car of an integer or move forward a character at the end of the buffer. You can also signal errors explicitly with the functions error and signal. Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See Section 20.11 [Quitting], page 358. Every error specifies an error message, one way or another. The message should state what is wrong (“File does not exist”), not how things ought to be (“File must exist”). The convention in Emacs Lisp is that error messages should start with a capital letter, but should not end with any sort of punctuation. error format-string &rest args [Function] This function signals an error with an error message constructed by applying format (see Section 4.7 [Formatting Strings], page 57) to format-string and args. These examples show typical uses of error: Chapter 10: Control Structures 135 (error "That is an error -- try something else") error That is an error -- try something else (error "You have committed %d errors" 10) error You have committed 10 errors error works by calling signal with two arguments: the error symbol error, and a list containing the string returned by format. Warning: If you want to use your own string as an error message verbatim, don’t just write (error string). If string contains ‘%’, it will be interpreted as a format specifier, with undesirable results. Instead, use (error "%s" string). signal error-symbol data [Function] This function signals an error named by error-symbol. The argument data is a list of additional Lisp objects relevant to the circumstances of the error. The argument error-symbol must be an error symbol—a symbol defined with define-error. This is how Emacs Lisp classifies different sorts of errors. See Section 10.5.3.4 [Error Symbols], page 140, for a description of error symbols, error conditions and condition names. If the error is not handled, the two arguments are used in printing the error message. Normally, this error message is provided by the error-message property of errorsymbol. If data is non-nil, this is followed by a colon and a comma separated list of the unevaluated elements of data. For error, the error message is the car of data (that must be a string). Subcategories of file-error are handled specially. The number and significance of the objects in data depends on error-symbol. For example, with a wrong-type-argument error, there should be two objects in the list: a predicate that describes the type that was expected, and the object that failed to fit that type. Both error-symbol and data are available to any error handlers that handle the error: condition-case binds a local variable to a list of the form (error-symbol . data) (see Section 10.5.3.3 [Handling Errors], page 136). The function signal never returns. (signal ’wrong-number-of-arguments ’(x y)) error Wrong number of arguments: x, y (signal ’no-such-error ’("My unknown error condition")) error peculiar error: "My unknown error condition" user-error format-string &rest args [Function] This function behaves exactly like error, except that it uses the error symbol user-error rather than error. As the name suggests, this is intended to report errors on the part of the user, rather than errors in the code itself. For example, if you try to use the command Info-history-back (l) to move back beyond the start of your Info browsing history, Emacs signals a user-error. Such errors do not cause entry to the debugger, even when debug-on-error is non-nil. See Section 17.1.1 [Error Debugging], page 249. Chapter 10: Control Structures 136 Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors. 10.5.3.2 How Emacs Processes Errors When an error is signaled, signal searches for an active handler for the error. A handler is a sequence of Lisp expressions designated to be executed if an error happens in part of the Lisp program. If the error has an applicable handler, the handler is executed, and control resumes following the handler. The handler executes in the environment of the condition-case that established it; all functions called within that condition-case have already been exited, and the handler cannot return to them. If there is no applicable handler for the error, it terminates the current command and returns control to the editor command loop. (The command loop has an implicit handler for all kinds of errors.) The command loop’s handler uses the error symbol and associated data to print an error message. You can use the variable command-error-function to control how this is done: command-error-function [Variable] This variable, if non-nil, specifies a function to use to handle errors that return control to the Emacs command loop. The function should take three arguments: data, a list of the same form that condition-case would bind to its variable; context, a string describing the situation in which the error occurred, or (more often) nil; and caller, the Lisp function which called the primitive that signaled the error. An error that has no explicit handler may call the Lisp debugger. The debugger is enabled if the variable debug-on-error (see Section 17.1.1 [Error Debugging], page 249) is non-nil. Unlike error handlers, the debugger runs in the environment of the error, so that you can examine values of variables precisely as they were at the time of the error. 10.5.3.3 Writing Code to Handle Errors The usual effect of signaling an error is to terminate the command that is running and return immediately to the Emacs editor command loop. You can arrange to trap errors occurring in a part of your program by establishing an error handler, with the special form condition-case. A simple example looks like this: (condition-case nil (delete-file filename) (error nil)) This deletes the file named filename, catching any error and returning nil if an error occurs. (You can use the macro ignore-errors for a simple case like this; see below.) The condition-case construct is often used to trap errors that are predictable, such as failure to open a file in a call to insert-file-contents. It is also used to trap errors that are totally unpredictable, such as when the program evaluates an expression read from the user. The second argument of condition-case is called the protected form. (In the example above, the protected form is a call to delete-file.) The error handlers go into effect when this form begins execution and are deactivated when this form returns. They remain in effect for all the intervening time. In particular, they are in effect during the execution Chapter 10: Control Structures 137 of functions called by this form, in their subroutines, and so on. This is a good thing, since, strictly speaking, errors can be signaled only by Lisp primitives (including signal and error) called by the protected form, not by the protected form itself. The arguments after the protected form are handlers. Each handler lists one or more condition names (which are symbols) to specify which errors it will handle. The error symbol specified when an error is signaled also defines a list of condition names. A handler applies to an error if they have any condition names in common. In the example above, there is one handler, and it specifies one condition name, error, which covers all errors. The search for an applicable handler checks all the established handlers starting with the most recently established one. Thus, if two nested condition-case forms offer to handle the same error, the inner of the two gets to handle it. If an error is handled by some condition-case form, this ordinarily prevents the debugger from being run, even if debug-on-error says this error should invoke the debugger. If you want to be able to debug errors that are caught by a condition-case, set the variable debug-on-signal to a non-nil value. You can also specify that a particular handler should let the debugger run first, by writing debug among the conditions, like this: (condition-case nil (delete-file filename) ((debug error) nil)) The effect of debug here is only to prevent condition-case from suppressing the call to the debugger. Any given error will invoke the debugger only if debug-on-error and the other usual filtering mechanisms say it should. See Section 17.1.1 [Error Debugging], page 249. condition-case-unless-debug var protected-form handlers. . . [Macro] The macro condition-case-unless-debug provides another way to handle debug- ging of such forms. It behaves exactly like condition-case, unless the variable debug-on-error is non-nil, in which case it does not handle any errors at all. Once Emacs decides that a certain handler handles the error, it returns control to that handler. To do so, Emacs unbinds all variable bindings made by binding constructs that are being exited, and executes the cleanups of all unwind-protect forms that are being exited. Once control arrives at the handler, the body of the handler executes normally. After execution of the handler body, execution returns from the condition-case form. Because the protected form is exited completely before execution of the handler, the handler cannot resume execution at the point of the error, nor can it examine variable bindings that were made within the protected form. All it can do is clean up and proceed. Error signaling and handling have some resemblance to throw and catch (see Section 10.5.1 [Catch and Throw], page 131), but they are entirely separate facilities. An error cannot be caught by a catch, and a throw cannot be handled by an error handler (though using throw when there is no suitable catch signals an error that can be handled). condition-case var protected-form handlers. . . [Special Form] This special form establishes the error handlers handlers around the execution of protected-form. If protected-form executes without error, the value it returns becomes the value of the condition-case form; in this case, the condition-case has no effect. Chapter 10: Control Structures 138 The condition-case form makes a difference when an error occurs during protectedform. Each of the handlers is a list of the form (conditions body...). Here conditions is an error condition name to be handled, or a list of condition names (which can include debug to allow the debugger to run before the handler); body is one or more Lisp expressions to be executed when this handler handles an error. Here are examples of handlers: (error nil) (arith-error (message "Division by zero")) ((arith-error file-error) (message "Either division by zero or failure to open a file")) Each error that occurs has an error symbol that describes what kind of error it is, and which describes also a list of condition names (see Section 10.5.3.4 [Error Symbols], page 140). Emacs searches all the active condition-case forms for a handler that specifies one or more of these condition names; the innermost matching condition-case handles the error. Within this condition-case, the first applicable handler handles the error. After executing the body of the handler, the condition-case returns normally, using the value of the last form in the handler body as the overall value. The argument var is a variable. condition-case does not bind this variable when executing the protected-form, only when it handles an error. At that time, it binds var locally to an error description, which is a list giving the particulars of the error. The error description has the form (error-symbol . data). The handler can refer to this list to decide what to do. For example, if the error is for failure opening a file, the file name is the second element of data—the third element of the error description. If var is nil, that means no variable is bound. Then the error symbol and associated data are not available to the handler. Sometimes it is necessary to re-throw a signal caught by condition-case, for some outer-level handler to catch. Here’s how to do that: (signal (car err) (cdr err)) where err is the error description variable, the first argument to condition-case whose error condition you want to re-throw. See [Definition of signal], page 135. error-message-string error-descriptor [Function] This function returns the error message string for a given error descriptor. It is useful if you want to handle an error by printing the usual error message for that error. See [Definition of signal], page 135. Here is an example of using condition-case to handle the error that results from dividing by zero. The handler displays the error message (but without a beep), then returns a very large number. Chapter 10: Control Structures 139 (defun safe-divide (dividend divisor) (condition-case err ;; Protected form. (/ dividend divisor) ;; The handler. (arith-error ; Condition. ;; Display the usual message for this error. (message "%s" (error-message-string err)) 1000000))) ⇒ safe-divide (safe-divide 5 0) Arithmetic error: (arith-error) ⇒ 1000000 The handler specifies condition name arith-error so that it will handle only division-byzero errors. Other kinds of errors will not be handled (by this condition-case). Thus: (safe-divide nil 3) error Wrong type argument: number-or-marker-p, nil Here is a condition-case that catches all kinds of errors, including those from error: (setq baz 34) ⇒ 34 (condition-case err (if (eq baz 35) t ;; This is a call to the function error. (error "Rats! The variable %s was %s, not 35" ’baz baz)) ;; This is the handler; it is not a form. (error (princ (format "The error was: %s" err)) 2)) The error was: (error "Rats! The variable baz was 34, not 35") ⇒2 ignore-errors body. . . [Macro] This construct executes body, ignoring any errors that occur during its execution. If the execution is without error, ignore-errors returns the value of the last form in body; otherwise, it returns nil. Here’s the example at the beginning of this subsection rewritten using ignore-errors: (ignore-errors (delete-file filename)) with-demoted-errors format body. . . [Macro] This macro is like a milder version of ignore-errors. Rather than suppressing errors altogether, it converts them into messages. It uses the string format to format the message. format should contain a single ‘%’-sequence; e.g., "Error: %S". Use Chapter 10: Control Structures 140 with-demoted-errors around code that is not expected to signal errors, but should be robust if one does occur. Note that this macro uses condition-case-unlessdebug rather than condition-case. 10.5.3.4 Error Symbols and Condition Names When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language. These narrow classifications are grouped into a hierarchy of wider classes called error conditions, identified by condition names. The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name error which takes in all kinds of errors (but not quit). Thus, each error has one or more condition names: error, the error symbol if that is distinct from error, and perhaps some intermediate classifications. define-error name message &optional parent [Function] In order for a symbol to be an error symbol, it must be defined with define-error which takes a parent condition (defaults to error). This parent defines the conditions that this kind of error belongs to. The transitive set of parents always includes the error symbol itself, and the symbol error. Because quitting is not considered an error, the set of parents of quit is just (quit). In addition to its parents, the error symbol has a message which is a string to be printed when that error is signaled but not handled. If that message is not valid, the error message ‘peculiar error’ is used. See [Definition of signal], page 135. Internally, the set of parents is stored in the error-conditions property of the error symbol and the message is stored in the error-message property of the error symbol. Here is how we define a new error symbol, new-error: (define-error ’new-error "A new error" ’my-own-errors) This error has several condition names: new-error, the narrowest classification; my-ownerrors, which we imagine is a wider classification; and all the conditions of my-own-errors which should include error, which is the widest of all. The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs. Naturally, Emacs will never signal new-error on its own; only an explicit call to signal (see [Definition of signal], page 135) in your code can do this: (signal ’new-error ’(x y)) error A new error: x, y This error can be handled through any of its condition names. This example handles new-error and any other errors in the class my-own-errors: (condition-case foo (bar nil t) (my-own-errors nil)) The significant way that errors are classified is by their condition names—the names used to match errors with handlers. An error symbol serves only as a convenient way to Chapter 10: Control Structures 141 specify the intended error message and list of condition names. It would be cumbersome to give signal a list of condition names rather than one error symbol. By contrast, using only error symbols without condition names would seriously decrease the power of condition-case. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification. See Appendix F [Standard Errors], page 1024, for a list of the main error symbols and their conditions. 10.5.4 Cleaning Up from Nonlocal Exits The unwind-protect construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to make the data consistent again in the event of an error or throw. (Another more specific cleanup construct that is used only for changes in buffer contents is the atomic change group; Section 31.27 [Atomic Changes], page 714.) unwind-protect body-form cleanup-forms. . . [Special Form] unwind-protect executes body-form with a guarantee that the cleanup-forms will be evaluated if control leaves body-form, no matter how that happens. body-form may complete normally, or execute a throw out of the unwind-protect, or cause an error; in all cases, the cleanup-forms will be evaluated. If body-form finishes normally, unwind-protect returns the value of body-form, after it evaluates the cleanup-forms. If body-form does not finish, unwind-protect does not return any value in the normal sense. Only body-form is protected by the unwind-protect. If any of the cleanup-forms themselves exits nonlocally (via a throw or an error), unwind-protect is not guaranteed to evaluate the rest of them. If the failure of one of the cleanup-forms has the potential to cause trouble, then protect it with another unwind-protect around that form. The number of currently active unwind-protect forms counts, together with the number of local variable bindings, against the limit max-specpdl-size (see [Local Variables], page 145). For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing: (let ((buffer (get-buffer-create " *temp*"))) (with-current-buffer buffer (unwind-protect body-form (kill-buffer buffer)))) You might think that we could just as well write (kill-buffer (current-buffer)) and dispense with the variable buffer. However, the way shown above is safer, if body-form happens to get an error after switching to a different buffer! (Alternatively, you could write a save-current-buffer around body-form, to ensure that the temporary buffer becomes current again in time to kill it.) Chapter 10: Control Structures 142 Emacs includes a standard macro called with-temp-buffer which expands into more or less the code shown above (see [Current Buffer], page 525). Several of the macros defined in this manual use unwind-protect in this way. Here is an actual example derived from an FTP package. It creates a process (see Chapter 36 [Processes], page 792) to try to establish a connection to a remote machine. As the function ftp-login is highly susceptible to numerous problems that the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, Emacs might fill up with useless subprocesses. (let ((win nil)) (unwind-protect (progn (setq process (ftp-setup-buffer host file)) (if (setq win (ftp-login process host user password)) (message "Logged in") (error "Ftp login failed"))) (or win (and process (delete-process process))))) This example has a small bug: if the user types C-g to quit, and the quit happens immediately after the function ftp-setup-buffer returns but before the variable process is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely. Chapter 11: Variables 143 11 Variables A variable is a name used in a program to stand for a value. In Lisp, each variable is represented by a Lisp symbol (see Chapter 8 [Symbols], page 105). The variable name is simply the symbol’s name, and the variable’s value is stored in the symbol’s value cell1. See Section 8.1 [Symbol Components], page 105. In Emacs Lisp, the use of a symbol as a variable is independent of its use as a function name. As previously noted in this manual, a Lisp program is represented primarily by Lisp objects, and only secondarily as text. The textual form of a Lisp program is given by the read syntax of the Lisp objects that constitute the program. Hence, the textual form of a variable in a Lisp program is written using the read syntax for the symbol representing the variable. 11.1 Global Variables The simplest way to use a variable is globally. This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable. You specify a value for a symbol with setq. For example, (setq x ’(a b)) gives the variable x the value (a b). Note that setq is a special form (see Section 9.1.7 [Special Forms], page 117); it does not evaluate its first argument, the name of the variable, but it does evaluate the second argument, the new value. Once the variable has a value, you can refer to it by using the symbol itself as an expression. Thus, x ⇒ (a b) assuming the setq form shown above has already been executed. If you do set the same variable again, the new value replaces the old one: x ⇒ (a b) (setq x 4) ⇒4 x ⇒4 11.2 Variables that Never Change In Emacs Lisp, certain symbols normally evaluate to themselves. These include nil and t, as well as any symbol whose name starts with ‘:’ (these are called keywords). These symbols cannot be rebound, nor can their values be changed. Any attempt to set or bind nil or t signals a setting-constant error. The same is true for a keyword (a symbol 1 To be precise, under the default dynamic scoping rule, the value cell always holds the variable’s current value, but this is not the case under the lexical scoping rule. See Section 11.9 [Variable Scoping], page 152, for details. Chapter 11: Variables 144 whose name starts with ‘:’), if it is interned in the standard obarray, except that setting such a symbol to itself is not an error. nil ≡ ’nil ⇒ nil (setq nil 500) error Attempt to set constant symbol: nil keywordp object [Function] function returns t if object is a symbol whose name starts with ‘:’, interned in the standard obarray, and returns nil otherwise. These constants are fundamentally different from the “constants” defined using the defconst special form (see Section 11.5 [Defining Variables], page 147). A defconst form serves to inform human readers that you do not intend to change the value of a variable, but Emacs does not raise an error if you actually change it. 11.3 Local Variables Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to give a variable a local value—a value that takes effect only within a certain part of a Lisp program. When a variable has a local value, we say that it is locally bound to that value, and that it is a local variable. For example, when a function is called, its argument variables receive local values, which are the actual arguments supplied to the function call; these local bindings take effect within the body of the function. To take another example, the let special form explicitly establishes local bindings for specific variables, which take effect within the body of the let form. We also speak of the global binding, which is where (conceptually) the global value is kept. Establishing a local binding saves away the variable’s previous value (or lack of one). We say that the previous value is shadowed. Both global and local values may be shadowed. If a local binding is in effect, using setq on the local variable stores the specified value in the local binding. When that local binding is no longer in effect, the previously shadowed value (or lack of one) comes back. A variable can have more than one local binding at a time (e.g., if there are nested let forms that bind the variable). The current binding is the local binding that is actually in effect. It determines the value returned by evaluating the variable symbol, and it is the binding acted on by setq. For most purposes, you can think of the current binding as the “innermost” local binding, or the global binding if there is no local binding. To be more precise, a rule called the scoping rule determines where in a program a local binding takes effect. The default scoping rule in Emacs Lisp is called dynamic scoping, which simply states that the current binding at any given point in the execution of a program is the most recently-created binding for that variable that still exists. For details about dynamic scoping, and an alternative scoping rule called lexical scoping, See Section 11.9 [Variable Scoping], page 152. The special forms let and let* exist to create local bindings: Chapter 11: Variables 145 let (bindings. . . ) forms. . . [Special Form] This special form sets up local bindings for a certain set of variables, as specified by bindings, and then evaluates all of the forms in textual order. Its return value is the value of the last form in forms. Each of the bindings is either (i) a symbol, in which case that symbol is locally bound to nil; or (ii) a list of the form (symbol value-form), in which case symbol is locally bound to the result of evaluating value-form. If value-form is omitted, nil is used. All of the value-forms in bindings are evaluated in the order they appear and before binding any of the symbols to them. Here is an example of this: z is bound to the old value of y, which is 2, not the new value of y, which is 1. (setq y 2) ⇒2 (let ((y 1) (z y)) (list y z)) ⇒ (1 2) let* (bindings. . . ) forms. . . [Special Form] This special form is like let, but it binds each variable right after computing its local value, before computing the local value for the next variable. Therefore, an expression in bindings can refer to the preceding symbols bound in this let* form. Compare the following example with the example above for let. (setq y 2) ⇒2 (let* ((y 1) (z y)) (list y z)) ⇒ (1 1) ; Use the just-established value of y. Here is a complete list of the other facilities that create local bindings: • Function calls (see Chapter 12 [Functions], page 172). • Macro calls (see Chapter 13 [Macros], page 198). • condition-case (see Section 10.5.3 [Errors], page 134). Variables can also have buffer-local bindings (see Section 11.10 [Buffer-Local Variables], page 157); a few variables have terminal-local bindings (see Section 28.2 [Multiple Terminals], page 598). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on “where” you are in Emacs. max-specpdl-size [User Option] This variable defines the limit on the total number of local variable bindings and unwind-protect cleanups (see Section 10.5.4 [Cleaning Up from Nonlocal Exits], page 141) that are allowed before Emacs signals an error (with data "Variable binding depth exceeds max-specpdl-size"). Chapter 11: Variables 146 This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function. max-lisp-eval-depth provides another limit on depth of nesting. See [Eval], page 122. The default value is 1300. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute. 11.4 When a Variable is “Void” We say that a variable is void if its symbol has an unassigned value cell (see Section 8.1 [Symbol Components], page 105). Under Emacs Lisp’s default dynamic scoping rule (see Section 11.9 [Variable Scoping], page 152), the value cell stores the variable’s current (local or global) value. Note that an unassigned value cell is not the same as having nil in the value cell. The symbol nil is a Lisp object and can be the value of a variable, just as any other object can be; but it is still a value. If a variable is void, trying to evaluate the variable signals a void-variable error, instead of returning a value. Under the optional lexical scoping rule, the value cell only holds the variable’s global value—the value outside of any lexical binding construct. When a variable is lexically bound, the local value is determined by the lexical environment; hence, variables can have local values even if their symbols’ value cells are unassigned. makunbound symbol [Function] This function empties out the value cell of symbol, making the variable void. It returns symbol. If symbol has a dynamic local binding, makunbound voids the current binding, and this voidness lasts only as long as the local binding is in effect. Afterwards, the previously shadowed local or global binding is reexposed; then the variable will no longer be void, unless the reexposed binding is void too. Here are some examples (assuming dynamic binding is in effect): (setq x 1) ; Put a value in the global binding. ⇒1 (let ((x 2)) ; Locally bind it. (makunbound ’x) ; Void the local binding. x) error Symbol’s value as variable is void: x x ; The global binding is unchanged. ⇒1 (let ((x 2)) ; Locally bind it. (let ((x 3)) ; And again. (makunbound ’x) ; Void the innermost-local binding. x)) ; And refer: it’s void. error Symbol’s value as variable is void: x (let ((x 2)) (let ((x 3)) (makunbound ’x)) x) ⇒2 ; Void inner binding, then remove it. ; Now outer let binding is visible. Chapter 11: Variables 147 boundp variable [Function] This function returns t if variable (a symbol) is not void, and nil if it is void. Here are some examples (assuming dynamic binding is in effect): (boundp ’abracadabra) ⇒ nil (let ((abracadabra 5)) (boundp ’abracadabra)) ⇒t (boundp ’abracadabra) ⇒ nil (setq abracadabra 5) ⇒5 (boundp ’abracadabra) ⇒t ; Starts out void. ; Locally bind it. ; Still globally void. ; Make it globally nonvoid. 11.5 Defining Global Variables A variable definition is a construct that announces your intention to use a symbol as a global variable. It uses the special forms defvar or defconst, which are documented below. A variable definition serves three purposes. First, it informs people who read the code that the symbol is intended to be used a certain way (as a variable). Second, it informs the Lisp system of this, optionally supplying an initial value and a documentation string. Third, it provides information to programming tools such as etags, allowing them to find where the variable was defined. The difference between defconst and defvar is mainly a matter of intent, serving to inform human readers of whether the value should ever change. Emacs Lisp does not actually prevent you from changing the value of a variable defined with defconst. One notable difference between the two forms is that defconst unconditionally initializes the variable, whereas defvar initializes it only if it is originally void. To define a customizable variable, you should use defcustom (which calls defvar as a subroutine). See Section 14.3 [Variable Definitions], page 209. defvar symbol [value [doc-string]] [Special Form] This special form defines symbol as a variable. Note that symbol is not evaluated; the symbol to be defined should appear explicitly in the defvar form. The vari- able is marked as special, meaning that it should always be dynamically bound (see Section 11.9 [Variable Scoping], page 152). If value is specified, and symbol is void (i.e., it has no dynamically bound value; see Section 11.4 [Void Variables], page 146), then value is evaluated and symbol is set to the result. But if symbol is not void, value is not evaluated, and symbol’s value is left unchanged. If value is omitted, the value of symbol is not changed in any case. If symbol has a buffer-local binding in the current buffer, defvar acts on the default value, which is buffer-independent, rather than the buffer-local binding. It sets the default value if the default value is void. See Section 11.10 [Buffer-Local Variables], page 157. If symbol is already lexically bound (e.g., if the defvar form occurs in a let form with lexical binding enabled), then defvar sets the dynamic value. The lexical binding Chapter 11: Variables 148 remains in effect until its binding construct exits. See Section 11.9 [Variable Scoping], page 152. When you evaluate a top-level defvar form with C-M-x in Emacs Lisp mode (eval-defun), a special feature of eval-defun arranges to set the variable unconditionally, without testing whether its value is void. If the doc-string argument is supplied, it specifies the documentation string for the variable (stored in the symbol’s variable-documentation property). See Chapter 23 [Documentation], page 461. Here are some examples. This form defines foo but does not initialize it: (defvar foo) ⇒ foo This example initializes the value of bar to 23, and gives it a documentation string: (defvar bar 23 "The normal weight of a bar.") ⇒ bar The defvar form returns symbol, but it is normally used at top level in a file where its value does not matter. defconst symbol value [doc-string] [Special Form] This special form defines symbol as a value and initializes it. It informs a person reading your code that symbol has a standard global value, established here, that should not be changed by the user or by other programs. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the defconst. The defconst form, like defvar, marks the variable as special, meaning that it should always be dynamically bound (see Section 11.9 [Variable Scoping], page 152). In addition, it marks the variable as risky (see Section 11.11 [File Local Variables], page 163). defconst always evaluates value, and sets the value of symbol to the result. If symbol does have a buffer-local binding in the current buffer, defconst sets the default value, not the buffer-local value. (But you should not be making buffer-local bindings for a symbol that is defined with defconst.) An example of the use of defconst is Emacs’s definition of float-pi—the mathematical constant pi, which ought not to be changed by anyone (attempts by the Indiana State Legislature notwithstanding). As the second form illustrates, however, defconst is only advisory. (defconst float-pi 3.141592653589793 "The value of Pi.") ⇒ float-pi (setq float-pi 3) ⇒ float-pi float-pi ⇒3 Warning: If you use a defconst or defvar special form while the variable has a local binding (made with let, or a function argument), it sets the local binding rather than the global binding. This is not what you usually want. To prevent this, use these special forms Chapter 11: Variables 149 at top level in a file, where normally no local binding is in effect, and make sure to load the file before making a local binding for the variable. 11.6 Tips for Defining Variables Robustly When you define a variable whose value is a function, or a list of functions, use a name that ends in ‘-function’ or ‘-functions’, respectively. There are several other variable name conventions; here is a complete list: ‘...-hook’ The variable is a normal hook (see Section 22.1 [Hooks], page 406). ‘...-function’ The value is a function. ‘...-functions’ The value is a list of functions. ‘...-form’ The value is a form (an expression). ‘...-forms’ The value is a list of forms (expressions). ‘...-predicate’ The value is a predicate—a function of one argument that returns non-nil for “good” arguments and nil for “bad” arguments. ‘...-flag’ The value is significant only as to whether it is nil or not. Since such variables often end up acquiring more values over time, this convention is not strongly recommended. ‘...-program’ The value is a program name. ‘...-command’ The value is a whole shell command. ‘...-switches’ The value specifies options for a command. When you define a variable, always consider whether you should mark it as “safe” or “risky”; see Section 11.11 [File Local Variables], page 163. When defining and initializing a variable that holds a complicated value (such as a keymap with bindings in it), it’s best to put the entire computation of the value into the defvar, like this: (defvar my-mode-map (let ((map (make-sparse-keymap))) (define-key map "\C-c\C-a" ’my-command) ... map) docstring) Chapter 11: Variables 150 This method has several benefits. First, if the user quits while loading the file, the variable is either still uninitialized or initialized properly, never in-between. If it is still uninitialized, reloading the file will initialize it properly. Second, reloading the file once the variable is initialized will not alter it; that is important if the user has run hooks to alter part of the contents (such as, to rebind keys). Third, evaluating the defvar form with C-M-x will reinitialize the map completely. Putting so much code in the defvar form has one disadvantage: it puts the documentation string far away from the line which names the variable. Here’s a safe way to avoid that: (defvar my-mode-map nil docstring) (unless my-mode-map (let ((map (make-sparse-keymap))) (define-key map "\C-c\C-a" ’my-command) ... (setq my-mode-map map))) This has all the same advantages as putting the initialization inside the defvar, except that you must type C-M-x twice, once on each form, if you do want to reinitialize the variable. 11.7 Accessing Variable Values The usual way to reference a variable is to write the symbol which names it. See Section 9.1.2 [Symbol Forms], page 114. Occasionally, you may want to reference a variable which is only determined at run time. In that case, you cannot specify the variable name in the text of the program. You can use the symbol-value function to extract the value. symbol-value symbol [Function] This function returns the value stored in symbol’s value cell. This is where the variable’s current (dynamic) value is stored. If the variable has no local binding, this is simply its global value. If the variable is void, a void-variable error is signaled. If the variable is lexically bound, the value reported by symbol-value is not necessarily the same as the variable’s lexical value, which is determined by the lexical environment rather than the symbol’s value cell. See Section 11.9 [Variable Scoping], page 152. (setq abracadabra 5) ⇒5 (setq foo 9) ⇒9 ;; Here the symbol abracadabra ;; is the symbol whose value is examined. (let ((abracadabra ’foo)) (symbol-value ’abracadabra)) ⇒ foo Chapter 11: Variables 151 ;; Here, the value of abracadabra, ;; which is foo, ;; is the symbol whose value is examined. (let ((abracadabra ’foo)) (symbol-value abracadabra)) ⇒9 (symbol-value ’abracadabra) ⇒5 11.8 Setting Variable Values The usual way to change the value of a variable is with the special form setq. When you need to compute the choice of variable at run time, use the function set. setq [symbol form]. . . [Special Form] This special form is the most common method of changing a variable’s value. Each symbol is given a new value, which is the result of evaluating the corresponding form. The current binding of the symbol is changed. setq does not evaluate symbol; it sets the symbol that you write. We say that this argument is automatically quoted. The ‘q’ in setq stands for “quoted”. The value of the setq form is the value of the last form. (setq x (1+ 2)) ⇒3 x ⇒3 (let ((x 5)) (setq x 6) x) ⇒6 x ⇒3 ; x now has a global value. ; The local binding of x is set. ; The global value is unchanged. Note that the first form is evaluated, then the first symbol is set, then the second form is evaluated, then the second symbol is set, and so on: (setq x 10 y (1+ x)) ⇒ 11 ; Notice that x is set before ; the value of y is computed. set symbol value [Function] This function puts value in the value cell of symbol. Since it is a function rather than a special form, the expression written for symbol is evaluated to obtain the symbol to set. The return value is value. When dynamic variable binding is in effect (the default), set has the same effect as setq, apart from the fact that set evaluates its symbol argument whereas setq does not. But when a variable is lexically bound, set affects its dynamic value, whereas setq affects its current (lexical) value. See Section 11.9 [Variable Scoping], page 152. Chapter 11: Variables 152 (set one 1) error Symbol’s value as variable is void: one (set ’one 1) ⇒1 (set ’two ’one) ⇒ one (set two 2) ; two evaluates to symbol one. ⇒2 one ; So it is one that was set. ⇒2 (let ((one 1)) ; This binding of one is set, (set ’one 3) ; not the global value. one) ⇒3 one ⇒2 If symbol is not actually a symbol, a wrong-type-argument error is signaled. (set ’(x y) ’z) error Wrong type argument: symbolp, (x y) 11.9 Scoping Rules for Variable Bindings When you create a local binding for a variable, that binding takes effect only within a limited portion of the program (see Section 11.3 [Local Variables], page 144). This section describes exactly what this means. Each local binding has a certain scope and extent. Scope refers to where in the textual source code the binding can be accessed. Extent refers to when, as the program is executing, the binding exists. By default, the local bindings that Emacs creates are dynamic bindings. Such a binding has dynamic scope, meaning that any part of the program can potentially access the variable binding. It also has dynamic extent, meaning that the binding lasts only while the binding construct (such as the body of a let form) is being executed. Emacs can optionally create lexical bindings. A lexical binding has lexical scope, meaning that any reference to the variable must be located textually within the binding construct2. It also has indefinite extent, meaning that under some circumstances the binding can live on even after the binding construct has finished executing, by means of special objects called closures. The following subsections describe dynamic binding and lexical binding in greater detail, and how to enable lexical binding in Emacs Lisp programs. 11.9.1 Dynamic Binding By default, the local variable bindings made by Emacs are dynamic bindings. When a variable is dynamically bound, its current binding at any point in the execution of the Lisp 2 With some exceptions; for instance, a lexical binding can also be accessed from the Lisp debugger. Chapter 11: Variables 153 program is simply the most recently-created dynamic local binding for that symbol, or the global binding if there is no such local binding. Dynamic bindings have dynamic scope and extent, as shown by the following example: (defvar x -99) ; x receives an initial value of −99. (defun getx () x) ; x is used ‘‘free” in this function. (let ((x 1)) (getx)) ⇒1 ; x is dynamically bound. ;; After the let form finishes, x reverts to its ;; previous value, which is −99. (getx) ⇒ -99 The function getx refers to x. This is a “free” reference, in the sense that there is no binding for x within that defun construct itself. When we call getx from within a let form in which x is (dynamically) bound, it retrieves the local value (i.e., 1). But when we call getx outside the let form, it retrieves the global value (i.e., −99). Here is another example, which illustrates setting a dynamically bound variable using setq: (defvar x -99) ; x receives an initial value of −99. (defun addx () (setq x (1+ x))) ; Add 1 to x and return its new value. (let ((x 1)) (addx) (addx)) ⇒3 ; The two addx calls add to x twice. ;; After the let form finishes, x reverts to its ;; previous value, which is −99. (addx) ⇒ -98 Dynamic binding is implemented in Emacs Lisp in a simple way. Each symbol has a value cell, which specifies its current dynamic value (or absence of value). See Section 8.1 [Symbol Components], page 105. When a symbol is given a dynamic local binding, Emacs records the contents of the value cell (or absence thereof) in a stack, and stores the new local value in the value cell. When the binding construct finishes executing, Emacs pops the old value off the stack, and puts it in the value cell. Chapter 11: Variables 154 11.9.2 Proper Use of Dynamic Binding Dynamic binding is a powerful feature, as it allows programs to refer to variables that are not defined within their local textual scope. However, if used without restraint, this can also make programs hard to understand. There are two clean ways to use this technique: • If a variable has no global definition, use it as a local variable only within a binding construct, such as the body of the let form where the variable was bound. If this convention is followed consistently throughout a program, the value of the variable will not affect, nor be affected by, any uses of the same variable symbol elsewhere in the program. • Otherwise, define the variable with defvar, defconst, or defcustom. See Section 11.5 [Defining Variables], page 147. Usually, the definition should be at top-level in an Emacs Lisp file. As far as possible, it should include a documentation string which explains the meaning and purpose of the variable. You should also choose the variable’s name to avoid name conflicts (see Section D.1 [Coding Conventions], page 987). Then you can bind the variable anywhere in a program, knowing reliably what the effect will be. Wherever you encounter the variable, it will be easy to refer back to the definition, e.g., via the C-h v command (provided the variable definition has been loaded into Emacs). See Section “Name Help” in The GNU Emacs Manual. For example, it is common to use local bindings for customizable variables like case-fold-search: (defun search-for-abc () "Search for the string \"abc\", ignoring case differences." (let ((case-fold-search nil)) (re-search-forward "abc"))) 11.9.3 Lexical Binding Lexical binding was introduced to Emacs, as an optional feature, in version 24.1. We expect its importance to increase in the future. Lexical binding opens up many more opportunities for optimization, so programs using it are likely to run faster in future Emacs versions. Lexical binding is also more compatible with concurrency, which we want to add to Emacs in the future. A lexically-bound variable has lexical scope, meaning that any reference to the variable must be located textually within the binding construct. Here is an example (see the next subsection, for how to actually enable lexical binding): (let ((x 1)) (+ x 3)) ⇒4 ; x is lexically bound. (defun getx () x) ; x is used ‘‘free” in this function. (let ((x 1)) ; x is lexically bound. (getx)) error Symbol’s value as variable is void: x Chapter 11: Variables 155 Here, the variable x has no global value. When it is lexically bound within a let form, it can be used in the textual confines of that let form. But it can not be used from within a getx function called from the let form, since the function definition of getx occurs outside the let form itself. Here is how lexical binding works. Each binding construct defines a lexical environment, specifying the symbols that are bound within the construct and their local values. When the Lisp evaluator wants the current value of a variable, it looks first in the lexical environment; if the variable is not specified in there, it looks in the symbol’s value cell, where the dynamic value is stored. (Internally, the lexical environment is an alist of symbol-value pairs, with the final element in the alist being the symbol t rather than a cons cell. Such an alist can be passed as the second argument to the eval function, in order to specify a lexical environment in which to evaluate a form. See Section 9.4 [Eval], page 120. Most Emacs Lisp programs, however, should not interact directly with lexical environments in this way; only specialized programs like debuggers.) Lexical bindings have indefinite extent. Even after a binding construct has finished executing, its lexical environment can be “kept around” in Lisp objects called closures. A closure is created when you define a named or anonymous function with lexical binding enabled. See Section 12.9 [Closures], page 185, for details. When a closure is called as a function, any lexical variable references within its definition use the retained lexical environment. Here is an example: (defvar my-ticker nil) ; We will use this dynamically bound ; variable to store a closure. (let ((x 0)) ; x is lexically bound. (setq my-ticker (lambda () (setq x (1+ x))))) ⇒ (closure ((x . 0) t) () (setq x (1+ x))) (funcall my-ticker) ⇒1 (funcall my-ticker) ⇒2 (funcall my-ticker) ⇒3 x ; Note that x has no global value. error Symbol’s value as variable is void: x The let binding defines a lexical environment in which the variable x is locally bound to 0. Within this binding construct, we define a lambda expression which increments x by one and returns the incremented value. This lambda expression is automatically turned into a closure, in which the lexical environment lives on even after the let binding construct has Chapter 11: Variables 156 exited. Each time we evaluate the closure, it increments x, using the binding of x in that lexical environment. Note that functions like symbol-value, boundp, and set only retrieve or modify a variable’s dynamic binding (i.e., the contents of its symbol’s value cell). Also, the code in the body of a defun or defmacro cannot refer to surrounding lexical variables. 11.9.4 Using Lexical Binding When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical binding is enabled if the buffer-local variable lexical-binding is non-nil: lexical-binding [Variable] If this buffer-local variable is non-nil, Emacs Lisp files and buffers are evaluated using lexical binding instead of dynamic binding. (However, special variables are still dynamically bound; see below.) If nil, dynamic binding is used for all local variables. This variable is typically set for a whole Emacs Lisp file, as a file local variable (see Section 11.11 [File Local Variables], page 163). Note that unlike other such variables, this one must be set in the first line of a file. When evaluating Emacs Lisp code directly using an eval call, lexical binding is enabled if the lexical argument to eval is non-nil. See Section 9.4 [Eval], page 120. Even when lexical binding is enabled, certain variables will continue to be dynamically bound. These are called special variables. Every variable that has been defined with defvar, defcustom or defconst is a special variable (see Section 11.5 [Defining Variables], page 147). All other variables are subject to lexical binding. special-variable-p symbol [Function] This function returns non-nil if symbol is a special variable (i.e., it has a defvar, defcustom, or defconst variable definition). Otherwise, the return value is nil. The use of a special variable as a formal argument in a function is discouraged. Doing so gives rise to unspecified behavior when lexical binding mode is enabled (it may use lexical binding sometimes, and dynamic binding other times). Converting an Emacs Lisp program to lexical binding is easy. First, add a file-local variable setting of lexical-binding to t in the header line of the Emacs Lisp source file (see Section 11.11 [File Local Variables], page 163). Second, check that every variable in the program which needs to be dynamically bound has a variable definition, so that it is not inadvertently bound lexically. A simple way to find out which variables need a variable definition is to byte-compile the source file. See Chapter 16 [Byte Compilation], page 239. If a non-special variable is used outside of a let form, the byte-compiler will warn about reference or assignment to a “free variable”. If a non-special variable is bound but not used within a let form, the byte-compiler will warn about an “unused lexical variable”. The byte-compiler will also issue a warning if you use a special variable as a function argument. (To silence byte-compiler warnings about unused variables, just use a variable name that start with an underscore. The byte-compiler interprets this as an indication that this is a variable known not to be used.) Chapter 11: Variables 157 11.10 Buffer-Local Variables Global and local variable bindings are found in most programming languages in one form or another. Emacs, however, also supports additional, unusual kinds of variable binding, such as buffer-local bindings, which apply only in one buffer. Having different values for a variable in different buffers is an important customization method. (Variables can also have bindings that are local to each terminal. See Section 28.2 [Multiple Terminals], page 598.) 11.10.1 Introduction to Buffer-Local Variables A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it. The variable’s ordinary binding, which is not associated with any specific buffer, is called the default binding. In most cases, this is the global binding. A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don’t have their own bindings for the variable. (This includes all newly-created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding, so the new value is visible in all the buffers that see the default binding. The most common use of buffer-local bindings is for major modes to change variables that control the behavior of commands. For example, C mode and Lisp mode both set the variable paragraph-start to specify that only blank lines separate paragraphs. They do this by making the variable buffer-local in the buffer that is being put into C mode or Lisp mode, and then setting it to the new value for that mode. See Section 22.2 [Major Modes], page 408. The usual way to make a buffer-local binding is with make-local-variable, which is what major mode commands typically use. This affects just the current buffer; all other buffers (including those yet to be created) will continue to share the default value unless they are explicitly given their own buffer-local bindings. A more powerful operation is to mark the variable as automatically buffer-local by calling make-variable-buffer-local. You can think of this as making the variable local in all buffers, even those yet to be created. More precisely, the effect is that setting the variable automatically makes the variable local to the current buffer if it is not already so. All buffers start out by sharing the default value of the variable as usual, but setting the variable creates a buffer-local binding for the current buffer. The new value is stored in the buffer-local binding, leaving the default binding untouched. This means that the default value cannot be changed with setq in any buffer; the only way to change it is with setq-default. Warning: When a variable has buffer-local bindings in one or more buffers, let rebinds the binding that’s currently in effect. For instance, if the current buffer has a buffer-local value, let temporarily rebinds that. If no buffer-local bindings are in effect, let rebinds the default value. If inside the let you then change to a different current buffer in which a different binding is in effect, you won’t see the let binding any more. And if you exit the Chapter 11: Variables 158 let while still in the other buffer, you won’t see the unbinding occur (though it will occur properly). Here is an example to illustrate: (setq foo ’g) (set-buffer "a") (make-local-variable ’foo) (setq foo ’a) (let ((foo ’temp)) ;; foo ⇒ ’temp ; let binding in buffer ‘a’ (set-buffer "b") ;; foo ⇒ ’g ; the global value since foo is not local in ‘b’ body...) foo ⇒ ’g ; exiting restored the local value in buffer ‘a’, ; but we don’t see that in buffer ‘b’ (set-buffer "a") ; verify the local value was restored foo ⇒ ’a Note that references to foo in body access the buffer-local binding of buffer ‘b’. When a file specifies local variable values, these become buffer-local values when you visit the file. See Section “File Variables” in The GNU Emacs Manual. A buffer-local variable cannot be made terminal-local (see Section 28.2 [Multiple Terminals], page 598). 11.10.2 Creating and Deleting Buffer-Local Bindings make-local-variable variable [Command] This function creates a buffer-local binding in the current buffer for variable (a sym- bol). Other buffers are not affected. The value returned is variable. The buffer-local value of variable starts out as the same value variable previously had. If variable was void, it remains void. ;; In buffer ‘b1’: (setq foo 5) ⇒5 (make-local-variable ’foo) ⇒ foo foo ⇒5 (setq foo 6) ⇒6 foo ⇒6 ; Affects all buffers. ; Now it is local in ‘b1’. ; That did not change ; the value. ; Change the value ; in ‘b1’. ;; In buffer ‘b2’, the value hasn’t changed. (with-current-buffer "b2" foo) ⇒5 Making a variable buffer-local within a let-binding for that variable does not work reliably, unless the buffer in which you do this is not current either on entry to or Chapter 11: Variables 159 exit from the let. This is because let does not distinguish between different kinds of bindings; it knows only which variable the binding was made for. If the variable is terminal-local (see Section 28.2 [Multiple Terminals], page 598), this function signals an error. Such variables cannot have buffer-local bindings as well. Warning: do not use make-local-variable for a hook variable. The hook variables are automatically made buffer-local as needed if you use the local argument to add-hook or remove-hook. setq-local variable value [Macro] This macro creates a buffer-local binding in the current buffer for variable, and gives it the buffer-local value value. It is equivalent to calling make-local-variable followed by setq. variable should be an unquoted symbol. make-variable-buffer-local variable [Command] This function marks variable (a symbol) automatically buffer-local, so that any sub- sequent attempt to set it will make it local to the current buffer at the time. Unlike make-local-variable, with which it is often confused, this cannot be undone, and affects the behavior of the variable in all buffers. A peculiar wrinkle of this feature is that binding the variable (with let or other binding constructs) does not create a buffer-local binding for it. Only setting the variable (with set or setq), while the variable does not have a let-style binding that was made in the current buffer, does so. If variable does not have a default value, then calling this command will give it a default value of nil. If variable already has a default value, that value remains unchanged. Subsequently calling makunbound on variable will result in a void bufferlocal value and leave the default value unaffected. The value returned is variable. Warning: Don’t assume that you should use make-variable-buffer-local for useroption variables, simply because users might want to customize them differently in different buffers. Users can make any variable local, when they wish to. It is better to leave the choice to them. The time to use make-variable-buffer-local is when it is crucial that no two buffers ever share the same binding. For example, when a variable is used for internal purposes in a Lisp program which depends on having separate values in separate buffers, then using make-variable-buffer-local can be the best solution. defvar-local variable value &optional docstring [Macro] This macro defines variable as a variable with initial value value and docstring, and marks it as automatically buffer-local. It is equivalent to calling defvar followed by make-variable-buffer-local. variable should be an unquoted symbol. local-variable-p variable &optional buffer [Function] This returns t if variable is buffer-local in buffer buffer (which defaults to the current buffer); otherwise, nil. Chapter 11: Variables 160 local-variable-if-set-p variable &optional buffer [Function] This returns t if variable either has a buffer-local value in buffer buffer, or is auto- matically buffer-local. Otherwise, it returns nil. If omitted or nil, buffer defaults to the current buffer. buffer-local-value variable buffer [Function] This function returns the buffer-local binding of variable (a symbol) in buffer buffer. If variable does not have a buffer-local binding in buffer buffer, it returns the default value (see Section 11.10.3 [Default Value], page 161) of variable instead. buffer-local-variables &optional buffer [Function] This function returns a list describing the buffer-local variables in buffer buffer. (If buffer is omitted, the current buffer is used.) Normally, each list element has the form (sym . val), where sym is a buffer-local variable (a symbol) and val is its buffer-local value. But when a variable’s buffer-local binding in buffer is void, its list element is just sym. (make-local-variable ’foobar) (makunbound ’foobar) (make-local-variable ’bind-me) (setq bind-me 69) (setq lcl (buffer-local-variables)) ;; First, built-in variables local in all buffers: ⇒ ((mark-active . nil) (buffer-undo-list . nil) (mode-name . "Fundamental") ... ;; Next, non-built-in buffer-local variables. ;; This one is buffer-local and void: foobar ;; This one is buffer-local and nonvoid: (bind-me . 69)) Note that storing new values into the cdrs of cons cells in this list does not change the buffer-local values of the variables. kill-local-variable variable [Command] This function deletes the buffer-local binding (if any) for variable (a symbol) in the current buffer. As a result, the default binding of variable becomes visible in this buffer. This typically results in a change in the value of variable, since the default value is usually different from the buffer-local value just eliminated. If you kill the buffer-local binding of a variable that automatically becomes bufferlocal when set, this makes the default value visible in the current buffer. However, if you set the variable again, that will once again create a buffer-local binding for it. kill-local-variable returns variable. This function is a command because it is sometimes useful to kill one buffer-local variable interactively, just as it is useful to create buffer-local variables interactively. Chapter 11: Variables 161 kill-all-local-variables [Function] This function eliminates all the buffer-local variable bindings of the current buffer except for variables marked as “permanent” and local hook functions that have a non- nil permanent-local-hook property (see Section 22.1.2 [Setting Hooks], page 407). As a result, the buffer will see the default values of most variables. This function also resets certain other information pertaining to the buffer: it sets the local keymap to nil, the syntax table to the value of (standard-syntax-table), the case table to (standard-case-table), and the abbrev table to the value of fundamental-mode-abbrev-table. The very first thing this function does is run the normal hook change-major-modehook (see below). Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent. kill-all-local-variables returns nil. change-major-mode-hook [Variable] The function kill-all-local-variables runs this normal hook before it does any- thing else. This gives major modes a way to arrange for something special to be done if the user switches to a different major mode. It is also useful for buffer-specific minor modes that should be forgotten if the user changes the major mode. For best results, make this variable buffer-local, so that it will disappear after doing its job and will not interfere with the subsequent major mode. See Section 22.1 [Hooks], page 406. A buffer-local variable is permanent if the variable name (a symbol) has a permanent-local property that is non-nil. Such variables are unaffected by kill-alllocal-variables, and their local bindings are therefore not cleared by changing major modes. Permanent locals are appropriate for data pertaining to where the file came from or how to save it, rather than with how to edit the contents. 11.10.3 The Default Value of a Buffer-Local Variable The global value of a variable with buffer-local bindings is also called the default value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable. The functions default-value and setq-default access and change a variable’s default value regardless of whether the current buffer has a buffer-local binding. For example, you could use setq-default to change the default setting of paragraph-start for most buffers; and this would work even when you are in a C or Lisp mode buffer that has a buffer-local value for this variable. The special forms defvar and defconst also set the default value (if they set the variable at all), rather than any buffer-local value. default-value symbol [Function] This function returns symbol’s default value. This is the value that is seen in buffers and frames that do not have their own values for this variable. If symbol is not Chapter 11: Variables 162 buffer-local, this is equivalent to symbol-value (see Section 11.7 [Accessing Variables], page 150). default-boundp symbol [Function] The function default-boundp tells you whether symbol’s default value is nonvoid. If (default-boundp ’foo) returns nil, then (default-value ’foo) would get an error. default-boundp is to default-value as boundp is to symbol-value. setq-default [symbol form]. . . [Special Form] This special form gives each symbol a new default value, which is the result of eval- uating the corresponding form. It does not evaluate symbol, but does evaluate form. The value of the setq-default form is the value of the last form. If a symbol is not buffer-local for the current buffer, and is not marked automatically buffer-local, setq-default has the same effect as setq. If symbol is buffer-local for the current buffer, then this changes the value that other buffers will see (as long as they don’t have a buffer-local value), but not the value that the current buffer sees. ;; In buffer ‘foo’: (make-local-variable ’buffer-local) ⇒ buffer-local (setq buffer-local ’value-in-foo) ⇒ value-in-foo (setq-default buffer-local ’new-default) ⇒ new-default buffer-local ⇒ value-in-foo (default-value ’buffer-local) ⇒ new-default ;; In (the new) buffer ‘bar’: buffer-local ⇒ new-default (default-value ’buffer-local) ⇒ new-default (setq buffer-local ’another-default) ⇒ another-default (default-value ’buffer-local) ⇒ another-default ;; Back in buffer ‘foo’: buffer-local ⇒ value-in-foo (default-value ’buffer-local) ⇒ another-default set-default symbol value [Function] This function is like setq-default, except that symbol is an ordinary evaluated argument. Chapter 11: Variables 163 (set-default (car ’(a b c)) 23) ⇒ 23 (default-value ’a) ⇒ 23 11.11 File Local Variables A file can specify local variable values; Emacs uses these to create buffer-local bindings for those variables in the buffer visiting that file. See Section “Local Variables in Files” in The GNU Emacs Manual, for basic information about file-local variables. This section describes the functions and variables that affect how file-local variables are processed. If a file-local variable could specify an arbitrary function or Lisp expression that would be called later, visiting a file could take over your Emacs. Emacs protects against this by automatically setting only those file-local variables whose specified values are known to be safe. Other file-local variables are set only if the user agrees. For additional safety, read-circle is temporarily bound to nil when Emacs reads filelocal variables (see Section 18.3 [Input Functions], page 282). This prevents the Lisp reader from recognizing circular and shared Lisp structures (see Section 2.5 [Circular Objects], page 27). enable-local-variables [User Option] This variable controls whether to process file-local variables. The possible values are: t (the default) Set the safe variables, and query (once) about any unsafe variables. :safe Set only the safe variables and do not query. :all Set all the variables and do not query. nil Don’t set any variables. anything else Query (once) about all the variables. inhibit-local-variables-regexps [Variable] This is a list of regular expressions. If a file has a name matching an element of this list, then it is not scanned for any form of file-local variable. For examples of why you might want to use this, see Section 22.2.2 [Auto Major Mode], page 412. hack-local-variables &optional mode-only [Function] This function parses, and binds or evaluates as appropriate, any local variables spec- ified by the contents of the current buffer. The variable enable-local-variables has its effect here. However, this function does not look for the ‘mode:’ local variable in the ‘-*-’ line. set-auto-mode does that, also taking enable-local-variables into account (see Section 22.2.2 [Auto Major Mode], page 412). This function works by walking the alist stored in file-local-variables-alist and applying each local variable in turn. It calls before-hack-local-variables-hook and hack-local-variables-hook before and after applying the variables, respectively. It only calls the before-hook if the alist is non-nil; it always calls the other Chapter 11: Variables 164 hook. This function ignores a ‘mode’ element if it specifies the same major mode as the buffer already has. If the optional argument mode-only is non-nil, then all this function does is return a symbol specifying the major mode, if the ‘-*-’ line or the local variables list specifies one, and nil otherwise. It does not set the mode nor any other file-local variable. file-local-variables-alist [Variable] This buffer-local variable holds the alist of file-local variable settings. Each element of the alist is of the form (var . value), where var is a symbol of the local variable and value is its value. When Emacs visits a file, it first collects all the file-local variables into this alist, and then the hack-local-variables function applies them one by one. before-hack-local-variables-hook [Variable] Emacs calls this hook immediately before applying file-local variables stored in file-local-variables-alist. hack-local-variables-hook [Variable] Emacs calls this hook immediately after it finishes applying file-local variables stored in file-local-variables-alist. You can specify safe values for a variable with a safe-local-variable property. The property has to be a function of one argument; any value is safe if the function returns non-nil given that value. Many commonly-encountered file variables have safe-localvariable properties; these include fill-column, fill-prefix, and indent-tabs-mode. For boolean-valued variables that are safe, use booleanp as the property value. When defining a user option using defcustom, you can set its safe-local-variable property by adding the arguments :safe function to defcustom (see Section 14.3 [Variable Definitions], page 209). safe-local-variable-values [User Option] This variable provides another way to mark some variable values as safe. It is a list of cons cells (var . val), where var is a variable name and val is a value which is safe for that variable. When Emacs asks the user whether or not to obey a set of file-local variable specifications, the user can choose to mark them as safe. Doing so adds those variable/value pairs to safe-local-variable-values, and saves it to the user’s custom file. safe-local-variable-p sym val [Function] This function returns non-nil if it is safe to give sym the value val, based on the above criteria. Some variables are considered risky. If a variable is risky, it is never entered automatically into safe-local-variable-values; Emacs always queries before setting a risky variable, unless the user explicitly allows a value by customizing safe-local-variablevalues directly. Any variable whose name has a non-nil risky-local-variable property is considered risky. When you define a user option using defcustom, you can set its Chapter 11: Variables 165 risky-local-variable property by adding the arguments :risky value to defcustom (see Section 14.3 [Variable Definitions], page 209). In addition, any variable whose name ends in any of ‘-command’, ‘-frame-alist’, ‘-function’, ‘-functions’, ‘-hook’, ‘-hooks’, ‘-form’, ‘-forms’, ‘-map’, ‘-map-alist’, ‘-mode-alist’, ‘-program’, or ‘-predicate’ is automatically considered risky. The variables ‘font-lock-keywords’, ‘font-lock-keywords’ followed by a digit, and ‘font-lock-syntactic-keywords’ are also considered risky. risky-local-variable-p sym [Function] This function returns non-nil if sym is a risky variable, based on the above criteria. ignored-local-variables [Variable] This variable holds a list of variables that should not be given local values by files. Any value specified for one of these variables is completely ignored. The ‘Eval:’ “variable” is also a potential loophole, so Emacs normally asks for confirmation before handling it. enable-local-eval [User Option] This variable controls processing of ‘Eval:’ in ‘-*-’ lines or local variables lists in files being visited. A value of t means process them unconditionally; nil means ignore them; anything else means ask the user what to do for each file. The default value is maybe. safe-local-eval-forms [User Option] This variable holds a list of expressions that are safe to evaluate when found in the ‘Eval:’ “variable” in a file local variables list. If the expression is a function call and the function has a safe-local-eval-function property, the property value determines whether the expression is safe to evaluate. The property value can be a predicate to call to test the expression, a list of such predicates (it’s safe if any predicate succeeds), or t (always safe provided the arguments are constant). Text properties are also potential loopholes, since their values could include functions to call. So Emacs discards all text properties from string values specified for file-local variables. 11.12 Directory Local Variables A directory can specify local variable values common to all files in that directory; Emacs uses these to create buffer-local bindings for those variables in buffers visiting any file in that directory. This is useful when the files in the directory belong to some project and therefore share the same local variables. There are two different methods for specifying directory local variables: by putting them in a special file, or by defining a project class for that directory. dir-locals-file [Constant] This constant is the name of the file where Emacs expects to find the directory- local variables. The name of the file is .dir-locals.el3. A file by that name in a 3 The MS-DOS version of Emacs uses _dir-locals.el instead, due to limitations of the DOS filesystems. Chapter 11: Variables 166 directory causes Emacs to apply its settings to any file in that directory or any of its subdirectories (optionally, you can exclude subdirectories; see below). If some of the subdirectories have their own .dir-locals.el files, Emacs uses the settings from the deepest file it finds starting from the file’s directory and moving up the directory tree. The file specifies local variables as a specially formatted list; see Section “Per-directory Local Variables” in The GNU Emacs Manual, for more details. hack-dir-local-variables [Function] This function reads the .dir-locals.el file and stores the directory-local vari- ables in file-local-variables-alist that is local to the buffer visiting any file in the directory, without applying them. It also stores the directory-local settings in dir-locals-class-alist, where it defines a special class for the directory in which .dir-locals.el file was found. This function works by calling dir-locals-set- class-variables and dir-locals-set-directory-class, described below. hack-dir-local-variables-non-file-buffer [Function] This function looks for directory-local variables, and immediately applies them in the current buffer. It is intended to be called in the mode commands for non-file buffers, such as Dired buffers, to let them obey directory-local variable settings. For non-file buffers, Emacs looks for directory-local variables in default-directory and its parent directories. dir-locals-set-class-variables class variables [Function] This function defines a set of variable settings for the named class, which is a symbol. You can later assign the class to one or more directories, and Emacs will apply those variable settings to all files in those directories. The list in variables can be of one of the two forms: (major-mode . alist) or (directory . list). With the first form, if the file’s buffer turns on a mode that is derived from major-mode, then the all the variables in the associated alist are applied; alist should be of the form (name . value). A special value nil for major-mode means the settings are applicable to any mode. In alist, you can use a special name: subdirs. If the associated value is nil, the alist is only applied to files in the relevant directory, not to those in any subdirectories. With the second form of variables, if directory is the initial substring of the file’s directory, then list is applied recursively by following the above rules; list should be of one of the two forms accepted by this function in variables. dir-locals-set-directory-class directory class &optional mtime [Function] This function assigns class to all the files in directory and its subdirectories. There- after, all the variable settings specified for class will be applied to any visited file in directory and its children. class must have been already defined by dir-locals-set- class-variables. Emacs uses this function internally when it loads directory variables from a .dir-locals.el file. In that case, the optional argument mtime holds the file modification time (as returned by file-attributes). Emacs uses this time to check stored local variables are still valid. If you are assigning a class directly, not via a file, this argument should be nil. Chapter 11: Variables 167 dir-locals-class-alist [Variable] This alist holds the class symbols and the associated variable settings. It is updated by dir-locals-set-class-variables. dir-locals-directory-cache [Variable] This alist holds directory names, their assigned class names, and modification times of the associated directory local variables file (if there is one). The function dir-locals- set-directory-class updates this list. enable-dir-local-variables [Variable] If nil, directory-local variables are ignored. This variable may be useful for modes that want to ignore directory-locals while still respecting file-local variables (see Section 11.11 [File Local Variables], page 163). 11.13 Variable Aliases It is sometimes useful to make two variables synonyms, so that both variables always have the same value, and changing either one also changes the other. Whenever you change the name of a variable—either because you realize its old name was not well chosen, or because its meaning has partly changed—it can be useful to keep the old name as an alias of the new one for compatibility. You can do this with defvaralias. defvaralias new-alias base-variable &optional docstring [Function] This function defines the symbol new-alias as a variable alias for symbol base-variable. This means that retrieving the value of new-alias returns the value of base-variable, and changing the value of new-alias changes the value of base-variable. The two aliased variable names always share the same value and the same bindings. If the docstring argument is non-nil, it specifies the documentation for new-alias; otherwise, the alias gets the same documentation as base-variable has, if any, unless base-variable is itself an alias, in which case new-alias gets the documentation of the variable at the end of the chain of aliases. This function returns base-variable. Variable aliases are convenient for replacing an old name for a variable with a new name. make-obsolete-variable declares that the old name is obsolete and therefore that it may be removed at some stage in the future. make-obsolete-variable obsolete-name current-name when &optional [Function] access-type This function makes the byte compiler warn that the variable obsolete-name is obsolete. If current-name is a symbol, it is the variable’s new name; then the warning message says to use current-name instead of obsolete-name. If current-name is a string, this is the message and there is no replacement variable. when should be a string indicating when the variable was first made obsolete (usually a version number string). The optional argument access-type, if non-nil, should should specify the kind of access that will trigger obsolescence warnings; it can be either get or set. Chapter 11: Variables 168 You can make two variables synonyms and declare one obsolete at the same time using the macro define-obsolete-variable-alias. define-obsolete-variable-alias obsolete-name current-name [Macro] &optional when docstring This macro marks the variable obsolete-name as obsolete and also makes it an alias for the variable current-name. It is equivalent to the following: (defvaralias obsolete-name current-name docstring) (make-obsolete-variable obsolete-name current-name when) indirect-variable variable [Function] This function returns the variable at the end of the chain of aliases of variable. If variable is not a symbol, or if variable is not defined as an alias, the function returns variable. This function signals a cyclic-variable-indirection error if there is a loop in the chain of symbols. (defvaralias ’foo ’bar) (indirect-variable ’foo) ⇒ bar (indirect-variable ’bar) ⇒ bar (setq bar 2) bar ⇒2 foo ⇒2 (setq foo 0) bar ⇒0 foo ⇒0 11.14 Variables with Restricted Values Ordinary Lisp variables can be assigned any value that is a valid Lisp object. However, certain Lisp variables are not defined in Lisp, but in C. Most of these variables are defined in the C code using DEFVAR_LISP. Like variables defined in Lisp, these can take on any value. However, some variables are defined using DEFVAR_INT or DEFVAR_BOOL. See [Writing Emacs Primitives], page 1010, in particular the description of functions of the type syms_ of_filename, for a brief discussion of the C implementation. Variables of type DEFVAR_BOOL can only take on the values nil or t. Attempting to assign them any other value will set them to t: (let ((display-hourglass 5)) display-hourglass) ⇒t byte-boolean-vars This variable holds a list of all variables of type DEFVAR_BOOL. [Variable] Chapter 11: Variables 169 Variables of type DEFVAR_INT can take on only integer values. Attempting to assign them any other value will result in an error: (setq undo-limit 1000.0) error Wrong type argument: integerp, 1000.0 11.15 Generalized Variables A generalized variable or place form is one of the many places in Lisp memory where values can be stored. The simplest place form is a regular Lisp variable. But the cars and cdrs of lists, elements of arrays, properties of symbols, and many other locations are also places where Lisp values are stored. Generalized variables are analogous to “lvalues” in the C language, where ‘x = a[i]’ gets an element from an array and ‘a[i] = x’ stores an element using the same notation. Just as certain forms like a[i] can be lvalues in C, there is a set of forms that can be generalized variables in Lisp. 11.15.1 The setf Macro The setf macro is the most basic way to operate on generalized variables. The setf form is like setq, except that it accepts arbitrary place forms on the left side rather than just symbols. For example, (setf (car a) b) sets the car of a to b, doing the same operation as (setcar a b), but without having to remember two separate functions for setting and accessing every type of place. setf [place form]. . . [Macro] This macro evaluates form and stores it in place, which must be a valid generalized variable form. If there are several place and form pairs, the assignments are done sequentially just as with setq. setf returns the value of the last form. The following Lisp forms will work as generalized variables, and so may appear in the place argument of setf: • A symbol naming a variable. In other words, (setf x y) is exactly equivalent to (setq x y), and setq itself is strictly speaking redundant given that setf exists. Many programmers continue to prefer setq for setting simple variables, though, purely for stylistic or historical reasons. The macro (setf x y) actually expands to (setq x y), so there is no performance penalty for using it in compiled code. • A call to any of the following standard Lisp functions: aref car caar cadr cdr cdar cddr elt get gethash nth nthcdr symbol-function symbol-plist symbol-value • A call to any of the following Emacs-specific functions: default-value frame-parameter terminal-parameter keymap-parent match-data overlay-get process-get process-sentinel window-buffer window-display-table window-dedicated-p window-hscroll Chapter 11: Variables 170 overlay-start overlay-end process-buffer process-filter window-parameter window-point window-start setf signals an error if you pass a place form that it does not know how to handle. Note that for nthcdr, the list argument of the function must itself be a valid place form. For example, (setf (nthcdr 0 foo) 7) will set foo itself to 7. The macros push (see Section 5.5 [List Variables], page 71) and pop (see Section 5.3 [List Elements], page 65) can manipulate generalized variables, not just lists. (pop place) removes and returns the first element of the list stored in place. It is analogous to (prog1 (car place) (setf place (cdr place))), except that it takes care to evaluate all subforms only once. (push x place) inserts x at the front of the list stored in place. It is analogous to (setf place (cons x place)), except for evaluation of the subforms. Note that push and pop on an nthcdr place can be used to insert or delete at any position in a list. The cl-lib library defines various extensions for generalized variables, including additional setf places. See Section “Generalized Variables” in Common Lisp Extensions. 11.15.2 Defining new setf forms This section describes how to define new forms that setf can operate on. gv-define-simple-setter name setter &optional fix-return [Macro] This macro enables you to easily define setf methods for simple cases. name is the name of a function, macro, or special form. You can use this macro whenever name has a directly corresponding setter function that updates it, e.g., (gv-define- simple-setter car setcar). This macro translates a call of the form (setf (name args...) value) into (setter args... value) Such a setf call is documented to return value. This is no problem with, e.g., car and setcar, because setcar returns the value that it set. If your setter function does not return value, use a non-nil value for the fix-return argument of gv-definesimple-setter. This expands into something equivalent to (let ((temp value)) (setter args... temp) temp) so ensuring that it returns the correct result. gv-define-setter name arglist &rest body [Macro] This macro allows for more complex setf expansions than the previous form. You may need to use this form, for example, if there is no simple setter function to call, or if there is one but it requires different arguments to the place form. This macro expands the form (setf (name args...) value) by first binding the setf argument forms (value args...) according to arglist, and then executing body. body should return a Lisp form that does the assignment, and finally returns the value that was set. An example of using this macro is: Chapter 11: Variables 171 (gv-define-setter caar (val x) ‘(setcar (car ,x) ,val)) For more control over the expansion, see the macro gv-define-expander. The macro gv-letplace can be useful in defining macros that perform similarly to setf; for example, the incf macro of Common Lisp. Consult the source file gv.el for more details. Common Lisp note: Common Lisp defines another way to specify the setf behavior of a function, namely “setf functions”, whose names are lists (setf name) rather than symbols. For example, (defun (setf foo) ...) defines the function that is used when setf is applied to foo. Emacs does not support this. It is a compile-time error to use setf on a form that has not already had an appropriate expansion defined. In Common Lisp, this is not an error since the function (setf func) might be defined later. Chapter 12: Functions 172 12 Functions A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them. 12.1 What Is a Function? In a general sense, a function is a rule for carrying out a computation given input values called arguments. The result of the computation is called the value or return value of the function. The computation can also have side effects, such as lasting changes in the values of variables or the contents of data structures. In most computer languages, every function has a name. But in Lisp, a function in the strictest sense has no name: it is an object which can optionally be associated with a symbol (e.g., car) that serves as the function name. See Section 12.3 [Function Names], page 177. When a function has been given a name, we usually also refer to that symbol as a “function” (e.g., we refer to “the function car”). In this manual, the distinction between a function name and the function object itself is usually unimportant, but we will take note wherever it is relevant. Certain function-like objects, called special forms and macros, also accept arguments to carry out computations. However, as explained below, these are not considered functions in Emacs Lisp. Here are important terms for functions and function-like objects: lambda expression A function (in the strict sense, i.e., a function object) which is written in Lisp. These are described in the following section. primitive A function which is callable from Lisp but is actually written in C. Primitives are also called built-in functions, or subrs. Examples include functions like car and append. In addition, all special forms (see below) are also considered primitives. Usually, a function is implemented as a primitive because it is a fundamental part of Lisp (e.g., car), or because it provides a low-level interface to operating system services, or because it needs to run fast. Unlike functions defined in Lisp, primitives can be modified or added only by changing the C sources and recompiling Emacs. See Section E.6 [Writing Emacs Primitives], page 1007. special form A primitive that is like a function but does not evaluate all of its arguments in the usual way. It may evaluate only some of the arguments, or may evaluate them in an unusual order, or several times. Examples include if, and, and while. See Section 9.1.7 [Special Forms], page 117. macro A construct defined in Lisp, which differs from a function in that it translates a Lisp expression into another expression which is to be evaluated instead of the original expression. Macros enable Lisp programmers to do the sorts of things that special forms can do. See Chapter 13 [Macros], page 198. Chapter 12: Functions 173 command closure An object which can be invoked via the command-execute primitive, usually due to the user typing in a key sequence bound to that command. See Section 20.3 [Interactive Call], page 328. A command is usually a function; if the function is written in Lisp, it is made into a command by an interactive form in the function definition (see Section 20.2 [Defining Commands], page 322). Commands that are functions can also be called from Lisp expressions, just like other functions. Keyboard macros (strings and vectors) are commands also, even though they are not functions. See Section 20.16 [Keyboard Macros], page 365. We say that a symbol is a command if its function cell contains a command (see Section 8.1 [Symbol Components], page 105); such a named command can be invoked with M-x. A function object that is much like a lambda expression, except that it also encloses an “environment” of lexical variable bindings. See Section 12.9 [Closures], page 185. byte-code function A function that has been compiled by the byte compiler. See Section 2.3.16 [Byte-Code Type], page 23. autoload object A place-holder for a real function. If the autoload object is called, Emacs loads the file containing the definition of the real function, and then calls the real function. See Section 15.5 [Autoload], page 230. You can use the function functionp to test if an object is a function: functionp object [Function] This function returns t if object is any kind of function, i.e., can be passed to funcall. Note that functionp returns t for symbols that are function names, and returns nil for special forms. Unlike functionp, the next three functions do not treat a symbol as its function definition. subrp object [Function] This function returns t if object is a built-in function (i.e., a Lisp primitive). (subrp ’message) ; message is a symbol, ⇒ nil ; not a subr object. (subrp (symbol-function ’message)) ⇒t byte-code-function-p object This function returns t if object is a byte-code function. For example: (byte-code-function-p (symbol-function ’next-line)) ⇒t [Function] subr-arity subr [Function] This function provides information about the argument list of a primitive, subr. The returned value is a pair (min . max). min is the minimum number of args. max is the maximum number or the symbol many, for a function with &rest arguments, or the symbol unevalled if subr is a special form. Chapter 12: Functions 174 12.2 Lambda Expressions A lambda expression is a function object written in Lisp. Here is an example: (lambda (x) "Return the hyperbolic cosine of X." (* 0.5 (+ (exp x) (exp (- x))))) In Emacs Lisp, such a list is a valid expression which evaluates to a function object. A lambda expression, by itself, has no name; it is an anonymous function. Although lambda expressions can be used this way (see Section 12.7 [Anonymous Functions], page 182), they are more commonly associated with symbols to make named functions (see Section 12.3 [Function Names], page 177). Before going into these details, the following subsections describe the components of a lambda expression and what they do. 12.2.1 Components of a Lambda Expression A lambda expression is a list that looks like this: (lambda (arg-variables...) [documentation-string] [interactive-declaration] body-forms...) The first element of a lambda expression is always the symbol lambda. This indicates that the list represents a function. The reason functions are defined to start with lambda is so that other lists, intended for other uses, will not accidentally be valid as functions. The second element is a list of symbols—the argument variable names. This is called the lambda list. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. See Section 11.3 [Local Variables], page 144. The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. See Section 12.2.4 [Function Documentation], page 176. The interactive declaration is a list of the form (interactive code-string). This declares how to provide arguments if the function is used interactively. Functions with this declaration are called commands; they can be called using M-x or bound to a key. Functions not intended to be called in this way should not have interactive declarations. See Section 20.2 [Defining Commands], page 322, for how to write an interactive declaration. The rest of the elements are the body of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, “a list of Lisp forms to evaluate”). The value returned by the function is the value returned by the last element of the body. 12.2.2 A Simple Lambda Expression Example Consider the following example: (lambda (a b c) (+ a b c)) We can call this function by passing it to funcall, like this: (funcall (lambda (a b c) (+ a b c)) 1 2 3) Chapter 12: Functions 175 This call evaluates the body of the lambda expression with the variable a bound to 1, b bound to 2, and c bound to 3. Evaluation of the body adds these three numbers, producing the result 6; therefore, this call to the function returns the value 6. Note that the arguments can be the results of other function calls, as in this example: (funcall (lambda (a b c) (+ a b c)) 1 (* 2 3) (- 5 4)) This evaluates the arguments 1, (* 2 3), and (- 5 4) from left to right. Then it applies the lambda expression to the argument values 1, 6 and 1 to produce the value 8. As these examples show, you can use a form with a lambda expression as its car to make local variables and give them values. In the old days of Lisp, this technique was the only way to bind and initialize local variables. But nowadays, it is clearer to use the special form let for this purpose (see Section 11.3 [Local Variables], page 144). Lambda expressions are mainly used as anonymous functions for passing as arguments to other functions (see Section 12.7 [Anonymous Functions], page 182), or stored as symbol function definitions to produce named functions (see Section 12.3 [Function Names], page 177). 12.2.3 Other Features of Argument Lists Our simple sample function, (lambda (a b c) (+ a b c)), specifies three argument variables, so it must be called with three arguments: if you try to call it with only two arguments or four arguments, you get a wrong-number-of-arguments error. It is often convenient to write a function that allows certain arguments to be omitted. For example, the function substring accepts three arguments—a string, the start index and the end index—but the third argument defaults to the length of the string if you omit it. It is also convenient for certain functions to accept an indefinite number of arguments, as the functions list and + do. To specify optional arguments that may be omitted when a function is called, simply include the keyword &optional before the optional arguments. To specify a list of zero or more extra arguments, include the keyword &rest before one final argument. Thus, the complete syntax for an argument list is as follows: (required-vars... [&optional optional-vars...] [&rest rest-var]) The square brackets indicate that the &optional and &rest clauses, and the variables that follow them, are optional. A call to the function requires one actual argument for each of the required-vars. There may be actual arguments for zero or more of the optional-vars, and there cannot be any actual arguments beyond that unless the lambda list uses &rest. In that case, there may be any number of extra actual arguments. If actual arguments for the optional and rest variables are omitted, then they always default to nil. There is no way for the function to distinguish between an explicit argument of nil and an omitted argument. However, the body of the function is free to consider nil an abbreviation for some other meaningful value. This is what substring does; nil as the third argument to substring means to use the length of the string supplied. Chapter 12: Functions 176 Common Lisp note: Common Lisp allows the function to specify what default value to use when an optional argument is omitted; Emacs Lisp always uses nil. Emacs Lisp does not support “supplied-p” variables that tell you whether an argument was explicitly passed. For example, an argument list that looks like this: (a b &optional c d &rest e) binds a and b to the first two actual arguments, which are required. If one or two more arguments are provided, c and d are bound to them respectively; any arguments after the first four are collected into a list and e is bound to that list. If there are only two arguments, c is nil; if two or three arguments, d is nil; if four arguments or fewer, e is nil. There is no way to have required arguments following optional ones—it would not make sense. To see why this must be so, suppose that c in the example were optional and d were required. Suppose three actual arguments are given; which variable would the third argument be for? Would it be used for the c, or for d? One can argue for both possibilities. Similarly, it makes no sense to have any more arguments (either required or optional) after a &rest argument. Here are some examples of argument lists and proper calls: (funcall (lambda (n) (1+ n)) ; One required: 1) ; requires exactly one argument. ⇒2 (funcall (lambda (n &optional n1) ; One required and one optional: (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments. 1 2) ⇒3 (funcall (lambda (n &rest ns) ; One required and one rest: (+ n (apply ’+ ns))) ; 1 or more arguments. 1 2 3 4 5) ⇒ 15 12.2.4 Documentation Strings of Functions A lambda expression may optionally have a documentation string just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. See Chapter 23 [Documentation], page 461, for how the documentation string is accessed. It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access. The first line of the documentation string should stand on its own, because apropos displays just this first line. It should consist of one or two complete sentences that summarize the function’s purpose. The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. That is a mistake. The indentation of the following lines is inside Chapter 12: Functions 177 the string; what looks nice in the source code will look ugly when displayed by the help commands. You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation. The last line of the documentation string can specify calling conventions different from the actual function arguments. Write text like this: \(fn arglist) following a blank line, at the beginning of the line, with no newline following it inside the documentation string. (The ‘\’ is used to avoid confusing the Emacs motion commands.) The calling convention specified in this way appears in help messages in place of the one derived from the actual arguments of the function. This feature is particularly useful for macro definitions, since the arguments written in a macro definition often do not correspond to the way users think of the parts of the macro call. 12.3 Naming a Function A symbol can serve as the name of a function. This happens when the symbol’s function cell (see Section 8.1 [Symbol Components], page 105) contains a function object (e.g., a lambda expression). Then the symbol itself becomes a valid, callable function, equivalent to the function object in its function cell. The contents of the function cell are also called the symbol’s function definition. The procedure of using a symbol’s function definition in place of the symbol is called symbol function indirection; see Section 9.1.4 [Function Indirection], page 115. If you have not given a symbol a function definition, its function cell is said to be void, and it cannot be used as a function. In practice, nearly all functions have names, and are referred to by their names. You can create a named Lisp function by defining a lambda expression and putting it in a function cell (see Section 12.8 [Function Cells], page 184). However, it is more common to use the defun special form, described in the next section. We give functions names because it is convenient to refer to them by their names in Lisp expressions. Also, a named Lisp function can easily refer to itself—it can be recursive. Furthermore, primitives can only be referred to textually by their names, since primitive function objects (see Section 2.3.15 [Primitive Function Type], page 22) have no read syntax. A function need not have a unique name. A given function object usually appears in the function cell of only one symbol, but this is just a convention. It is easy to store it in several symbols using fset; then each of the symbols is a valid name for the same function. Note that a symbol used as a function name may also be used as a variable; these two uses of a symbol are independent and do not conflict. (This is not the case in some dialects of Lisp, like Scheme.) Chapter 12: Functions 178 12.4 Defining Functions We usually give a name to a function when it is first created. This is called defining a function, and it is done with the defun macro. defun name args [doc] [declare] [interactive] body. . . [Macro] defun is the usual way to define new Lisp functions. It defines the symbol name as a function with argument list args and body forms given by body. Neither name nor args should be quoted. doc, if present, should be a string specifying the function’s documentation string (see Section 12.2.4 [Function Documentation], page 176). declare, if present, should be a declare form specifying function metadata (see Section 12.13 [Declare Form], page 193). interactive, if present, should be an interactive form specifying how the function is to be called interactively (see Section 20.3 [Interactive Call], page 328). The return value of defun is undefined. Here are some examples: (defun foo () 5) (foo) ⇒5 (defun bar (a &optional b &rest c) (list a b c)) (bar 1 2 3 4 5) ⇒ (1 2 (3 4 5)) (bar 1) ⇒ (1 nil nil) (bar) error Wrong number of arguments. (defun capitalize-backwards () "Upcase the last letter of the word at point." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) Be careful not to redefine existing functions unintentionally. defun redefines even primitive functions such as car without any hesitation or notification. Emacs does not prevent you from doing this, because redefining a function is sometimes done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition. defalias name definition &optional doc [Function] This function defines the symbol name as a function, with definition definition (which can be any valid Lisp function). Its return value is undefined. If doc is non-nil, it becomes the function documentation of name. Otherwise, any documentation provided by definition is used. Chapter 12: Functions 179 Internally, defalias normally uses fset to set the definition. If name has a defalias-fset-function property, however, the associated value is used as a function to call in place of fset. The proper place to use defalias is where a specific function name is being defined— especially where that name appears explicitly in the source file being loaded. This is because defalias records which file defined the function, just like defun (see Section 15.9 [Unloading], page 237). By contrast, in programs that manipulate function definitions for other purposes, it is better to use fset, which does not keep such records. See Section 12.8 [Function Cells], page 184. You cannot create a new primitive function with defun or defalias, but you can use them to change the function definition of any symbol, even one such as car or x-popupmenu whose normal definition is a primitive. However, this is risky: for instance, it is next to impossible to redefine car without breaking Lisp completely. Redefining an obscure function such as x-popup-menu is less dangerous, but it still may not work as you expect. If there are calls to the primitive from C code, they call the primitive’s C definition directly, so changing the symbol’s definition will have no effect on them. See also defsubst, which defines a function like defun and tells the Lisp compiler to perform inline expansion on it. See Section 12.12 [Inline Functions], page 193. 12.5 Calling Functions Defining functions is only half the battle. Functions don’t do anything until you call them, i.e., tell them to run. Calling a function is also known as invocation. The most common way of invoking a function is by evaluating a list. For example, evaluating the list (concat "a" "b") calls the function concat with arguments "a" and "b". See Chapter 9 [Evaluation], page 113, for a description of evaluation. When you write a list as an expression in your program, you specify which function to call, and how many arguments to give it, in the text of the program. Usually that’s just what you want. Occasionally you need to compute at run time which function to call. To do that, use the function funcall. When you also need to determine at run time how many arguments to pass, use apply. funcall function &rest arguments [Function] funcall calls function with arguments, and returns whatever function returns. Since funcall is a function, all of its arguments, including function, are evaluated before funcall is called. This means that you can use any expression to obtain the function to be called. It also means that funcall does not see the expressions you write for the arguments, only their values. These values are not evaluated a second time in the act of calling function; the operation of funcall is like the normal procedure for calling a function, once its arguments have already been evaluated. The argument function must be either a Lisp function or a primitive function. Special forms and macros are not allowed, because they make sense only when given the “unevaluated” argument expressions. funcall cannot provide these because, as we saw above, it never knows them in the first place. Chapter 12: Functions 180 (setq f ’list) ⇒ list (funcall f ’x ’y ’z) ⇒ (x y z) (funcall f ’x ’y ’(z)) ⇒ (x y (z)) (funcall ’and t nil) error Invalid function: # Compare these examples with the examples of apply. apply function &rest arguments [Function] apply calls function with arguments, just like funcall but with one difference: the last of arguments is a list of objects, which are passed to function as separate ar- guments, rather than a single list. We say that apply spreads this list so that each individual element becomes an argument. apply returns the result of calling function. As with funcall, function must either be a Lisp function or a primitive function; special forms and macros do not make sense in apply. (setq f ’list) ⇒ list (apply f ’x ’y ’z) error Wrong type argument: listp, z (apply ’+ 1 2 ’(3 4)) ⇒ 10 (apply ’+ ’(1 2 3 4)) ⇒ 10 (apply ’append ’((a b c) nil (x y z) nil)) ⇒ (a b c x y z) For an interesting example of using apply, see [Definition of mapcar], page 181. Sometimes it is useful to fix some of the function’s arguments at certain values, and leave the rest of arguments for when the function is actually called. The act of fixing some of the function’s arguments is called partial application of the function1. The result is a new function that accepts the rest of arguments and calls the original function with all the arguments combined. Here’s how to do partial application in Emacs Lisp: apply-partially func &rest args [Function] This function returns a new function which, when called, will call func with the list of arguments composed from args and additional arguments specified at the time of the call. If func accepts n arguments, then a call to apply-partially with m < n arguments will produce a new function of n - m arguments. Here’s how we could define the built-in function 1+, if it didn’t exist, using apply-partially and +, another built-in function: 1 This is related to, but different from currying, which transforms a function that takes multiple arguments in such a way that it can be called as a chain of functions, each one with a single argument. Chapter 12: Functions 181 (defalias ’1+ (apply-partially ’+ 1) "Increment argument by one.") (1+ 10) ⇒ 11 It is common for Lisp functions to accept functions as arguments or find them in data structures (especially in hook variables and property lists) and call them using funcall or apply. Functions that accept function arguments are often called functionals. Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function: identity arg This function returns arg and has no side effects. [Function] ignore &rest args This function ignores any arguments and returns nil. [Function] Some functions are user-visible commands, which can be called interactively (usually by a key sequence). It is possible to invoke such a command exactly as though it was called interactively, by using the call-interactively function. See Section 20.3 [Interactive Call], page 328. 12.6 Mapping Functions A mapping function applies a given function (not a special form or macro) to each element of a list or other collection. Emacs Lisp has several such functions; this section describes mapcar, mapc, and mapconcat, which map over a list. See [Definition of mapatoms], page 109, for the function mapatoms which maps over the symbols in an obarray. See [Definition of maphash], page 103, for the function maphash which maps over key/value associations in a hash table. These mapping functions do not allow char-tables because a char-table is a sparse array whose nominal range of indices is very large. To map over a char-table in a way that deals properly with its sparse nature, use the function map-char-table (see Section 6.6 [Char-Tables], page 94). mapcar function sequence [Function] mapcar applies function to each element of sequence in turn, and returns a list of the results. The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. The result is always a list. The length of the result is the same as the length of sequence. For example: (mapcar ’car ’((a b) (c d) (e f))) ⇒ (a c e) (mapcar ’1+ [1 2 3]) ⇒ (2 3 4) (mapcar ’string "abc") ⇒ ("a" "b" "c") Chapter 12: Functions 182 ;; Call each function in my-hooks. (mapcar ’funcall my-hooks) (defun mapcar* (function &rest args) "Apply FUNCTION to successive cars of all ARGS. Return the list of results." ;; If no list is exhausted, (if (not (memq nil args)) ;; apply function to cars. (cons (apply function (mapcar ’car args)) (apply ’mapcar* function ;; Recurse for rest of elements. (mapcar ’cdr args))))) (mapcar* ’cons ’(a b c) ’(1 2 3 4)) ⇒ ((a . 1) (b . 2) (c . 3)) mapc function sequence [Function] mapc is like mapcar except that function is used for side-effects only—the values it returns are ignored, not collected into a list. mapc always returns sequence. mapconcat function sequence separator [Function] mapconcat applies function to each element of sequence: the results, which must be strings, are concatenated. Between each pair of result strings, mapconcat inserts the string separator. Usually separator contains a space or comma or other suitable punctuation. The argument function must be a function that can take one argument and return a string. The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. (mapconcat ’symbol-name ’(The cat in the hat) " ") ⇒ "The cat in the hat" (mapconcat (function (lambda (x) (format "%c" (1+ x)))) "HAL-8000" "") ⇒ "IBM.9111" 12.7 Anonymous Functions Although functions are usually defined with defun and given names at the same time, it is sometimes convenient to use an explicit lambda expression—an anonymous function. Anonymous functions are valid wherever function names are. They are often assigned as variable values, or as arguments to functions; for instance, you might pass one as the function argument to mapcar, which applies that function to each element of a list (see Section 12.6 [Mapping Functions], page 181). See [describe-symbols example], page 462, for a realistic example of this. Chapter 12: Functions 183 When defining a lambda expression that is to be used as an anonymous function, you can in principle use any method to construct the list. But typically you should use the lambda macro, or the function special form, or the #’ read syntax: lambda args [doc] [interactive] body. . . [Macro] This macro returns an anonymous function with argument list args, documentation string doc (if any), interactive spec interactive (if any), and body forms given by body. In effect, this macro makes lambda forms “self-quoting”: evaluating a form whose car is lambda yields the form itself: (lambda (x) (* x x)) ⇒ (lambda (x) (* x x)) The lambda form has one other effect: it tells the Emacs evaluator and byte-compiler that its argument is a function, by using function as a subroutine (see below). function function-object [Special Form] This special form returns function-object without evaluating it. In this, it is similar to quote (see Section 9.2 [Quoting], page 119). But unlike quote, it also serves as a note to the Emacs evaluator and byte-compiler that function-object is intended to be used as a function. Assuming function-object is a valid lambda expression, this has two effects: • When the code is byte-compiled, function-object is compiled into a byte-code function object (see Chapter 16 [Byte Compilation], page 239). • When lexical binding is enabled, function-object is converted into a closure. See Section 12.9 [Closures], page 185. The read syntax #’ is a short-hand for using function. The following forms are all equivalent: (lambda (x) (* x x)) (function (lambda (x) (* x x))) #’(lambda (x) (* x x)) In the following example, we define a change-property function that takes a function as its third argument, followed by a double-property function that makes use of change-property by passing it an anonymous function: (defun change-property (symbol prop function) (let ((value (get symbol prop))) (put symbol prop (funcall function value)))) (defun double-property (symbol prop) (change-property symbol prop (lambda (x) (* 2 x)))) Note that we do not quote the lambda form. If you compile the above code, the anonymous function is also compiled. This would not happen if, say, you had constructed the anonymous function by quoting it as a list: (defun double-property (symbol prop) (change-property symbol prop ’(lambda (x) (* 2 x)))) Chapter 12: Functions 184 In that case, the anonymous function is kept as a lambda expression in the compiled code. The byte-compiler cannot assume this list is a function, even though it looks like one, since it does not know that change-property intends to use it as a function. 12.8 Accessing Function Cell Contents The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols. See also the function indirect-function. See [Definition of indirect-function], page 116. symbol-function symbol [Function] This returns the object in the function cell of symbol. It does not check that the returned object is a legitimate function. If the function cell is void, the return value is nil. To distinguish between a function cell that is void and one set to nil, use fboundp (see below). (defun bar (n) (+ n 2)) (symbol-function ’bar) ⇒ (lambda (n) (+ n 2)) (fset ’baz ’bar) ⇒ bar (symbol-function ’baz) ⇒ bar If you have never given a symbol any function definition, we say that that symbol’s function cell is void. In other words, the function cell does not have any Lisp object in it. If you try to call the symbol as a function, Emacs signals a void-function error. Note that void is not the same as nil or the symbol void. The symbols nil and void are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with defun). A void function cell contains no object whatsoever. You can test the voidness of a symbol’s function definition with fboundp. After you have given a symbol a function definition, you can make it void once more using fmakunbound. fboundp symbol [Function] This function returns t if the symbol has an object in its function cell, nil otherwise. It does not check that the object is a legitimate function. fmakunbound symbol [Function] This function makes symbol’s function cell void, so that a subsequent attempt to access this cell will cause a void-function error. It returns symbol. (See also makunbound, in Section 11.4 [Void Variables], page 146.) (defun foo (x) x) (foo 1) ⇒1 (fmakunbound ’foo) ⇒ foo (foo 1) error Symbol’s function definition is void: foo Chapter 12: Functions 185 fset symbol definition [Function] This function stores definition in the function cell of symbol. The result is definition. Normally definition should be a function or the name of a function, but this is not checked. The argument symbol is an ordinary evaluated argument. The primary use of this function is as a subroutine by constructs that define or alter functions, like defun or advice-add (see Section 12.10 [Advising Functions], page 185). You can also use it to give a symbol a function definition that is not a function, e.g., a keyboard macro (see Section 20.16 [Keyboard Macros], page 365): ;; Define a named keyboard macro. (fset ’kill-two-lines "\^u2\^k") ⇒ "\^u2\^k" It you wish to use fset to make an alternate name for a function, consider using defalias instead. See [Definition of defalias], page 178. 12.9 Closures As explained in Section 11.9 [Variable Scoping], page 152, Emacs can optionally enable lexical binding of variables. When lexical binding is enabled, any named function that you create (e.g., with defun), as well as any anonymous function that you create using the lambda macro or the function special form or the #’ syntax (see Section 12.7 [Anonymous Functions], page 182), is automatically converted into a closure. A closure is a function that also carries a record of the lexical environment that existed when the function was defined. When it is invoked, any lexical variable references within its definition use the retained lexical environment. In all other respects, closures behave much like ordinary functions; in particular, they can be called in the same way as ordinary functions. See Section 11.9.3 [Lexical Binding], page 154, for an example of using a closure. Currently, an Emacs Lisp closure object is represented by a list with the symbol closure as the first element, a list representing the lexical environment as the second element, and the argument list and body forms as the remaining elements: ;; lexical binding is enabled. (lambda (x) (* x x)) ⇒ (closure (t) (x) (* x x)) However, the fact that the internal structure of a closure is “exposed” to the rest of the Lisp world is considered an internal implementation detail. For this reason, we recommend against directly examining or altering the structure of closure objects. 12.10 Advising Emacs Lisp Functions When you need to modify a function defined in another library, or when you need to modify a hook like foo-function, a process filter, or basically any variable or object field which holds a function value, you can use the appropriate setter function, such as fset or defun for named functions, setq for hook variables, or set-process-filter for process filters, but those are often too blunt, completely throwing away the previous value. The advice feature lets you add to the existing definition of a function, by advising the function. This is a cleaner method than redefining the whole function. Chapter 12: Functions 186 Emacs’s advice system provides two sets of primitives for that: the core set, for function values held in variables and object fields (with the corresponding primitives being add-function and remove-function) and another set layered on top of it for named functions (with the main primitives being advice-add and advice-remove). For example, in order to trace the calls to the process filter of a process proc, you could use: (defun my-tracing-function (proc string) (message "Proc %S received %S" proc string)) (add-function :before (process-filter proc) #’my-tracing-function) This will cause the process’s output to be passed to my-tracing-function before being passed to the original process filter. my-tracing-function receives the same arguments as the original function. When you’re done with it, you can revert to the untraced behavior with: (remove-function (process-filter proc) #’my-tracing-function) Similarly, if you want to trace the execution of the function named display-buffer, you could use: (defun his-tracing-function (orig-fun &rest args) (message "display-buffer called with args %S" args) (let ((res (apply orig-fun args))) (message "display-buffer returned %S" res) res)) (advice-add ’display-buffer :around #’his-tracing-function) Here, his-tracing-function is called instead of the original function and receives the original function (additionally to that function’s arguments) as argument, so it can call it if and when it needs to. When you’re tired of seeing this output, you can revert to the untraced behavior with: (advice-remove ’display-buffer #’his-tracing-function) The arguments :before and :around used in the above examples specify how the two functions are composed, since there are many different ways to do it. The added function is also called an advice. 12.10.1 Primitives to manipulate advices add-function where place function &optional props [Macro] This macro is the handy way to add the advice function to the function stored in place (see Section 11.15 [Generalized Variables], page 169). where determines how function is composed with the existing function, e.g. whether function should be called before, or after the original function. See Section 12.10.3 [Advice combinators], page 189, for the list of available ways to compose the two functions. When modifying a variable (whose name will usually end with -function), you can choose whether function is used globally or only in the current buffer: if place is just a symbol, then function is added to the global value of place. Whereas if place is of Chapter 12: Functions 187 the form (local symbol), where symbol is an expression which returns the variable name, then function will only be added in the current buffer. Finally, if you want to modify a lexical variable, you will have to use (var variable). Every function added with add-function can be accompanied by an association list of properties props. Currently only two of those properties have a special meaning: name This gives a name to the advice, which remove-function can use to identify which function to remove. Typically used when function is an anonymous function. depth This specifies how to order the advices, in case several advices are present. By default, the depth is 0. A depth of 100 indicates that this advice should be kept as deep as possible, whereas a depth of -100 indicates that it should stay as the outermost advice. When two advices specify the same depth, the most recently added advice will be outermost. For a :before advice, being outermost means that this advice will be run first, before any other advice, whereas being innermost means that it will run right before the original function, with no other advice run between itself and the original function. Similarly, for an :after advice innermost means that it will run right after the original function, with no other advice run in between, whereas outermost means that it will be run very last after all other advices. An innermost :override advice will only override the original function and other advices will apply to it, whereas an outermost :override advice will override not only the original function but all other advices applied to it as well. If function is not interactive, then the combined function will inherit the interactive spec, if any, of the original function. Else, the combined function will be interactive and will use the interactive spec of function. One exception: if the interactive spec of function is a function (rather than an expression or a string), then the interactive spec of the combined function will be a call to that function with as sole argument the interactive spec of the original function. To interpret the spec received as argument, use advice-eval-interactive-spec. Note: The interactive spec of function will apply to the combined function and should hence obey the calling convention of the combined function rather than that of function. In many cases, it makes no difference since they are identical, but it does matter for :around, :filter-args, and filter-return, where function. remove-function place function [Macro] This macro removes function from the function stored in place. This only works if function was added to place using add-function. function is compared with functions added to place using equal, to try and make it work also with lambda expressions. It is additionally compared also with the name property of the functions added to place, which can be more reliable than comparing lambda expressions using equal. Chapter 12: Functions 188 advice-function-member-p advice function-def [Function] Return non-nil if advice is already in function-def. Like for remove-function above, instead of advice being the actual function, it can also be the name of the piece of advice. advice-function-mapc f function-def [Function] Call the function f for every advice that was added to function-def. f is called with two arguments: the advice function and its properties. advice-eval-interactive-spec spec [Function] Evaluate the interactive spec just like an interactive call to a function with such a spec would, and then return the corresponding list of arguments that was built. E.g. (advice-eval-interactive-spec "r\nP") will return a list of three elements, containing the boundaries of the region and the current prefix argument. 12.10.2 Advising Named Functions A common use of advice is for named functions and macros. You could just use add-function as in: (add-function :around (symbol-function ’fun) #’his-tracing-function) But you should use advice-add and advice-remove for that instead. This separate set of functions to manipulate pieces of advice applied to named functions, offers the following extra features compared to add-function: they know how to deal with macros and autoloaded functions, they let describe-function preserve the original docstring as well as document the added advice, and they let you add and remove advices before a function is even defined. advice-add can be useful for altering the behavior of existing calls to an existing function without having to redefine the whole function. However, it can be a source of bugs, since existing callers to the function may assume the old behavior, and work incorrectly when the behavior is changed by advice. Advice can also cause confusion in debugging, if the person doing the debugging does not notice or remember that the function has been modified by advice. For these reasons, advice should be reserved for the cases where you cannot modify a function’s behavior in any other way. If it is possible to do the same thing via a hook, that is preferable (see Section 22.1 [Hooks], page 406). If you simply want to change what a particular key does, it may be better to write a new command, and remap the old command’s key bindings to the new one (see Section 21.13 [Remapping Commands], page 385). In particular, Emacs’s own source files should not put advice on functions in Emacs. (There are currently a few exceptions to this convention, but we aim to correct them.) Special forms (see Section 9.1.7 [Special Forms], page 117) cannot be advised, however macros can be advised, in much the same way as functions. Of course, this will not affect code that has already been macro-expanded, so you need to make sure the advice is installed before the macro is expanded. It is possible to advise a primitive (see Section 12.1 [What Is a Function], page 172), but one should typically not do so, for two reasons. Firstly, some primitives are used by the advice mechanism, and advising them could cause an infinite recursion. Secondly, many Chapter 12: Functions 189 primitives are called directly from C, and such calls ignore advice; hence, one ends up in a confusing situation where some calls (occurring from Lisp code) obey the advice and other calls (from C code) do not. advice-add symbol where function &optional props [Function] Add the advice function to the named function symbol. where and props have the same meaning as for add-function (see Section 12.10.1 [Core Advising Primitives], page 186). advice-remove symbol function [Function] Remove the advice function from the named function symbol. function can also be the name of an advice. advice-member-p function symbol [Function] Return non-nil if the advice function is already in the named function symbol. func- tion can also be the name of an advice. advice-mapc function symbol [Function] Call function for every advice that was added to the named function symbol. function is called with two arguments: the advice function and its properties. 12.10.3 Ways to compose advices Here are the different possible values for the where argument of add-function and advice-add, specifying how the advice function and the original function should be composed. :before Call function before the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like: (lambda (&rest r) (apply function r) (apply oldfun r)) (add-function :before funvar function) is comparable for single-function hooks to (add-hook ’hookvar function) for normal hooks. :after Call function after the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like: (lambda (&rest r) (prog1 (apply oldfun r) (apply function r))) (add-function :after funvar function) is comparable for single-function hooks to (add-hook ’hookvar function ’append) for normal hooks. :override This completely replaces the old function with the new one. The old function can of course be recovered if you later call remove-function. :around Call function instead of the old function, but provide the old function as an extra argument to function. This is the most flexible composition. For example, it lets you call the old function with different arguments, or many times, or within a let-binding, or you can sometimes delegate the work to the old function and sometimes override it completely. More specifically, the composition of the two functions behaves like: Chapter 12: Functions 190 (lambda (&rest r) (apply function oldfun r)) :before-while Call function before the old function and don’t call the old function if function returns nil. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like: (lambda (&rest r) (and (apply function r) (apply oldfun r))) (add-function :before-while funvar function) is comparable for singlefunction hooks to (add-hook ’hookvar function) when hookvar is run via run-hook-with-args-until-failure. :before-until Call function before the old function and only call the old function if function returns nil. More specifically, the composition of the two functions behaves like: (lambda (&rest r) (or (apply function r) (apply oldfun r))) (add-function :before-until funvar function) is comparable for singlefunction hooks to (add-hook ’hookvar function) when hookvar is run via run-hook-with-args-until-success. :after-while Call function after the old function and only if the old function returned nonnil. Both functions receive the same arguments, and the return value of the composition is the return value of function. More specifically, the composition of the two functions behaves like: (lambda (&rest r) (and (apply oldfun r) (apply function r))) (add-function :after-while funvar function) is comparable for single-function hooks to (add-hook ’hookvar function ’append) when hookvar is run via run-hook-with-args-until-failure. :after-until Call function after the old function and only if the old function returned nil. More specifically, the composition of the two functions behaves like: (lambda (&rest r) (or (apply oldfun r) (apply function r))) (add-function :after-until funvar function) is comparable for single-function hooks to (add-hook ’hookvar function ’append) when hookvar is run via run-hook-with-args-until-success. :filter-args Call function first and use the result (which should be a list) as the new arguments to pass to the old function. More specifically, the composition of the two functions behaves like: (lambda (&rest r) (apply oldfun (funcall function r))) :filter-return Call the old function first and pass the result to function. More specifically, the composition of the two functions behaves like: (lambda (&rest r) (funcall function (apply oldfun r))) Chapter 12: Functions 191 12.10.4 Adapting code using the old defadvice A lot of code uses the old defadvice mechanism, which is largely made obsolete by the new advice-add, whose implementation and semantics is significantly simpler. An old advice such as: (defadvice previous-line (before next-line-at-end (&optional arg try-vscroll)) "Insert an empty line when moving up from the top line." (if (and next-line-add-newlines (= arg 1) (save-excursion (beginning-of-line) (bobp))) (progn (beginning-of-line) (newline)))) could be translated in the new advice mechanism into a plain function: (defun previous-line--next-line-at-end (&optional arg try-vscroll) "Insert an empty line when moving up from the top line." (if (and next-line-add-newlines (= arg 1) (save-excursion (beginning-of-line) (bobp))) (progn (beginning-of-line) (newline)))) Obviously, this does not actually modify previous-line. For that the old advice needed: (ad-activate ’previous-line) whereas the new advice mechanism needs: (advice-add ’previous-line :before #’previous-line--next-line-at-end) Note that ad-activate had a global effect: it activated all pieces of advice enabled for that specified function. If you wanted to only activate or deactivate a particular advice, you needed to enable or disable that advice with ad-enable-advice and ad-disable-advice. The new mechanism does away with this distinction. An around advice such as: (defadvice foo (around foo-around) "Ignore case in ‘foo’." (let ((case-fold-search t)) ad-do-it)) (ad-activate ’foo) could translate into: (defun foo--foo-around (orig-fun &rest args) "Ignore case in ‘foo’." (let ((case-fold-search t)) (apply orig-fun args))) (advice-add ’foo :around #’foo--foo-around) Regarding the advice’s class, note that the new :before is not quite equivalent to the old before, because in the old advice you could modify the function’s arguments (e.g., with ad-set-arg), and that would affect the argument values seen by the original function, Chapter 12: Functions 192 whereas in the new :before, modifying an argument via setq in the advice has no effect on the arguments seen by the original function. When porting a before advice which relied on this behavior, you’ll need to turn it into a new :around or :filter-args advice instead. Similarly an old after advice could modify the returned value by changing ad-returnvalue, whereas a new :after advice cannot, so when porting such an old after advice, you’ll need to turn it into a new :around or :filter-return advice instead. 12.11 Declaring Functions Obsolete You can mark a named function as obsolete, meaning that it may be removed at some point in the future. This causes Emacs to warn that the function is obsolete whenever it byte-compiles code containing that function, and whenever it displays the documentation for that function. In all other respects, an obsolete function behaves like any other function. The easiest way to mark a function as obsolete is to put a (declare (obsolete ...)) form in the function’s defun definition. See Section 12.13 [Declare Form], page 193. Alternatively, you can use the make-obsolete function, described below. A macro (see Chapter 13 [Macros], page 198) can also be marked obsolete with make-obsolete; this has the same effects as for a function. An alias for a function or macro can also be marked as obsolete; this makes the alias itself obsolete, not the function or macro which it resolves to. make-obsolete obsolete-name current-name &optional when [Function] This function marks obsolete-name as obsolete. obsolete-name should be a symbol naming a function or macro, or an alias for a function or macro. If current-name is a symbol, the warning message says to use current-name instead of obsolete-name. current-name does not need to be an alias for obsolete-name; it can be a different function with similar functionality. current-name can also be a string, which serves as the warning message. The message should begin in lower case, and end with a period. It can also be nil, in which case the warning message provides no additional details. If provided, when should be a string indicating when the function was first made obsolete—for example, a date or a release number. define-obsolete-function-alias obsolete-name current-name [Macro] &optional when doc This convenience macro marks the function obsolete-name obsolete and also defines it as an alias for the function current-name. It is equivalent to the following: (defalias obsolete-name current-name doc) (make-obsolete obsolete-name current-name when) In addition, you can mark a certain a particular calling convention for a function as obsolete: set-advertised-calling-convention function signature when [Function] This function specifies the argument list signature as the correct way to call function. This causes the Emacs byte compiler to issue a warning whenever it comes across an Emacs Lisp program that calls function any other way (however, it will still allow the Chapter 12: Functions 193 code to be byte compiled). when should be a string indicating when the variable was first made obsolete (usually a version number string). For instance, in old versions of Emacs the sit-for function accepted three arguments, like this (sit-for seconds milliseconds nodisp) However, calling sit-for this way is considered obsolete (see Section 20.10 [Waiting], page 357). The old calling convention is deprecated like this: (set-advertised-calling-convention ’sit-for ’(seconds &optional nodisp) "22.1") 12.12 Inline Functions An inline function is a function that works just like an ordinary function, except for one thing: when you byte-compile a call to the function (see Chapter 16 [Byte Compilation], page 239), the function’s definition is expanded into the caller. To define an inline function, use defsubst instead of defun. defsubst name args [doc] [declare] [interactive] body. . . [Macro] This macro defines an inline function. Its syntax is exactly the same as defun (see Section 12.4 [Defining Functions], page 178). Making a function inline often makes its function calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them. Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline. Also, inline functions do not behave well with respect to debugging, tracing, and advising (see Section 12.10 [Advising Functions], page 185). Since ease of debugging and the flexibility of redefining functions are important features of Emacs, you should not make a function inline, even if it’s small, unless its speed is really crucial, and you’ve timed the code to verify that using defun actually has performance problems. It’s possible to define a macro to expand into the same code that an inline function would execute (see Chapter 13 [Macros], page 198). But the macro would be limited to direct use in expressions—a macro cannot be called with apply, mapcar and so on. Also, it takes some work to convert an ordinary function into a macro. To convert it into an inline function is easy; just replace defun with defsubst. Since each argument of an inline function is evaluated exactly once, you needn’t worry about how many times the body uses the arguments, as you do for macros. After an inline function is defined, its inline expansion can be performed later on in the same file, just like macros. 12.13 The declare Form declare is a special macro which can be used to add “meta” properties to a function or macro: for example, marking it as obsolete, or giving its forms a special TAB indentation convention in Emacs Lisp mode. Chapter 12: Functions 194 declare specs. . . [Macro] This macro ignores its arguments and evaluates to nil; it has no run-time effect. How- ever, when a declare form occurs in the declare argument of a defun or defsubst function definition (see Section 12.4 [Defining Functions], page 178) or a defmacro macro definition (see Section 13.4 [Defining Macros], page 200), it appends the prop- erties specified by specs to the function or macro. This work is specially performed by defun, defsubst, and defmacro. Each element in specs should have the form (property args...), which should not be quoted. These have the following effects: (advertised-calling-convention signature when) This acts like a call to set-advertised-calling-convention (see Section 12.11 [Obsolete Functions], page 192); signature specifies the correct argument list for calling the function or macro, and when should be a string indicating when the old argument list was first made obsolete. (debug edebug-form-spec) This is valid for macros only. When stepping through the macro with Edebug, use edebug-form-spec. See Section 17.2.15.1 [Instrumenting Macro Calls], page 270. (doc-string n) This is used when defining a function or macro which itself will be used to define entities like functions, macros, or variables. It indicates that the nth argument, if any, should be considered as a documentation string. (indent indent-spec) Indent calls to this function or macro according to indent-spec. This is typically used for macros, though it works for functions too. See Section 13.6 [Indenting Macros], page 205. (obsolete current-name when) Mark the function or macro as obsolete, similar to a call to make-obsolete (see Section 12.11 [Obsolete Functions], page 192). current-name should be a symbol (in which case the warning message says to use that instead), a string (specifying the warning message), or nil (in which case the warning message gives no extra details). when should be a string indicating when the function or macro was first made obsolete. (compiler-macro expander) This can only be used for functions, and tells the compiler to use expander as an optimization function. When encountering a call to the function, of the form (function args...), the macro expander will call expander with that form as well as with args . . . , and expander can either return a new expression to use instead of the function call, or it can return just the form unchanged, to indicate that the function call should be left alone. expander can be a symbol, or it can be a form (lambda (arg) body) in which case arg will hold the original function call expression, and the (unevaluated) arguments to the function can be accessed using the function’s formal arguments. Chapter 12: Functions 195 (gv-expander expander) Declare expander to be the function to handle calls to the macro (or function) as a generalized variable, similarly to gv-define-expander. expander can be a symbol or it can be of the form (lambda (arg) body) in which case that function will additionally have access to the macro (or function)’s arguments. (gv-setter setter) Declare setter to be the function to handle calls to the macro (or function) as a generalized variable. setter can be a symbol in which case it will be passed to gv-define-simple-setter, or it can be of the form (lambda (arg) body) in which case that function will additionally have access to the macro (or function)’s arguments and it will passed to gv-definesetter. 12.14 Telling the Compiler that a Function is Defined Byte-compiling a file often produces warnings about functions that the compiler doesn’t know about (see Section 16.6 [Compiler Errors], page 244). Sometimes this indicates a real problem, but usually the functions in question are defined in other files which would be loaded if that code is run. For example, byte-compiling fortran.el used to warn: In end of data: fortran.el:2152:1:Warning: the function ‘gud-find-c-expr’ is not known to be defined. In fact, gud-find-c-expr is only used in the function that Fortran mode uses for the local value of gud-find-expr-function, which is a callback from GUD; if it is called, the GUD functions will be loaded. When you know that such a warning does not indicate a real problem, it is good to suppress the warning. That makes new warnings which might mean real problems more visible. You do that with declare-function. All you need to do is add a declare-function statement before the first use of the function in question: (declare-function gud-find-c-expr "gud.el" nil) This says that gud-find-c-expr is defined in gud.el (the ‘.el’ can be omitted). The compiler takes for granted that that file really defines the function, and does not check. The optional third argument specifies the argument list of gud-find-c-expr. In this case, it takes no arguments (nil is different from not specifying a value). In other cases, this might be something like (file &optional overwrite). You don’t have to specify the argument list, but if you do the byte compiler can check that the calls match the declaration. declare-function function file &optional arglist fileonly [Macro] Tell the byte compiler to assume that function is defined, with arguments arglist, and that the definition should come from the file file. fileonly non-nil means only check that file exists, not that it actually defines function. To verify that these functions really are declared where declare-function says they are, use check-declare-file to check all declare-function calls in one source file, or use check-declare-directory check all the files in and under a certain directory. Chapter 12: Functions 196 These commands find the file that ought to contain a function’s definition using locate-library; if that finds no file, they expand the definition file name relative to the directory of the file that contains the declare-function call. You can also say that a function is a primitive by specifying a file name ending in ‘.c’ or ‘.m’. This is useful only when you call a primitive that is defined only on certain systems. Most primitives are always defined, so they will never give you a warning. Sometimes a file will optionally use functions from an external package. If you prefix the filename in the declare-function statement with ‘ext:’, then it will be checked if it is found, otherwise skipped without error. There are some function definitions that ‘check-declare’ does not understand (e.g., defstruct and some other macros). In such cases, you can pass a non-nil fileonly argument to declare-function, meaning to only check that the file exists, not that it actually defines the function. Note that to do this without having to specify an argument list, you should set the arglist argument to t (because nil means an empty argument list, as opposed to an unspecified one). 12.15 Determining whether a Function is Safe to Call Some major modes, such as SES, call functions that are stored in user files. (See Info file ses, node ‘Top’, for more information on SES.) User files sometimes have poor pedigrees— you can get a spreadsheet from someone you’ve just met, or you can get one through email from someone you’ve never met. So it is risky to call a function whose source code is stored in a user file until you have determined that it is safe. unsafep form &optional unsafep-vars [Function] Returns nil if form is a safe Lisp expression, or returns a list that describes why it might be unsafe. The argument unsafep-vars is a list of symbols known to have temporary bindings at this point; it is mainly used for internal recursive calls. The current buffer is an implicit argument, which provides a list of buffer-local bindings. Being quick and simple, unsafep does a very light analysis and rejects many Lisp expressions that are actually safe. There are no known cases where unsafep returns nil for an unsafe expression. However, a “safe” Lisp expression can return a string with a display property, containing an associated Lisp expression to be executed after the string is inserted into a buffer. This associated expression can be a virus. In order to be safe, you must delete properties from all strings calculated by user code before inserting them into buffers. 12.16 Other Topics Related to Functions Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here. apply See Section 12.5 [Calling Functions], page 179. autoload See Section 15.5 [Autoload], page 230. call-interactively See Section 20.3 [Interactive Call], page 328. called-interactively-p See Section 20.4 [Distinguish Interactive], page 330. Chapter 12: Functions 197 commandp See Section 20.3 [Interactive Call], page 328. documentation See Section 23.2 [Accessing Documentation], page 462. eval See Section 9.4 [Eval], page 120. funcall See Section 12.5 [Calling Functions], page 179. function See Section 12.7 [Anonymous Functions], page 182. ignore See Section 12.5 [Calling Functions], page 179. indirect-function See Section 9.1.4 [Function Indirection], page 115. interactive See Section 20.2.1 [Using Interactive], page 322. interactive-p See Section 20.4 [Distinguish Interactive], page 330. mapatoms See Section 8.3 [Creating Symbols], page 107. mapcar See Section 12.6 [Mapping Functions], page 181. map-char-table See Section 6.6 [Char-Tables], page 94. mapconcat See Section 12.6 [Mapping Functions], page 181. undefined See Section 21.11 [Functions for Key Lookup], page 380. Chapter 13: Macros 198 13 Macros Macros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro. Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them. If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See Section 12.12 [Inline Functions], page 193. 13.1 A Simple Example of a Macro Suppose we would like to define a Lisp construct to increment a variable value, much like the ++ operator in C. We would like to write (inc x) and have the effect of (setq x (1+ x)). Here’s a macro definition that does the job: (defmacro inc (var) (list ’setq var (list ’1+ var))) When this is called with (inc x), the argument var is the symbol x—not the value of x, as it would be in a function. The body of the macro uses this to construct the expansion, which is (setq x (1+ x)). Once the macro definition returns this expansion, Lisp proceeds to evaluate it, thus incrementing x. macrop object [Function] This predicate tests whether its argument is a macro, and returns t if so, nil other- wise. 13.2 Expansion of a Macro Call A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro. Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list. Having obtained the arguments, Lisp invokes the macro definition just as a function is invoked. The argument variables of the macro are bound to the argument values from the macro call, or to a list of them in the case of a &rest argument. And the macro body executes and returns its value just as a function body does. The second crucial difference between macros and functions is that the value returned by the macro body is an alternate Lisp expression, also known as the expansion of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro. Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual. Chapter 13: Macros 199 Note that Emacs tries to expand macros when loading an uncompiled Lisp file. This is not always possible, but if it is, it speeds up subsequent execution. See Section 15.1 [How Programs Do Loading], page 225. You can see the expansion of a given macro call by calling macroexpand. macroexpand form &optional environment [Function] This function expands form, if it is a macro call. If the result is another macro call, it is expanded in turn, until something which is not a macro call results. That is the value returned by macroexpand. If form is not a macro call to begin with, it is returned as given. Note that macroexpand does not look at the subexpressions of form (although some macro definitions may do so). Even if they are macro calls themselves, macroexpand does not expand them. The function macroexpand does not expand calls to inline functions. Normally there is no need for that, since a call to an inline function is no harder to understand than a call to an ordinary function. If environment is provided, it specifies an alist of macro definitions that shadow the currently defined macros. Byte compilation uses this feature. (defmacro inc (var) (list ’setq var (list ’1+ var))) (macroexpand ’(inc r)) ⇒ (setq r (1+ r)) (defmacro inc2 (var1 var2) (list ’progn (list ’inc var1) (list ’inc var2))) (macroexpand ’(inc2 r s)) ⇒ (progn (inc r) (inc s)) ; inc not expanded here. macroexpand-all form &optional environment [Function] macroexpand-all expands macros like macroexpand, but will look for and expand all macros in form, not just at the top-level. If no macros are expanded, the return value is eq to form. Repeating the example used for macroexpand above with macroexpand-all, we see that macroexpand-all does expand the embedded calls to inc: (macroexpand-all ’(inc2 r s)) ⇒ (progn (setq r (1+ r)) (setq s (1+ s))) 13.3 Macros and Byte Compilation You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation. When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead Chapter 13: Macros 200 of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself—they would be computed at compile time, which is not useful. In order for compilation of macro calls to work, the macros must already be defined in Lisp when the calls to them are compiled. The compiler has a special feature to help you do this: if a file being compiled contains a defmacro form, the macro is defined temporarily for the rest of the compilation of that file. Byte-compiling a file also executes any require calls at top-level in the file, so you can ensure that necessary macro definitions are available during compilation by requiring the files that define them (see Section 15.7 [Named Features], page 234). To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see Section 16.5 [Eval During Compile], page 243). 13.4 Defining Macros A Lisp macro object is a list whose car is macro, and whose cdr is a function. Expansion of the macro works by applying the function (with apply) to the list of unevaluated arguments from the macro call. It is possible to use an anonymous Lisp macro just like an anonymous function, but this is never done, because it does not make sense to pass an anonymous macro to functionals such as mapcar. In practice, all Lisp macros have names, and they are almost always defined with the defmacro macro. defmacro name args [doc] [declare] body. . . [Macro] defmacro defines the symbol name (which should not be quoted) as a macro that looks like this: (macro lambda args . body) (Note that the cdr of this list is a lambda expression.) This macro object is stored in the function cell of name. The meaning of args is the same as in a function, and the keywords &rest and &optional may be used (see Section 12.2.3 [Argument List], page 175). Neither name nor args should be quoted. The return value of defmacro is undefined. doc, if present, should be a string specifying the macro’s documentation string. declare, if present, should be a declare form specifying metadata for the macro (see Section 12.13 [Declare Form], page 193). Note that macros cannot have interactive declarations, since they cannot be called interactively. Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ‘‘’ syntax (see Section 9.3 [Backquote], page 119). For example: (defmacro t-becomes-nil (variable) ‘(if (eq ,variable t) (setq ,variable nil))) Chapter 13: Macros 201 (t-becomes-nil foo) ≡ (if (eq foo t) (setq foo nil)) The body of a macro definition can include a declare form, which specifies additional properties about the macro. See Section 12.13 [Declare Form], page 193. 13.5 Common Problems Using Macros Macro expansion can have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble. 13.5.1 Wrong Time The most common problem in writing macros is doing some of the real work prematurely— while expanding the macro, rather than in the expansion itself. For instance, one real package had this macro definition: (defmacro my-set-buffer-multibyte (arg) (if (fboundp ’set-buffer-multibyte) (set-buffer-multibyte arg))) With this erroneous macro definition, the program worked fine when interpreted but failed when compiled. This macro definition called set-buffer-multibyte during compilation, which was wrong, and then did nothing when the compiled package was run. The definition that the programmer really wanted was this: (defmacro my-set-buffer-multibyte (arg) (if (fboundp ’set-buffer-multibyte) ‘(set-buffer-multibyte ,arg))) This macro expands, if appropriate, into a call to set-buffer-multibyte that will be executed when the compiled program is actually run. 13.5.2 Evaluating Macro Arguments Repeatedly When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a “for” loop construct. (defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." (list ’let (list (list var init)) (cons ’while (cons (list ’<= var final) (append body (list (list ’inc var))))))) (for i from 1 to 3 do (setq square (* i i)) (princ (format "\n%d %d" i square))) → Chapter 13: Macros 202 (let ((i 1)) (while (<= i 3) (setq square (* i i)) (princ (format "\n%d %d" i square)) (inc i))) 1 1 2 4 3 9 ⇒ nil The arguments from, to, and do in this macro are “syntactic sugar”; they are entirely ignored. The idea is that you will write noise words (such as from, to, and do) in those positions in the macro call. Here’s an equivalent definition simplified through use of backquote: (defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." ‘(let ((,var ,init)) (while (<= ,var ,final) ,@body (inc ,var)))) Both forms of this definition (with backquote and without) suffer from the defect that final is evaluated on every iteration. If final is a constant, this is not a problem. If it is a more complex form, say (long-complex-calculation x), this can slow down the execution significantly. If final has side effects, executing it more than once is probably incorrect. A well-designed macro definition takes steps to avoid this problem by producing an expansion that evaluates the argument expressions exactly once unless repeated evaluation is part of the intended purpose of the macro. Here is a correct expansion for the for macro: (let ((i 1) (max 3)) (while (<= i max) (setq square (* i i)) (princ (format "%d (inc i))) %d" i square)) Here is a macro definition that creates this expansion: (defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." ‘(let ((,var ,init) (max ,final)) (while (<= ,var max) ,@body (inc ,var)))) Unfortunately, this fix introduces another problem, described in the following section. Chapter 13: Macros 203 13.5.3 Local Variables in Macro Expansions The new definition of for has a new problem: it introduces a local variable named max which the user does not expect. This causes trouble in examples such as the following: (let ((max 0)) (for x from 0 to 10 do (let ((this (frob x))) (if (< max this) (setq max this))))) The references to max inside the body of the for, which are supposed to refer to the user’s binding of max, really access the binding made by for. The way to correct this is to use an uninterned symbol instead of max (see Section 8.3 [Creating Symbols], page 107). The uninterned symbol can be bound and referred to just like any other symbol, but since it is created by for, we know that it cannot already appear in the user’s program. Since it is not interned, there is no way the user can put it into the program later. It will never appear anywhere except where put by for. Here is a definition of for that works this way: (defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." (let ((tempvar (make-symbol "max"))) ‘(let ((,var ,init) (,tempvar ,final)) (while (<= ,var ,tempvar) ,@body (inc ,var))))) This creates an uninterned symbol named max and puts it in the expansion instead of the usual interned symbol max that appears in expressions ordinarily. 13.5.4 Evaluating Macro Arguments in Expansion Another problem can happen if the macro definition itself evaluates any of the macro argument expressions, such as by calling eval (see Section 9.4 [Eval], page 120). If the argument is supposed to refer to the user’s variables, you may have trouble if the user happens to use a variable with the same name as one of the macro arguments. Inside the macro body, the macro argument binding is the most local binding of this variable, so any references inside the form being evaluated do refer to it. Here is an example: (defmacro foo (a) (list ’setq (eval a) t)) (setq x ’b) (foo x) → (setq b t) ⇒t ; and b has been set. ;; but (setq a ’c) (foo a) → (setq a t) ⇒t ; but this set a, not c. Chapter 13: Macros 204 It makes a difference whether the user’s variable is named a or x, because a conflicts with the macro argument variable a. Another problem with calling eval in a macro definition is that it probably won’t do what you intend in a compiled program. The byte compiler runs macro definitions while compiling the program, when the program’s own computations (which you might have wished to access with eval) don’t occur and its local variable bindings don’t exist. To avoid these problems, don’t evaluate an argument expression while computing the macro expansion. Instead, substitute the expression into the macro expansion, so that its value will be computed as part of executing the expansion. This is how the other examples in this chapter work. 13.5.5 How Many Times is the Macro Expanded? Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded. Therefore, you should avoid side effects in computation of the macro expansion, unless you really know what you are doing. One special kind of side effect can’t be avoided: constructing Lisp objects. Almost all macro expansions include constructed lists; that is the whole point of most macros. This is usually safe; there is just one case where you must be careful: when the object you construct is part of a quoted constant in the macro expansion. If the macro is expanded just once, in compilation, then the object is constructed just once, during compilation. But in interpreted execution, the macro is expanded each time the macro call runs, and this means a new object is constructed each time. In most clean Lisp code, this difference won’t matter. It can matter only if you perform side-effects on the objects constructed by the macro definition. Thus, to avoid trouble, avoid side effects on objects constructed by macro definitions. Here is an example of how such side effects can get you into trouble: (defmacro empty-object () (list ’quote (cons nil nil))) (defun initialize (condition) (let ((object (empty-object))) (if condition (setcar object condition)) object)) If initialize is interpreted, a new list (nil) is constructed each time initialize is called. Thus, no side effect survives between calls. If initialize is compiled, then the macro empty-object is expanded during compilation, producing a single “constant” (nil) that is reused and altered each time initialize is called. One way to avoid pathological cases like this is to think of empty-object as a funny kind of constant, not as a memory allocation construct. You wouldn’t use setcar on a constant such as ’(nil), so naturally you won’t use it on (empty-object) either. Chapter 13: Macros 205 13.6 Indenting Macros Within a macro definition, you can use the declare form (see Section 13.4 [Defining Macros], page 200) to specify how TAB should indent calls to the macro. An indentation specification is written like this: (declare (indent indent-spec)) Here are the possibilities for indent-spec: nil This is the same as no property—use the standard indentation pattern. defun Handle this function like a ‘def’ construct: treat the second line as the start of a body. an integer, number The first number arguments of the function are distinguished arguments; the rest are considered the body of the expression. A line in the expression is indented according to whether the first argument on it is distinguished or not. If the argument is part of the body, the line is indented lisp-body-indent more columns than the open-parenthesis starting the containing expression. If the argument is distinguished and is either the first or second argument, it is indented twice that many extra columns. If the argument is distinguished and not the first or second argument, the line uses the standard pattern. a symbol, symbol symbol should be a function name; that function is called to calculate the indentation of a line within this expression. The function receives two arguments: pos The position at which the line being indented begins. state The value returned by parse-partial-sexp (a Lisp primitive for indentation and nesting computation) when it parses up to the beginning of this line. It should return either a number, which is the number of columns of indentation for that line, or a list whose car is such a number. The difference between returning a number and returning a list is that a number says that all following lines at the same nesting level should be indented just like this one; a list says that following lines might call for different indentations. This makes a difference when the indentation is being computed by C-M-q; if the value is a number, C-M-q need not recalculate indentation for the following lines until the end of the list. Chapter 14: Customization Settings 206 14 Customization Settings Users of Emacs can customize variables and faces without writing Lisp code, by using the Customize interface. See Section “Easy Customization” in The GNU Emacs Manual. This chapter describes how to define customization items that users can interact with through the Customize interface. Customization items include customizable variables, which are defined with the defcustom macro; customizable faces, which are defined with defface (described separately in Section 37.12.2 [Defining Faces], page 865); and customization groups, defined with defgroup, which act as containers for groups of related customization items. 14.1 Common Item Keywords The customization declarations that we will describe in the next few sections—defcustom, defgroup, etc.—all accept keyword arguments (see Section 11.2 [Constant Variables], page 143) for specifying various information. This section describes keywords that apply to all types of customization declarations. All of these keywords, except :tag, can be used more than once in a given item. Each use of the keyword has an independent effect. The keyword :tag is an exception because any given item can only display one name. :tag label Use label, a string, instead of the item’s name, to label the item in customization menus and buffers. Don’t use a tag which is substantially different from the item’s real name; that would cause confusion. :group group Put this customization item in group group. When you use :group in a defgroup, it makes the new group a subgroup of group. If you use this keyword more than once, you can put a single item into more than one group. Displaying any of those groups will show this item. Please don’t overdo this, since the result would be annoying. :link link-data Include an external link after the documentation string for this item. This is a sentence containing a button that references some other documentation. There are several alternatives you can use for link-data: (custom-manual info-node) Link to an Info node; info-node is a string which specifies the node name, as in "(emacs)Top". The link appears as ‘[Manual]’ in the customization buffer and enters the built-in Info reader on infonode. (info-link info-node) Like custom-manual except that the link appears in the customization buffer with the Info node name. Chapter 14: Customization Settings 207 (url-link url) Link to a web page; url is a string which specifies the URL. The link appears in the customization buffer as url and invokes the WWW browser specified by browse-url-browser-function. (emacs-commentary-link library) Link to the commentary section of a library; library is a string which specifies the library name. See Section D.8 [Library Headers], page 997. (emacs-library-link library) Link to an Emacs Lisp library file; library is a string which specifies the library name. (file-link file) Link to a file; file is a string which specifies the name of the file to visit with find-file when the user invokes this link. (function-link function) Link to the documentation of a function; function is a string which specifies the name of the function to describe with describe-function when the user invokes this link. (variable-link variable) Link to the documentation of a variable; variable is a string which specifies the name of the variable to describe with describe-variable when the user invokes this link. (custom-group-link group) Link to another customization group. Invoking it creates a new customization buffer for group. You can specify the text to use in the customization buffer by adding :tag name after the first element of the link-data; for example, (info-link :tag "foo" "(emacs)Top") makes a link to the Emacs manual which appears in the buffer as ‘foo’. You can use this keyword more than once, to add multiple links. :load file Load file file (a string) before displaying this customization item (see Chapter 15 [Loading], page 225). Loading is done with load, and only if the file is not already loaded. :require feature Execute (require ’feature) when your saved customizations set the value of this item. feature should be a symbol. The most common reason to use :require is when a variable enables a feature such as a minor mode, and just setting the variable won’t have any effect unless the code which implements the mode is loaded. Chapter 14: Customization Settings 208 :version version This keyword specifies that the item was first introduced in Emacs version version, or that its default value was changed in that version. The value version must be a string. :package-version ’(package . version) This keyword specifies that the item was first introduced in package version version, or that its meaning or default value was changed in that version. This keyword takes priority over :version. package should be the official name of the package, as a symbol (e.g., MH-E). version should be a string. If the package package is released as part of Emacs, package and version should appear in the value of customize-package-emacsversion-alist. Packages distributed as part of Emacs that use the :package-version keyword must also update the customize-package-emacs-version-alist variable. customize-package-emacs-version-alist [Variable] This alist provides a mapping for the versions of Emacs that are associated with versions of a package listed in the :package-version keyword. Its elements are: (package (pversion . eversion)...) For each package, which is a symbol, there are one or more elements that contain a package version pversion with an associated Emacs version eversion. These versions are strings. For example, the MH-E package updates this alist with the following: (add-to-list ’customize-package-emacs-version-alist ’(MH-E ("6.0" . "22.1") ("6.1" . "22.1") ("7.0" . "22.1") ("7.1" . "22.1") ("7.2" . "22.1") ("7.3" . "22.1") ("7.4" . "22.1") ("8.0" . "22.1"))) The value of package needs to be unique and it needs to match the package value appearing in the :package-version keyword. Since the user might see the value in an error message, a good choice is the official name of the package, such as MH-E or Gnus. 14.2 Defining Customization Groups Each Emacs Lisp package should have one main customization group which contains all the options, faces and other groups in the package. If the package has a small number of options and faces, use just one group and put everything in it. When there are more than twenty or so options and faces, then you should structure them into subgroups, and put the subgroups under the package’s main customization group. It is OK to put some of the options and faces in the package’s main group alongside the subgroups. The package’s main or only group should be a member of one or more of the standard customization groups. (To display the full list of them, use M-x customize.) Choose one or more of them (but not too many), and add your group to each of them using the :group keyword. The way to declare new customization groups is with defgroup. Chapter 14: Customization Settings 209 defgroup group members doc [keyword value]. . . [Macro] Declare group as a customization group containing members. Do not quote the symbol group. The argument doc specifies the documentation string for the group. The argument members is a list specifying an initial set of customization items to be members of the group. However, most often members is nil, and you specify the group’s members by using the :group keyword when defining those members. If you want to specify group members through members, each element should have the form (name widget). Here name is a symbol, and widget is a widget type for editing that symbol. Useful widgets are custom-variable for a variable, custom-face for a face, and custom-group for a group. When you introduce a new group into Emacs, use the :version keyword in the defgroup; then you need not use it for the individual members of the group. In addition to the common keywords (see Section 14.1 [Common Keywords], page 206), you can also use this keyword in defgroup: :prefix prefix If the name of an item in the group starts with prefix, and the customizable variable custom-unlispify-remove-prefixes is non-nil, the item’s tag will omit prefix. A group can have any number of prefixes. custom-unlispify-remove-prefixes [User Option] If this variable is non-nil, the prefixes specified by a group’s :prefix keyword are omitted from tag names, whenever the user customizes the group. The default value is nil, i.e., the prefix-discarding feature is disabled. This is because discarding prefixes often leads to confusing names for options and faces. 14.3 Defining Customization Variables Customizable variables, also called user options, are global Lisp variables whose values can be set through the Customize interface. Unlike other global variables, which are defined with defvar (see Section 11.5 [Defining Variables], page 147), customizable variables are defined using the defcustom macro. In addition to calling defvar as a subroutine, defcustom states how the variable should be displayed in the Customize interface, the values it is allowed to take, etc. defcustom option standard doc [keyword value]. . . [Macro] This macro declares option as a user option (i.e., a customizable variable). You should not quote option. The argument standard is an expression that specifies the standard value for option. Evaluating the defcustom form evaluates standard, but does not necessarily bind the option to that value. If option already has a default value, it is left unchanged. If the user has already saved a customization for option, the user’s customized value is installed as the default value. Otherwise, the result of evaluating standard is installed as the default value. Like defvar, this macro marks option as a special variable, meaning that it should always be dynamically bound. If option is already lexically bound, that lexical binding Chapter 14: Customization Settings 210 remains in effect until the binding construct exits. See Section 11.9 [Variable Scoping], page 152. The expression standard can be evaluated at various other times, too—whenever the customization facility needs to know option’s standard value. So be sure to use an expression which is harmless to evaluate at any time. The argument doc specifies the documentation string for the variable. If a defcustom does not specify any :group, the last group defined with defgroup in the same file will be used. This way, most defcustom do not need an explicit :group. When you evaluate a defcustom form with C-M-x in Emacs Lisp mode (eval-defun), a special feature of eval-defun arranges to set the variable unconditionally, without testing whether its value is void. (The same feature applies to defvar, see Section 11.5 [Defining Variables], page 147.) Using eval-defun on a defcustom that is already defined calls the :set function (see below), if there is one. If you put a defcustom in a pre-loaded Emacs Lisp file (see Section E.1 [Building Emacs], page 1000), the standard value installed at dump time might be incorrect, e.g., because another variable that it depends on has not been assigned the right value yet. In that case, use custom-reevaluate-setting, described below, to re-evaluate the standard value after Emacs starts up. In addition to the keywords listed in Section 14.1 [Common Keywords], page 206, this macro accepts the following keywords: :type type Use type as the data type for this option. It specifies which values are legitimate, and how to display the value (see Section 14.4 [Customization Types], page 213). :options value-list Specify the list of reasonable values for use in this option. The user is not restricted to using only these values, but they are offered as convenient alternatives. This is meaningful only for certain types, currently including hook, plist and alist. See the definition of the individual types for a description of how to use :options. :set setfunction Specify setfunction as the way to change the value of this option when using the Customize interface. The function setfunction should take two arguments, a symbol (the option name) and the new value, and should do whatever is necessary to update the value properly for this option (which may not mean simply setting the option as a Lisp variable); preferably, though, it should not modify its value argument destructively. The default for setfunction is set-default. If you specify this keyword, the variable’s documentation string should describe how to do the same job in hand-written Lisp code. :get getfunction Specify getfunction as the way to extract the value of this option. The function getfunction should take one argument, a symbol, and should return whatever Chapter 14: Customization Settings 211 customize should use as the “current value” for that symbol (which need not be the symbol’s Lisp value). The default is default-value. You have to really understand the workings of Custom to use :get correctly. It is meant for values that are treated in Custom as variables but are not actually stored in Lisp variables. It is almost surely a mistake to specify getfunction for a value that really is stored in a Lisp variable. :initialize function function should be a function used to initialize the variable when the defcustom is evaluated. It should take two arguments, the option name (a symbol) and the value. Here are some predefined functions meant for use in this way: custom-initialize-set Use the variable’s :set function to initialize the variable, but do not reinitialize it if it is already non-void. custom-initialize-default Like custom-initialize-set, but use the function set-default to set the variable, instead of the variable’s :set function. This is the usual choice for a variable whose :set function enables or disables a minor mode; with this choice, defining the variable will not call the minor mode function, but customizing the variable will do so. custom-initialize-reset Always use the :set function to initialize the variable. If the variable is already non-void, reset it by calling the :set function using the current value (returned by the :get method). This is the default :initialize function. custom-initialize-changed Use the :set function to initialize the variable, if it is already set or has been customized; otherwise, just use set-default. custom-initialize-safe-set custom-initialize-safe-default These functions behave like custom-initialize-set (custom-initialize-default, respectively), but catch errors. If an error occurs during initialization, they set the variable to nil using set-default, and signal no error. These functions are meant for options defined in pre-loaded files, where the standard expression may signal an error because some required variable or function is not yet defined. The value normally gets updated in startup.el, ignoring the value computed by defcustom. After startup, if one unsets the value and reevaluates the defcustom, the standard expression can be evaluated without error. :risky value Set the variable’s risky-local-variable property to value (see Section 11.11 [File Local Variables], page 163). Chapter 14: Customization Settings 212 :safe function Set the variable’s safe-local-variable property to function (see Section 11.11 [File Local Variables], page 163). :set-after variables When setting variables according to saved customizations, make sure to set the variables variables before this one; i.e., delay setting this variable until after those others have been handled. Use :set-after if setting this variable won’t work properly unless those other variables already have their intended values. It is useful to specify the :require keyword for an option that “turns on” a certain feature. This causes Emacs to load the feature, if it is not already loaded, whenever the option is set. See Section 14.1 [Common Keywords], page 206. Here is an example, from the library saveplace.el: (defcustom save-place nil "Non-nil means automatically save place in each file..." :type ’boolean :require ’saveplace :group ’save-place) If a customization item has a type such as hook or alist, which supports :options, you can add additional values to the list from outside the defcustom declaration by calling custom-add-frequent-value. For example, if you define a function my-lisp-modeinitialization intended to be called from emacs-lisp-mode-hook, you might want to add that to the list of reasonable values for emacs-lisp-mode-hook, but not by editing its definition. You can do it thus: (custom-add-frequent-value ’emacs-lisp-mode-hook ’my-lisp-mode-initialization) custom-add-frequent-value symbol value [Function] For the customization option symbol, add value to the list of reasonable values. The precise effect of adding a value depends on the customization type of symbol. Internally, defcustom uses the symbol property standard-value to record the expression for the standard value, saved-value to record the value saved by the user with the customization buffer, and customized-value to record the value set by the user with the customization buffer, but not saved. See Section 8.4 [Symbol Properties], page 109. These properties are lists, the car of which is an expression that evaluates to the value. custom-reevaluate-setting symbol [Function] This function re-evaluates the standard value of symbol, which should be a user option declared via defcustom. If the variable was customized, this function re-evaluates the saved value instead. Then it sets the user option to that value (using the option’s :set property if that is defined). This is useful for customizable options that are defined before their value could be computed correctly. For example, during startup Emacs calls this function for some user options that were defined in pre-loaded Emacs Lisp files, but whose initial values depend on information available only at run-time. Chapter 14: Customization Settings 213 custom-variable-p arg [Function] This function returns non-nil if arg is a customizable variable. A customizable vari- able is either a variable that has a standard-value or custom-autoload property (usually meaning it was declared with defcustom), or an alias for another customiz- able variable. 14.4 Customization Types When you define a user option with defcustom, you must specify its customization type. That is a Lisp object which describes (1) which values are legitimate and (2) how to display the value in the customization buffer for editing. You specify the customization type in defcustom with the :type keyword. The argument of :type is evaluated, but only once when the defcustom is executed, so it isn’t useful for the value to vary. Normally we use a quoted constant. For example: (defcustom diff-command "diff" "The command to use to run diff." :type ’(string) :group ’diff) In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (see Section 14.4.4 [Type Keywords], page 219). Some type symbols do not use any arguments; those are called simple types. For a simple type, if you do not use any keyword-value pairs, you can omit the parentheses around the type symbol. For example just string as a customization type is equivalent to (string). All customization types are implemented as widgets; see Section “Introduction” in The Emacs Widget Library, for details. 14.4.1 Simple Types This section describes all the simple customization types. For several of these customization types, the customization widget provides inline completion with C-M-i or M-TAB. sexp The value may be any Lisp object that can be printed and read back. You can use sexp as a fall-back for any option, if you don’t want to take the time to work out a more specific type to use. integer The value must be an integer. number The value must be a number (floating point or integer). float The value must be floating point. string The value must be a string. The customization buffer shows the string without delimiting ‘"’ characters or ‘\’ quotes. regexp Like string except that the string must be a valid regular expression. character The value must be a character code. A character code is actually an integer, but this type shows the value by inserting the character in the buffer, rather than by showing the number. Chapter 14: Customization Settings 214 file The value must be a file name. The widget provides completion. (file :must-match t) The value must be a file name for an existing file. The widget provides completion. directory The value must be a directory name. The widget provides completion. hook The value must be a list of functions. This customization type is used for hook variables. You can use the :options keyword in a hook variable’s defcustom to specify a list of functions recommended for use in the hook; See Section 14.3 [Variable Definitions], page 209. symbol The value must be a symbol. It appears in the customization buffer as the symbol name. The widget provides completion. function The value must be either a lambda expression or a function name. The widget provides completion for function names. variable The value must be a variable name. The widget provides completion. face The value must be a symbol which is a face name. The widget provides completion. boolean The value is boolean—either nil or t. Note that by using choice and const together (see the next section), you can specify that the value must be nil or t, but also specify the text to describe each value in a way that fits the specific meaning of the alternative. key-sequence The value is a key sequence. The customization buffer shows the key sequence using the same syntax as the kbd function. See Section 21.1 [Key Sequences], page 367. coding-system The value must be a coding-system name, and you can do completion with M-TAB. color The value must be a valid color name. The widget provides completion for color names, as well as a sample and a button for selecting a color name from a list of color names shown in a *Colors* buffer. 14.4.2 Composite Types When none of the simple types is appropriate, you can use composite types, which build new types from other types or from specified data. The specified types or data are called the arguments of the composite type. The composite type normally looks like this: (constructor arguments...) but you can also add keyword-value pairs before the arguments, like this: (constructor {keyword value}... arguments...) Here is a table of constructors and how to use them to write composite types: Chapter 14: Customization Settings 215 (cons car-type cdr-type) The value must be a cons cell, its car must fit car-type, and its cdr must fit cdr-type. For example, (cons string symbol) is a customization type which matches values such as ("foo" . foo). In the customization buffer, the car and cdr are displayed and edited separately, each according to their specified type. (list element-types...) The value must be a list with exactly as many elements as the element-types given; and each element must fit the corresponding element-type. For example, (list integer string function) describes a list of three elements; the first element must be an integer, the second a string, and the third a function. In the customization buffer, each element is displayed and edited separately, according to the type specified for it. (group element-types...) This works like list except for the formatting of text in the Custom buffer. list labels each element value with its tag; group does not. (vector element-types...) Like list except that the value must be a vector instead of a list. The elements work the same as in list. (alist :key-type key-type :value-type value-type) The value must be a list of cons-cells, the car of each cell representing a key of customization type key-type, and the cdr of the same cell representing a value of customization type value-type. The user can add and delete key/value pairs, and edit both the key and the value of each pair. If omitted, key-type and value-type default to sexp. The user can add any key matching the specified key type, but you can give some keys a preferential treatment by specifying them with the :options (see Section 14.3 [Variable Definitions], page 209). The specified keys will always be shown in the customize buffer (together with a suitable value), with a checkbox to include or exclude or disable the key/value pair from the alist. The user will not be able to edit the keys specified by the :options keyword argument. The argument to the :options keywords should be a list of specifications for reasonable keys in the alist. Ordinarily, they are simply atoms, which stand for themselves. For example: :options ’("foo" "bar" "baz") specifies that there are three “known” keys, namely "foo", "bar" and "baz", which will always be shown first. You may want to restrict the value type for specific keys, for example, the value associated with the "bar" key can only be an integer. You can specify this by using a list instead of an atom in the list. The first element will specify the key, like before, while the second element will specify the value type. For example: :options ’("foo" ("bar" integer) "baz") Chapter 14: Customization Settings 216 Finally, you may want to change how the key is presented. By default, the key is simply shown as a const, since the user cannot change the special keys specified with the :options keyword. However, you may want to use a more specialized type for presenting the key, like function-item if you know it is a symbol with a function binding. This is done by using a customization type specification instead of a symbol for the key. :options ’("foo" ((function-item some-function) integer) "baz") Many alists use lists with two elements, instead of cons cells. For example, (defcustom list-alist ’(("foo" 1) ("bar" 2) ("baz" 3)) "Each element is a list of the form (KEY VALUE).") instead of (defcustom cons-alist ’(("foo" . 1) ("bar" . 2) ("baz" . 3)) "Each element is a cons-cell (KEY . VALUE).") Because of the way lists are implemented on top of cons cells, you can treat list-alist in the example above as a cons cell alist, where the value type is a list with a single element containing the real value. (defcustom list-alist ’(("foo" 1) ("bar" 2) ("baz" 3)) "Each element is a list of the form (KEY VALUE)." :type ’(alist :value-type (group integer))) The group widget is used here instead of list only because the formatting is better suited for the purpose. Similarly, you can have alists with more values associated with each key, using variations of this trick: (defcustom person-data ’(("brian" 50 t) ("dorith" 55 nil) ("ken" 52 t)) "Alist of basic info about people. Each element has the form (NAME AGE MALE-FLAG)." :type ’(alist :value-type (group integer boolean))) (plist :key-type key-type :value-type value-type) This customization type is similar to alist (see above), except that (i) the information is stored as a property list, (see Section 5.9 [Property Lists], page 86), and (ii) key-type, if omitted, defaults to symbol rather than sexp. (choice alternative-types...) The value must fit one of alternative-types. For example, (choice integer string) allows either an integer or a string. In the customization buffer, the user selects an alternative using a menu, and can then edit the value in the usual way for that alternative. Normally the strings in this menu are determined automatically from the choices; however, you can specify different strings for the menu by including Chapter 14: Customization Settings 217 the :tag keyword in the alternatives. For example, if an integer stands for a number of spaces, while a string is text to use verbatim, you might write the customization type this way, (choice (integer :tag "Number of spaces") (string :tag "Literal text")) so that the menu offers ‘Number of spaces’ and ‘Literal text’. In any alternative for which nil is not a valid value, other than a const, you should specify a valid default for that alternative using the :value keyword. See Section 14.4.4 [Type Keywords], page 219. If some values are covered by more than one of the alternatives, customize will choose the first alternative that the value fits. This means you should always list the most specific types first, and the most general last. Here’s an example of proper usage: (choice (const :tag "Off" nil) symbol (sexp :tag "Other")) This way, the special value nil is not treated like other symbols, and symbols are not treated like other Lisp expressions. (radio element-types...) This is similar to choice, except that the choices are displayed using ‘radio buttons’ rather than a menu. This has the advantage of displaying documentation for the choices when applicable and so is often a good choice for a choice between constant functions (function-item customization types). (const value) The value must be value—nothing else is allowed. The main use of const is inside of choice. For example, (choice integer (const nil)) allows either an integer or nil. :tag is often used with const, inside of choice. For example, (choice (const :tag "Yes" t) (const :tag "No" nil) (const :tag "Ask" foo)) describes a variable for which t means yes, nil means no, and foo means “ask”. (other value) This alternative can match any Lisp value, but if the user chooses this alternative, that selects the value value. The main use of other is as the last element of choice. For example, (choice (const :tag "Yes" t) (const :tag "No" nil) (other :tag "Ask" foo)) describes a variable for which t means yes, nil means no, and anything else means “ask”. If the user chooses ‘Ask’ from the menu of alternatives, that specifies the value foo; but any other value (not t, nil or foo) displays as ‘Ask’, just like foo. Chapter 14: Customization Settings 218 (function-item function) Like const, but used for values which are functions. This displays the documentation string as well as the function name. The documentation string is either the one you specify with :doc, or function’s own documentation string. (variable-item variable) Like const, but used for values which are variable names. This displays the documentation string as well as the variable name. The documentation string is either the one you specify with :doc, or variable’s own documentation string. (set types...) The value must be a list, and each element of the list must match one of the types specified. This appears in the customization buffer as a checklist, so that each of types may have either one corresponding element or none. It is not possible to specify two different elements that match the same one of types. For example, (set integer symbol) allows one integer and/or one symbol in the list; it does not allow multiple integers or multiple symbols. As a result, it is rare to use nonspecific types such as integer in a set. Most often, the types in a set are const types, as shown here: (set (const :bold) (const :italic)) Sometimes they describe possible elements in an alist: (set (cons :tag "Height" (const height) integer) (cons :tag "Width" (const width) integer)) That lets the user specify a height value optionally and a width value optionally. (repeat element-type) The value must be a list and each element of the list must fit the type elementtype. This appears in the customization buffer as a list of elements, with ‘[INS]’ and ‘[DEL]’ buttons for adding more elements or removing elements. (restricted-sexp :match-alternatives criteria) This is the most general composite type construct. The value may be any Lisp object that satisfies one of criteria. criteria should be a list, and each element should be one of these possibilities: • A predicate—that is, a function of one argument that has no side effects, and returns either nil or non-nil according to the argument. Using a predicate in the list says that objects for which the predicate returns nonnil are acceptable. • A quoted constant—that is, ’object. This sort of element in the list says that object itself is an acceptable value. For example, (restricted-sexp :match-alternatives (integerp ’t ’nil)) allows integers, t and nil as legitimate values. The customization buffer shows all legitimate values using their read syntax, and the user edits them textually. Chapter 14: Customization Settings 219 Here is a table of the keywords you can use in keyword-value pairs in a composite type: :tag tag Use tag as the name of this alternative, for user communication purposes. This is useful for a type that appears inside of a choice. :match-alternatives criteria Use criteria to match possible values. This is used only in restricted-sexp. :args argument-list Use the elements of argument-list as the arguments of the type construct. For instance, (const :args (foo)) is equivalent to (const foo). You rarely need to write :args explicitly, because normally the arguments are recognized automatically as whatever follows the last keyword-value pair. 14.4.3 Splicing into Lists The :inline feature lets you splice a variable number of elements into the middle of a list or vector customization type. You use it by adding :inline t to a type specification which is contained in a list or vector specification. Normally, each entry in a list or vector type specification describes a single element type. But when an entry contains :inline t, the value it matches is merged directly into the containing sequence. For example, if the entry matches a list with three elements, those become three elements of the overall sequence. This is analogous to ‘,@’ in a backquote construct (see Section 9.3 [Backquote], page 119). For example, to specify a list whose first element must be baz and whose remaining arguments should be zero or more of foo and bar, use this customization type: (list (const baz) (set :inline t (const foo) (const bar))) This matches values such as (baz), (baz foo), (baz bar) and (baz foo bar). When the element-type is a choice, you use :inline not in the choice itself, but in (some of) the alternatives of the choice. For example, to match a list which must start with a file name, followed either by the symbol t or two strings, use this customization type: (list file (choice (const t) (list :inline t string string))) If the user chooses the first alternative in the choice, then the overall list has two elements and the second element is t. If the user chooses the second alternative, then the overall list has three elements and the second and third must be strings. 14.4.4 Type Keywords You can specify keyword-argument pairs in a customization type after the type name symbol. Here are the keywords you can use, and their meanings: :value default Provide a default value. If nil is not a valid value for the alternative, then it is essential to specify a valid default with :value. Chapter 14: Customization Settings 220 If you use this for a type that appears as an alternative inside of choice; it specifies the default value to use, at first, if and when the user selects this alternative with the menu in the customization buffer. Of course, if the actual value of the option fits this alternative, it will appear showing the actual value, not default. :format format-string This string will be inserted in the buffer to represent the value corresponding to the type. The following ‘%’ escapes are available for use in format-string: ‘%[button%]’ Display the text button marked as a button. The :action attribute specifies what the button will do if the user invokes it; its value is a function which takes two arguments—the widget which the button appears in, and the event. There is no way to specify two different buttons with different actions. ‘%{sample%}’ Show sample in a special face specified by :sample-face. ‘%v’ Substitute the item’s value. How the value is represented depends on the kind of item, and (for variables) on the customization type. ‘%d’ Substitute the item’s documentation string. ‘%h’ Like ‘%d’, but if the documentation string is more than one line, add a button to control whether to show all of it or just the first line. ‘%t’ Substitute the tag here. You specify the tag with the :tag keyword. ‘%%’ Display a literal ‘%’. :action action Perform action if the user clicks on a button. :button-face face Use the face face (a face name or a list of face names) for button text displayed with ‘%[...%]’. :button-prefix prefix :button-suffix suffix These specify the text to display before and after a button. Each can be: nil No text is inserted. a string The string is inserted literally. a symbol The symbol’s value is used. :tag tag Use tag (a string) as the tag for the value (or part of the value) that corresponds to this type. Chapter 14: Customization Settings 221 :doc doc Use doc as the documentation string for this value (or part of the value) that corresponds to this type. In order for this to work, you must specify a value for :format, and use ‘%d’ or ‘%h’ in that value. The usual reason to specify a documentation string for a type is to provide more information about the meanings of alternatives inside a :choice type or the parts of some other composite type. :help-echo motion-doc When you move to this item with widget-forward or widget-backward, it will display the string motion-doc in the echo area. In addition, motion-doc is used as the mouse help-echo string and may actually be a function or form evaluated to yield a help string. If it is a function, it is called with one argument, the widget. :match function Specify how to decide whether a value matches the type. The corresponding value, function, should be a function that accepts two arguments, a widget and a value; it should return non-nil if the value is acceptable. :validate function Specify a validation function for input. function takes a widget as an argument, and should return nil if the widget’s current value is valid for the widget. Otherwise, it should return the widget containing the invalid data, and set that widget’s :error property to a string explaining the error. 14.4.5 Defining New Types In the previous sections we have described how to construct elaborate type specifications for defcustom. In some cases you may want to give such a type specification a name. The obvious case is when you are using the same type for many user options: rather than repeat the specification for each option, you can give the type specification a name, and use that name each defcustom. The other case is when a user option’s value is a recursive data structure. To make it possible for a datatype to refer to itself, it needs to have a name. Since custom types are implemented as widgets, the way to define a new customize type is to define a new widget. We are not going to describe the widget interface here in details, see Section “Introduction” in The Emacs Widget Library, for that. Instead we are going to demonstrate the minimal functionality needed for defining new customize types by a simple example. (define-widget ’binary-tree-of-string ’lazy "A binary tree made of cons-cells and strings." :offset 4 :tag "Node" :type ’(choice (string :tag "Leaf" :value "") (cons :tag "Interior" :value ("" . "") binary-tree-of-string binary-tree-of-string))) (defcustom foo-bar "" Chapter 14: Customization Settings 222 "Sample variable holding a binary tree of strings." :type ’binary-tree-of-string) The function to define a new widget is called define-widget. The first argument is the symbol we want to make a new widget type. The second argument is a symbol representing an existing widget, the new widget is going to be defined in terms of difference from the existing widget. For the purpose of defining new customization types, the lazy widget is perfect, because it accepts a :type keyword argument with the same syntax as the keyword argument to defcustom with the same name. The third argument is a documentation string for the new widget. You will be able to see that string with the M-x widget-browse RET binary-tree-of-string RET command. After these mandatory arguments follow the keyword arguments. The most important is :type, which describes the data type we want to match with this widget. Here a binary-tree-of-string is described as being either a string, or a cons-cell whose car and cdr are themselves both binary-tree-of-string. Note the reference to the widget type we are currently in the process of defining. The :tag attribute is a string to name the widget in the user interface, and the :offset argument is there to ensure that child nodes are indented four spaces relative to the parent node, making the tree structure apparent in the customization buffer. The defcustom shows how the new widget can be used as an ordinary customization type. The reason for the name lazy is that the other composite widgets convert their inferior widgets to internal form when the widget is instantiated in a buffer. This conversion is recursive, so the inferior widgets will convert their inferior widgets. If the data structure is itself recursive, this conversion is an infinite recursion. The lazy widget prevents the recursion: it convert its :type argument only when needed. 14.5 Applying Customizations The following functions are responsible for installing the user’s customization settings for variables and faces, respectively. When the user invokes ‘Save for future sessions’ in the Customize interface, that takes effect by writing a custom-set-variables and/or a custom-set-faces form into the custom file, to be evaluated the next time Emacs starts. custom-set-variables &rest args [Function] This function installs the variable customizations specified by args. Each argument in args should have the form (var expression [now [request [comment]]]) var is a variable name (a symbol), and expression is an expression which evaluates to the desired customized value. If the defcustom form for var has been evaluated prior to this custom-set-variables call, expression is immediately evaluated, and the variable’s value is set to the result. Otherwise, expression is stored into the variable’s saved-value property, to be evaluated when the relevant defcustom is called (usually when the library defining that variable is loaded into Emacs). The now, request, and comment entries are for internal use only, and may be omitted. now, if non-nil, means to set the variable’s value now, even if the variable’s defcustom Chapter 14: Customization Settings 223 form has not been evaluated. request is a list of features to be loaded immediately (see Section 15.7 [Named Features], page 234). comment is a string describing the customization. custom-set-faces &rest args [Function] This function installs the face customizations specified by args. Each argument in args should have the form (face spec [now [comment]]) face is a face name (a symbol), and spec is the customized face specification for that face (see Section 37.12.2 [Defining Faces], page 865). The now and comment entries are for internal use only, and may be omitted. now, if non-nil, means to install the face specification now, even if the defface form has not been evaluated. comment is a string describing the customization. 14.6 Custom Themes Custom themes are collections of settings that can be enabled or disabled as a unit. See Section “Custom Themes” in The GNU Emacs Manual. Each Custom theme is defined by an Emacs Lisp source file, which should follow the conventions described in this section. (Instead of writing a Custom theme by hand, you can also create one using a Customize-like interface; see Section “Creating Custom Themes” in The GNU Emacs Manual.) A Custom theme file should be named foo-theme.el, where foo is the theme name. The first Lisp form in the file should be a call to deftheme, and the last form should be a call to provide-theme. deftheme theme &optional doc [Macro] This macro declares theme (a symbol) as the name of a Custom theme. The optional argument doc should be a string describing the theme; this is the description shown when the user invokes the describe-theme command or types ? in the ‘*Custom Themes*’ buffer. Two special theme names are disallowed (using them causes an error): user is a “dummy” theme that stores the user’s direct customization settings, and changed is a “dummy” theme that stores changes made outside of the Customize system. provide-theme theme [Macro] This macro declares that the theme named theme has been fully specified. In between deftheme and provide-theme are Lisp forms specifying the theme settings: usually a call to custom-theme-set-variables and/or a call to custom-theme-set-faces. custom-theme-set-variables theme &rest args [Function] This function specifies the Custom theme theme’s variable settings. theme should be a symbol. Each argument in args should be a list of the form (var expression [now [request [comment]]]) where the list entries have the same meanings as in custom-set-variables. See Section 14.5 [Applying Customizations], page 222. Chapter 14: Customization Settings 224 custom-theme-set-faces theme &rest args [Function] This function specifies the Custom theme theme’s face settings. theme should be a symbol. Each argument in args should be a list of the form (face spec [now [comment]]) where the list entries have the same meanings as in custom-set-faces. See Section 14.5 [Applying Customizations], page 222. In theory, a theme file can also contain other Lisp forms, which would be evaluated when loading the theme, but that is “bad form”. To protect against loading themes containing malicious code, Emacs displays the source file and asks for confirmation from the user before loading any non-built-in theme for the first time. The following functions are useful for programmatically enabling and disabling themes: custom-theme-p theme [Function] This function return a non-nil value if theme (a symbol) is the name of a Custom theme (i.e., a Custom theme which has been loaded into Emacs, whether or not the theme is enabled). Otherwise, it returns nil. custom-known-themes [Variable] The value of this variable is a list of themes loaded into Emacs. Each theme is represented by a Lisp symbol (the theme name). The default value of this variable is a list containing two “dummy” themes: (user changed). The changed theme stores settings made before any Custom themes are applied (e.g., variables set outside of Customize). The user theme stores settings the user has customized and saved. Any additional themes declared with the deftheme macro are added to the front of this list. load-theme theme &optional no-confirm no-enable [Command] This function loads the Custom theme named theme from its source file, looking for the source file in the directories specified by the variable custom-theme-load- path. See Section “Custom Themes” in The GNU Emacs Manual. It also enables the theme (unless the optional argument no-enable is non-nil), causing its variable and face settings to take effect. It prompts the user for confirmation before loading the theme, unless the optional argument no-confirm is non-nil. enable-theme theme [Command] This function enables the Custom theme named theme. It signals an error if no such theme has been loaded. disable-theme theme [Command] This function disables the Custom theme named theme. The theme remains loaded, so that a subsequent call to enable-theme will re-enable it. Chapter 15: Loading 225 15 Loading Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file. Such a file is also called a Lisp library. The load functions evaluate all the expressions in a file just as the eval-buffer function evaluates all the expressions in a buffer. The difference is that the load functions read and evaluate the text in the file as found on disk, not the text in an Emacs buffer. The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a top-level form. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions. For on-demand loading of external libraries, see Section 38.20 [Dynamic Libraries], page 958. 15.1 How Programs Do Loading Emacs Lisp has several interfaces for loading. For example, autoload creates a placeholder object for a function defined in a file; trying to call the autoloading function loads the file to get the function’s real definition (see Section 15.5 [Autoload], page 230). require loads a file if it isn’t already loaded (see Section 15.7 [Named Features], page 234). Ultimately, all these facilities call the load function to do the work. load filename &optional missing-ok nomessage nosuffix must-suffix [Function] This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file. To find the file, load first looks for a file named filename.elc, that is, for a file whose name is filename with the extension ‘.elc’ appended. If such a file exists, it is loaded. If there is no file by that name, then load looks for a file named filename.el. If that file exists, it is loaded. Finally, if neither of those names is found, load looks for a file named filename with nothing appended, and loads it if it exists. (The load function is not clever about looking at filename. In the perverse case of a file named foo.el.el, evaluation of (load "foo.el") will indeed find it.) If Auto Compression mode is enabled, as it is by default, then if load can not find a file, it searches for a compressed version of the file before trying other file names. It decompresses and loads it if it exists. It looks for compressed versions by appending each of the suffixes in jka-compr-load-suffixes to the file name. The value of this variable must be a list of strings. Its standard value is (".gz"). If the optional argument nosuffix is non-nil, then load does not try the suffixes ‘.elc’ and ‘.el’. In this case, you must specify the precise file name you want, except that, if Auto Compression mode is enabled, load will still use jka-compr-load-suffixes to find compressed versions. By specifying the precise file name and using t for nosuffix, you can prevent file names like foo.el.el from being tried. If the optional argument must-suffix is non-nil, then load insists that the file name used must end in either ‘.el’ or ‘.elc’ (possibly extended with a compression suffix), unless it contains an explicit directory name. Chapter 15: Loading 226 If the option load-prefer-newer is non-nil, then when searching suffixes, load selects whichever version of a file (‘.elc’, ‘.el’, etc.) has been modified most recently. If filename is a relative file name, such as foo or baz/foo.bar, load searches for the file using the variable load-path. It appends filename to each of the directories listed in load-path, and loads the first file it finds whose name matches. The current default directory is tried only if it is specified in load-path, where nil stands for the default directory. load tries all three possible suffixes in the first directory in load-path, then all three suffixes in the second directory, and so on. See Section 15.3 [Library Search], page 228. Whatever the name under which the file is eventually found, and the directory where Emacs found it, Emacs sets the value of the variable load-file-name to that file’s name. If you get a warning that foo.elc is older than foo.el, it means you should consider recompiling foo.el. See Chapter 16 [Byte Compilation], page 239. When loading a source file (not compiled), load performs character set translation just as Emacs would do when visiting the file. See Section 32.10 [Coding Systems], page 728. When loading an uncompiled file, Emacs tries to expand any macros that the file contains (see Chapter 13 [Macros], page 198). We refer to this as eager macro expansion. Doing this (rather than deferring the expansion until the relevant code runs) can significantly speed up the execution of uncompiled code. Sometimes, this macro expansion cannot be done, owing to a cyclic dependency. In the simplest example of this, the file you are loading refers to a macro defined in another file, and that file in turn requires the file you are loading. This is generally harmless. Emacs prints a warning (‘Eager macro-expansion skipped due to cycle...’) giving details of the problem, but it still loads the file, just leaving the macro unexpanded for now. You may wish to restructure your code so that this does not happen. Loading a compiled file does not cause macroexpansion, because this should already have happened during compilation. See Section 13.3 [Compiling Macros], page 199. Messages like ‘Loading foo...’ and ‘Loading foo...done’ appear in the echo area during loading unless nomessage is non-nil. Any unhandled errors while loading a file terminate loading. If the load was done for the sake of autoload, any function definitions made during the loading are undone. If load can’t find the file to load, then normally it signals the error file-error (with ‘Cannot open load file filename’). But if missing-ok is non-nil, then load just returns nil. You can use the variable load-read-function to specify a function for load to use instead of read for reading expressions. See below. load returns t if the file loads successfully. load-file filename [Command] This command loads the file filename. If filename is a relative file name, then the current default directory is assumed. This command does not use load-path, and does not append suffixes. However, it does look for compressed versions (if Auto Compression Mode is enabled). Use this command if you wish to specify precisely the file name to load. Chapter 15: Loading 227 load-library library [Command] This command loads the library named library. It is equivalent to load, except for the way it reads its argument interactively. See Section “Lisp Libraries” in The GNU Emacs Manual. load-in-progress [Variable] This variable is non-nil if Emacs is in the process of loading a file, and it is nil otherwise. load-file-name [Variable] When Emacs is in the process of loading a file, this variable’s value is the name of that file, as Emacs found it during the search described earlier in this section. load-read-function [Variable] This variable specifies an alternate expression-reading function for load and eval-region to use instead of read. The function should accept one argument, just as read does. Normally, the variable’s value is nil, which means those functions should use read. Instead of using this variable, it is cleaner to use another, newer feature: to pass the function as the read-function argument to eval-region. See [Eval], page 121. For information about how load is used in building Emacs, see Section E.1 [Building Emacs], page 1000. 15.2 Load Suffixes We now describe some technical details about the exact suffixes that load tries. load-suffixes [Variable] This is a list of suffixes indicating (compiled or source) Emacs Lisp files. It should not include the empty string. load uses these suffixes in order when it appends Lisp suffixes to the specified file name. The standard value is (".elc" ".el") which produces the behavior described in the previous section. load-file-rep-suffixes [Variable] This is a list of suffixes that indicate representations of the same file. This list should normally start with the empty string. When load searches for a file it appends the suffixes in this list, in order, to the file name, before searching for another file. Enabling Auto Compression mode appends the suffixes in jka-compr-load-suffixes to this list and disabling Auto Compression mode removes them again. The standard value of load-file-rep-suffixes if Auto Compression mode is disabled is (""). Given that the standard value of jka-compr-load-suffixes is (".gz"), the standard value of load-file-rep-suffixes if Auto Compression mode is enabled is ("" ".gz"). get-load-suffixes [Function] This function returns the list of all suffixes that load should try, in order, when its must-suffix argument is non-nil. This takes both load-suffixes and load-file- rep-suffixes into account. If load-suffixes, jka-compr-load-suffixes and Chapter 15: Loading 228 load-file-rep-suffixes all have their standard values, this function returns (".elc" ".elc.gz" ".el" ".el.gz") if Auto Compression mode is enabled and (".elc" ".el") if Auto Compression mode is disabled. To summarize, load normally first tries the suffixes in the value of (get-load-suffixes) and then those in load-file-rep-suffixes. If nosuffix is non-nil, it skips the former group, and if must-suffix is non-nil, it skips the latter group. load-prefer-newer [User Option] If this option is non-nil, then rather than stopping at the first suffix that exists, load tests them all, and uses whichever file is the newest. 15.3 Library Search When Emacs loads a Lisp library, it searches for the library in a list of directories specified by the variable load-path. load-path [Variable] The value of this variable is a list of directories to search when loading files with load. Each element is a string (which must be a directory name) or nil (which stands for the current working directory). When Emacs starts up, it sets up the value of load-path in several steps. First, it initializes load-path using default locations set when Emacs was compiled. Normally, this is a directory something like "/usr/local/share/emacs/version/lisp" (In this and the following examples, replace /usr/local with the installation prefix appropriate for your Emacs.) These directories contain the standard Lisp files that come with Emacs. If Emacs cannot find them, it will not start correctly. If you run Emacs from the directory where it was built—that is, an executable that has not been formally installed—Emacs instead initializes load-path using the lisp directory in the directory containing the sources from which it was built. If you built Emacs in a separate directory from the sources, it also adds the lisp directories from the build directory. (In all cases, elements are represented as absolute file names.) Unless you start Emacs with the --no-site-lisp option, it then adds two more site-lisp directories to the front of load-path. These are intended for locally installed Lisp files, and are normally of the form: "/usr/local/share/emacs/version/site-lisp" and "/usr/local/share/emacs/site-lisp" The first one is for locally installed files for a specific Emacs version; the second is for locally installed files meant for use with all installed Emacs versions. (If Emacs is running uninstalled, it also adds site-lisp directories from the source and build directories, if they exist. Normally these directories do not contain site-lisp directories.) If the environment variable EMACSLOADPATH is set, it modifies the above initialization procedure. Emacs initializes load-path based on the value of the environment variable. Chapter 15: Loading 229 The syntax of EMACSLOADPATH is the same as used for PATH; directory names are separated by ‘:’ (or ‘;’, on some operating systems). Here is an example of how to set EMACSLOADPATH variable (from a sh-style shell): export EMACSLOADPATH=/home/foo/.emacs.d/lisp: An empty element in the value of the environment variable, whether trailing (as in the above example), leading, or embedded, is replaced by the default value of load-path as determined by the standard initialization procedure. If there are no such empty elements, then EMACSLOADPATH specifies the entire load-path. You must include either an empty element, or the explicit path to the directory containing the standard Lisp files, else Emacs will not function. (Another way to modify load-path is to use the -L command-line option when starting Emacs; see below.) For each directory in load-path, Emacs then checks to see if it contains a file subdirs.el, and if so, loads it. The subdirs.el file is created when Emacs is built/installed, and contains code that causes Emacs to add any subdirectories of those directories to load-path. Both immediate subdirectories and subdirectories multiple levels down are added. But it excludes subdirectories whose names do not start with a letter or digit, and subdirectories named RCS or CVS, and subdirectories containing a file named .nosearch. Next, Emacs adds any extra load directories that you specify using the -L commandline option (see Section “Action Arguments” in The GNU Emacs Manual). It also adds the directories where optional packages are installed, if any (see Section 39.1 [Packaging Basics], page 960). It is common to add code to one’s init file (see Section 38.1.2 [Init File], page 927) to add one or more directories to load-path. For example: (push "~/.emacs.d/lisp" load-path) Dumping Emacs uses a special value of load-path. If you use a site-load.el or site-init.el file to customize the dumped Emacs (see Section E.1 [Building Emacs], page 1000), any changes to load-path that these files make will be lost after dumping. locate-library library &optional nosuffix path interactive-call [Command] This command finds the precise file name for library library. It searches for the library in the same way load does, and the argument nosuffix has the same meaning as in load: don’t add suffixes ‘.elc’ or ‘.el’ to the specified name library. If the path is non-nil, that list of directories is used instead of load-path. When locate-library is called from a program, it returns the file name as a string. When the user runs locate-library interactively, the argument interactive-call is t, and this tells locate-library to display the file name in the echo area. list-load-path-shadows &optional stringp [Command] This command shows a list of shadowed Emacs Lisp files. A shadowed file is one that will not normally be loaded, despite being in a directory on load-path, due to the existence of another similarly-named file in a directory earlier on load-path. For instance, suppose load-path is set to ("/opt/emacs/site-lisp" "/usr/share/emacs/23.3/lisp") Chapter 15: Loading 230 and that both these directories contain a file named foo.el. Then (require ’foo) never loads the file in the second directory. Such a situation might indicate a problem in the way Emacs was installed. When called from Lisp, this function prints a message listing the shadowed files, instead of displaying them in a buffer. If the optional argument stringp is non-nil, it instead returns the shadowed files as a string. 15.4 Loading Non-ASCII Characters When Emacs Lisp programs contain string constants with non-ASCII characters, these can be represented within Emacs either as unibyte strings or as multibyte strings (see Section 32.1 [Text Representations], page 717). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. See Section 32.10 [Coding Systems], page 728. In most Emacs Lisp programs, the fact that non-ASCII strings are multibyte strings should not be noticeable, since inserting them in unibyte buffers converts them to unibyte automatically. However, if this does make a difference, you can force a particular Lisp file to be interpreted as unibyte by writing ‘coding: raw-text’ in a local variables section. With that designator, the file will unconditionally be interpreted as unibyte. This can matter when making keybindings to non-ASCII characters written as ?vliteral. 15.5 Autoload The autoload facility lets you register the existence of a function or macro, but put off loading the file that defines it. The first call to the function automatically loads the proper library, in order to install the real definition and other associated code, then runs the real definition as if it had been loaded all along. Autoloading can also be triggered by looking up the documentation of the function or macro (see Section 23.1 [Documentation Basics], page 461). There are two ways to set up an autoloaded function: by calling autoload, and by writing a special “magic” comment in the source before the real definition. autoload is the low-level primitive for autoloading; any Lisp program can call autoload at any time. Magic comments are the most convenient way to make a function autoload, for packages installed along with Emacs. These comments do nothing on their own, but they serve as a guide for the command update-file-autoloads, which constructs calls to autoload and arranges to execute them when Emacs is built. autoload function filename &optional docstring interactive type [Function] This function defines the function (or macro) named function so as to load automat- ically from filename. The string filename specifies the file to load to get the real definition of function. If filename does not contain either a directory name, or the suffix .el or .elc, this function insists on adding one of these suffixes, and it will not load from a file whose name is just filename with no added suffix. (The variable load-suffixes specifies the exact required suffixes.) Chapter 15: Loading 231 The argument docstring is the documentation string for the function. Specifying the documentation string in the call to autoload makes it possible to look at the documentation without loading the function’s real definition. Normally, this should be identical to the documentation string in the function definition itself. If it isn’t, the function definition’s documentation string takes effect when it is loaded. If interactive is non-nil, that says function can be called interactively. This lets completion in M-x work without loading function’s real definition. The complete interactive specification is not given here; it’s not needed unless the user actually calls function, and when that happens, it’s time to load the real definition. You can autoload macros and keymaps as well as ordinary functions. Specify type as macro if function is really a macro. Specify type as keymap if function is really a keymap. Various parts of Emacs need to know this information without loading the real definition. An autoloaded keymap loads automatically during key lookup when a prefix key’s binding is the symbol function. Autoloading does not occur for other kinds of access to the keymap. In particular, it does not happen when a Lisp program gets the keymap from the value of a variable and calls define-key; not even if the variable name is the same symbol function. If function already has a non-void function definition that is not an autoload object, this function does nothing and returns nil. Otherwise, it constructs an autoload object (see Section 2.3.17 [Autoload Type], page 23), and stores it as the function definition for function. The autoload object has this form: (autoload filename docstring interactive type) For example, (symbol-function ’run-prolog) ⇒ (autoload "prolog" 169681 t nil) In this case, "prolog" is the name of the file to load, 169681 refers to the documentation string in the emacs/etc/DOC file (see Section 23.1 [Documentation Basics], page 461), t means the function is interactive, and nil that it is not a macro or a keymap. autoloadp object [Function] This function returns non-nil if object is an autoload object. For example, to check if run-prolog is defined as an autoloaded function, evaluate (autoloadp (symbol-function ’run-prolog)) The autoloaded file usually contains other definitions and may require or provide one or more features. If the file is not completely loaded (due to an error in the evaluation of its contents), any function definitions or provide calls that occurred during the load are undone. This is to ensure that the next attempt to call any function autoloading from this file will try again to load the file. If not for this, then some of the functions in the file might be defined by the aborted load, but fail to work properly for the lack of certain subroutines not loaded successfully because they come later in the file. If the autoloaded file fails to define the desired Lisp function or macro, then an error is signaled with data "Autoloading failed to define function function-name". Chapter 15: Loading 232 A magic autoload comment (often called an autoload cookie) consists of ‘;;;###autoload’, on a line by itself, just before the real definition of the function in its autoloadable source file. The command M-x update-file-autoloads writes a corresponding autoload call into loaddefs.el. (The string that serves as the autoload cookie and the name of the file generated by update-file-autoloads can be changed from the above defaults, see below.) Building Emacs loads loaddefs.el and thus calls autoload. M-x update-directory-autoloads is even more powerful; it updates autoloads for all files in the current directory. The same magic comment can copy any kind of form into loaddefs.el. The form following the magic comment is copied verbatim, except if it is one of the forms which the autoload facility handles specially (e.g., by conversion into an autoload call). The forms which are not copied verbatim are the following: Definitions for function or function-like objects: defun and defmacro; also cl-defun and cl-defmacro (see Section “Argument Lists” in Common Lisp Extensions), and define-overloadable-function (see the commentary in mode-local.el). Definitions for major or minor modes: define-minor-mode, define-globalized-minor-mode, define-generic- mode, define-derived-mode, easy-mmode-define-minor-mode, easy-mmode-define-global-mode, define-compilation-mode, and define-global-minor-mode. Other definition types: defcustom, defgroup, defclass (see EIEIO), and define-skeleton (see the commentary in skeleton.el). You can also use a magic comment to execute a form at build time without executing it when the file itself is loaded. To do this, write the form on the same line as the magic comment. Since it is in a comment, it does nothing when you load the source file; but M-x update-file-autoloads copies it to loaddefs.el, where it is executed while building Emacs. The following example shows how doctor is prepared for autoloading with a magic comment: ;;;###autoload (defun doctor () "Switch to *doctor* buffer and start giving psychotherapy." (interactive) (switch-to-buffer "*doctor*") (doctor-mode)) Here’s what that produces in loaddefs.el: (autoload (quote doctor) "doctor" "\ Switch to *doctor* buffer and start giving psychotherapy. \(fn)" t nil) The backslash and newline immediately following the double-quote are a convention used only in the preloaded uncompiled Lisp files such as loaddefs.el; they tell make-docfile Chapter 15: Loading 233 to put the documentation string in the etc/DOC file. See Section E.1 [Building Emacs], page 1000. See also the commentary in lib-src/make-docfile.c. ‘(fn)’ in the usage part of the documentation string is replaced with the function’s name when the various help functions (see Section 23.5 [Help Functions], page 467) display it. If you write a function definition with an unusual macro that is not one of the known and recognized function definition methods, use of an ordinary magic autoload comment would copy the whole definition into loaddefs.el. That is not desirable. You can put the desired autoload call into loaddefs.el instead by writing this: ;;;###autoload (autoload ’foo "myfile") (mydefunmacro foo ...) You can use a non-default string as the autoload cookie and have the corresponding autoload calls written into a file whose name is different from the default loaddefs.el. Emacs provides two variables to control this: generate-autoload-cookie [Variable] The value of this variable should be a string whose syntax is a Lisp comment. M-x update-file-autoloads copies the Lisp form that follows the cookie into the au- toload file it generates. The default value of this variable is ";;;###autoload". generated-autoload-file [Variable] The value of this variable names an Emacs Lisp file where the autoload calls should go. The default value is loaddefs.el, but you can override that, e.g., in the “Local Variables” section of a .el file (see Section 11.11 [File Local Variables], page 163). The autoload file is assumed to contain a trailer starting with a formfeed character. The following function may be used to explicitly load the library specified by an autoload object: autoload-do-load autoload &optional name macro-only [Function] This function performs the loading specified by autoload, which should be an autoload object. The optional argument name, if non-nil, should be a symbol whose function value is autoload; in that case, the return value of this function is the symbol’s new function value. If the value of the optional argument macro-only is macro, this function avoids loading a function, only a macro. 15.6 Repeated Loading You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from. When you load or reload files, bear in mind that the load and load-library functions automatically load a byte-compiled file rather than a non-compiled file of similar name. If you rewrite a file that you intend to save and reinstall, you need to byte-compile the new version; otherwise Emacs will load the older, byte-compiled file instead of your newer, non-compiled file! If that happens, the message displayed when loading the file includes, ‘(compiled; note, source is newer)’, to remind you to recompile it. Chapter 15: Loading 234 When writing the forms in a Lisp library file, keep in mind that the file might be loaded more than once. For example, think about whether each variable should be reinitialized when you reload the library; defvar does not change the value if the variable is already initialized. (See Section 11.5 [Defining Variables], page 147.) The simplest way to add an element to an alist is like this: (push ’(leif-mode " Leif") minor-mode-alist) But this would add multiple elements if the library is reloaded. To avoid the problem, use add-to-list (see Section 5.5 [List Variables], page 71): (add-to-list ’minor-mode-alist ’(leif-mode " Leif")) Occasionally you will want to test explicitly whether a library has already been loaded. If the library uses provide to provide a named feature, you can use featurep earlier in the file to test whether the provide call has been executed before (see Section 15.7 [Named Features], page 234). Alternatively, you could use something like this: (defvar foo-was-loaded nil) (unless foo-was-loaded execute-first-time-only (setq foo-was-loaded t)) 15.7 Features provide and require are an alternative to autoload for loading files automatically. They work in terms of named features. Autoloading is triggered by calling a specific function, but a feature is loaded the first time another program asks for it by name. A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should provide the feature. Another program that uses them may ensure they are defined by requiring the feature. This loads the file of definitions if it hasn’t been loaded already. To require the presence of a feature, call require with the feature name as argument. require looks in the global variable features to see whether the desired feature has been provided already. If not, it loads the feature from the appropriate file. This file should call provide at the top level to add the feature to features; if it fails to do so, require signals an error. For example, in idlwave.el, the definition for idlwave-complete-filename includes the following code: (defun idlwave-complete-filename () "Use the comint stuff to complete a file name." (require ’comint) (let* ((comint-file-name-chars "~/A-Za-z0-9+@:_.$#%={}\\-") (comint-completion-addsuffix nil) ...) (comint-dynamic-complete-filename))) The expression (require ’comint) loads the file comint.el if it has not yet been loaded, ensuring that comint-dynamic-complete-filename is defined. Features are normally named after the files that provide them, so that require need not be given the file name. Chapter 15: Loading 235 (Note that it is important that the require statement be outside the body of the let. Loading a library while its variables are let-bound can have unintended consequences, namely the variables becoming unbound after the let exits.) The comint.el file contains the following top-level expression: (provide ’comint) This adds comint to the global features list, so that (require ’comint) will henceforth know that nothing needs to be done. When require is used at top level in a file, it takes effect when you byte-compile that file (see Chapter 16 [Byte Compilation], page 239) as well as when you load it. This is in case the required package contains macros that the byte compiler must know about. It also avoids byte compiler warnings for functions and variables defined in the file loaded with require. Although top-level calls to require are evaluated during byte compilation, provide calls are not. Therefore, you can ensure that a file of definitions is loaded before it is bytecompiled by including a provide followed by a require for the same feature, as in the following example. (provide ’my-feature) ; Ignored by byte compiler, ; evaluated by load. (require ’my-feature) ; Evaluated by byte compiler. The compiler ignores the provide, then processes the require by loading the file in question. Loading the file does execute the provide call, so the subsequent require call does nothing when the file is loaded. provide feature &optional subfeatures [Function] This function announces that feature is now loaded, or being loaded, into the current Emacs session. This means that the facilities associated with feature are or will be available for other Lisp programs. The direct effect of calling provide is if not already in features then to add feature to the front of that list and call any eval-after-load code waiting for it (see Section 15.10 [Hooks for Loading], page 238). The argument feature must be a symbol. provide returns feature. If provided, subfeatures should be a list of symbols indicating a set of specific subfeatures provided by this version of feature. You can test the presence of a subfeature using featurep. The idea of subfeatures is that you use them when a package (which is one feature) is complex enough to make it useful to give names to various parts or functionalities of the package, which might or might not be loaded, or might or might not be present in a given version. See Section 36.17.3 [Network Feature Testing], page 823, for an example. features ⇒ (bar bish) (provide ’foo) ⇒ foo features ⇒ (foo bar bish) Chapter 15: Loading 236 When a file is loaded to satisfy an autoload, and it stops due to an error in the evaluation of its contents, any function definitions or provide calls that occurred during the load are undone. See Section 15.5 [Autoload], page 230. require feature &optional filename noerror [Function] This function checks whether feature is present in the current Emacs session (using (featurep feature); see below). The argument feature must be a symbol. If the feature is not present, then require loads filename with load. If filename is not supplied, then the name of the symbol feature is used as the base file name to load. However, in this case, require insists on finding feature with an added ‘.el’ or ‘.elc’ suffix (possibly extended with a compression suffix); a file whose name is just feature won’t be used. (The variable load-suffixes specifies the exact required Lisp suffixes.) If noerror is non-nil, that suppresses errors from actual loading of the file. In that case, require returns nil if loading the file fails. Normally, require returns feature. If loading the file succeeds but does not provide feature, require signals an error, ‘Required feature feature was not provided’. featurep feature &optional subfeature [Function] This function returns t if feature has been provided in the current Emacs session (i.e., if feature is a member of features.) If subfeature is non-nil, then the function returns t only if that subfeature is provided as well (i.e., if subfeature is a member of the subfeature property of the feature symbol.) features [Variable] The value of this variable is a list of symbols that are the features loaded in the current Emacs session. Each symbol was put in this list with a call to provide. The order of the elements in the features list is not significant. 15.8 Which File Defined a Certain Symbol symbol-file symbol &optional type [Function] This function returns the name of the file that defined symbol. If type is nil, then any kind of definition is acceptable. If type is defun, defvar, or defface, that specifies function definition, variable definition, or face definition only. The value is normally an absolute file name. It can also be nil, if the definition is not associated with any file. If symbol specifies an autoloaded function, the value can be a relative file name without extension. The basis for symbol-file is the data in the variable load-history. load-history [Variable] The value of this variable is an alist that associates the names of loaded library files with the names of the functions and variables they defined, as well as the features they provided or required. Each element in this alist describes one loaded library (including libraries that are preloaded at startup). It is a list whose car is the absolute file name of the library (a string). The rest of the list elements have these forms: Chapter 15: Loading 237 var The symbol var was defined as a variable. (defun . fun) The function fun was defined. (t . fun) The function fun was previously an autoload before this library redefined it as a function. The following element is always (defun . fun), which represents defining fun as a function. (autoload . fun) The function fun was defined as an autoload. (defface . face) The face face was defined. (require . feature) The feature feature was required. (provide . feature) The feature feature was provided. The value of load-history may have one element whose car is nil. This element describes definitions made with eval-buffer on a buffer that is not visiting a file. The command eval-region updates load-history, but does so by adding the symbols defined to the element for the file being visited, rather than replacing that element. See Section 9.4 [Eval], page 120. 15.9 Unloading You can discard the functions and variables loaded by a library to reclaim memory for other Lisp objects. To do this, use the function unload-feature: unload-feature feature &optional force [Command] This command unloads the library that provided feature feature. It undefines all func- tions, macros, and variables defined in that library with defun, defalias, defsubst, defmacro, defconst, defvar, and defcustom. It then restores any autoloads for- merly associated with those symbols. (Loading saves these in the autoload property of the symbol.) Before restoring the previous definitions, unload-feature runs remove-hook to remove functions in the library from certain hooks. These hooks include variables whose names end in ‘-hook’ (or the deprecated suffix ‘-hooks’), plus those listed in unload-feature-special-hooks, as well as auto-mode-alist. This is to prevent Emacs from ceasing to function because important hooks refer to functions that are no longer defined. Standard unloading activities also undoes ELP profiling of functions in that library, unprovides any features provided by the library, and cancels timers held in variables defined by the library. If these measures are not sufficient to prevent malfunction, a library can define an explicit unloader named feature-unload-function. If that symbol is defined as a function, unload-feature calls it with no arguments before doing anything else. It can Chapter 15: Loading 238 do whatever is appropriate to unload the library. If it returns nil, unload-feature proceeds to take the normal unload actions. Otherwise it considers the job to be done. Ordinarily, unload-feature refuses to unload a library on which other loaded libraries depend. (A library a depends on library b if a contains a require for b.) If the optional argument force is non-nil, dependencies are ignored and you can unload any library. The unload-feature function is written in Lisp; its actions are based on the variable load-history. unload-feature-special-hooks [Variable] This variable holds a list of hooks to be scanned before unloading a library, to remove functions defined in the library. 15.10 Hooks for Loading You can ask for code to be executed each time Emacs loads a library, by using the variable after-load-functions: after-load-functions [Variable] This abnormal hook is run after loading a file. Each function in the hook is called with a single argument, the absolute filename of the file that was just loaded. If you want code to be executed when a particular library is loaded, use the macro with-eval-after-load: with-eval-after-load library body. . . [Macro] This macro arranges to evaluate body at the end of loading the file library, each time library is loaded. If library is already loaded, it evaluates body right away. You don’t need to give a directory or extension in the file name library. Normally, you just give a bare file name, like this: (with-eval-after-load "edebug" (def-edebug-spec c-point t)) To restrict which files can trigger the evaluation, include a directory or an extension or both in library. Only a file whose absolute true name (i.e., the name with all symbolic links chased out) matches all the given name components will match. In the following example, my_inst.elc or my_inst.elc.gz in some directory ..../foo/bar will trigger the evaluation, but not my_inst.el: (with-eval-after-load "foo/bar/my_inst.elc" ...) library can also be a feature (i.e., a symbol), in which case body is evaluated at the end of any file where (provide library) is called. An error in body does not undo the load, but does prevent execution of the rest of body. Normally, well-designed Lisp programs should not use eval-after-load. If you need to examine and set the variables defined in another library (those meant for outside use), you can do it immediately—there is no need to wait until the library is loaded. If you need to call functions defined by that library, you should load the library, preferably with require (see Section 15.7 [Named Features], page 234). Chapter 16: Byte Compilation 239 16 Byte Compilation Emacs Lisp has a compiler that translates functions written in Lisp into a special representation called byte-code that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the byte-code interpreter. Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine’s hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code. In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. If you do not want a Lisp file to be compiled, ever, put a file-local variable binding for no-byte-compile into it, like this: ;; -*-no-byte-compile: t; -*- 16.1 Performance of Byte-Compiled Code A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example: (defun silly-loop (n) "Return the time, in seconds, to run N iterations of a loop." (let ((t1 (float-time))) (while (> (setq n (1- n)) 0)) (- (float-time) t1))) ⇒ silly-loop (silly-loop 50000000) ⇒ 10.235304117202759 (byte-compile ’silly-loop) ⇒ [Compiled code not shown] (silly-loop 50000000) ⇒ 3.705854892730713 In this example, the interpreted code required 10 seconds to run, whereas the bytecompiled code required less than 4 seconds. These results are representative, but actual results may vary. 16.2 Byte-Compilation Functions You can byte-compile an individual function or macro definition with the byte-compile function. You can compile a whole file with byte-compile-file, or several files with byte-recompile-directory or batch-byte-compile. Sometimes, the byte compiler produces warning and/or error messages (see Section 16.6 [Compiler Errors], page 244, for details). These messages are recorded in a buffer called Chapter 16: Byte Compilation 240 *Compile-Log*, which uses Compilation mode. See Section “Compilation Mode” in The GNU Emacs Manual. Be careful when writing macro calls in files that you intend to byte-compile. Since macro calls are expanded when they are compiled, the macros need to be loaded into Emacs or the byte compiler will not do the right thing. The usual way to handle this is with require forms which specify the files containing the needed macro definitions (see Section 15.7 [Named Features], page 234). Normally, the byte compiler does not evaluate the code that it is compiling, but it handles require forms specially, by loading the specified libraries. To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see Section 16.5 [Eval During Compile], page 243). For more details, See Section 13.3 [Compiling Macros], page 199. Inline (defsubst) functions are less troublesome; if you compile a call to such a function before its definition is known, the call will still work right, it will just run slower. byte-compile symbol [Function] This function byte-compiles the function definition of symbol, replacing the previous definition with the compiled one. The function definition of symbol must be the actual code for the function; byte-compile does not handle function indirection. The return value is the byte-code function object which is the compiled definition of symbol (see Section 16.7 [Byte-Code Objects], page 245). (defun factorial (integer) "Compute factorial of INTEGER." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) ⇒ factorial (byte-compile ’factorial) ⇒ #[(integer) "^H\301U\203^H^@\301\207\302^H\303^HS!\"\207" [integer 1 * factorial] 4 "Compute factorial of INTEGER."] If symbol’s definition is a byte-code function object, byte-compile does nothing and returns nil. It does not “compile the symbol’s definition again”, since the original (non-compiled) code has already been replaced in the symbol’s function cell by the byte-compiled code. The argument to byte-compile can also be a lambda expression. In that case, the function returns the corresponding compiled code but does not store it anywhere. compile-defun &optional arg [Command] This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function. compile-defun normally displays the result of evaluation in the echo area, but if arg is non-nil, it inserts the result in the current buffer after the form it compiled. Chapter 16: Byte Compilation 241 byte-compile-file filename &optional load [Command] This function compiles a file of Lisp code named filename into a file of byte-code. The output file’s name is made by changing the ‘.el’ suffix into ‘.elc’; if filename does not end in ‘.el’, it adds ‘.elc’ to the end of filename. Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read. This command returns t if there were no errors and nil otherwise. When called interactively, it prompts for the file name. If load is non-nil, this command loads the compiled file after compiling it. Interactively, load is the prefix argument. $ ls -l push* -rw-r--r-- 1 lewis lewis 791 Oct 5 20:31 push.el (byte-compile-file "~/emacs/push.el") ⇒t $ ls -l push* -rw-r--r-- 1 lewis lewis 791 Oct 5 20:31 push.el -rw-rw-rw- 1 lewis lewis 638 Oct 8 20:25 push.elc byte-recompile-directory directory &optional flag force [Command] This command recompiles every ‘.el’ file in directory (or its subdirectories) that needs recompilation. A file needs recompilation if a ‘.elc’ file exists but is older than the ‘.el’ file. When a ‘.el’ file has no corresponding ‘.elc’ file, flag says what to do. If it is nil, this command ignores these files. If flag is 0, it compiles them. If it is neither nil nor 0, it asks the user whether to compile each such file, and asks about each subdirectory as well. Interactively, byte-recompile-directory prompts for directory and flag is the prefix argument. If force is non-nil, this command recompiles every ‘.el’ file that has a ‘.elc’ file. The returned value is unpredictable. batch-byte-compile &optional noforce [Function] This function runs byte-compile-file on files specified on the command line. This function must be used only in a batch execution of Emacs, as it kills Emacs on completion. An error in one file does not prevent processing of subsequent files, but no output file will be generated for it, and the Emacs process will terminate with a nonzero status code. If noforce is non-nil, this function does not recompile files that have an up-to-date ‘.elc’ file. $ emacs -batch -f batch-byte-compile *.el Chapter 16: Byte Compilation 242 16.3 Documentation Strings and Compilation When Emacs loads functions and variables from a byte-compiled file, it normally does not load their documentation strings into memory. Each documentation string is “dynamically” loaded from the byte-compiled file only when needed. This saves memory, and speeds up loading by skipping the processing of the documentation strings. This feature has a drawback: if you delete, move, or alter the compiled file (such as by compiling a new version), Emacs may no longer be able to access the documentation string of previously-loaded functions or variables. Such a problem normally only occurs if you build Emacs yourself, and happen to edit and/or recompile the Lisp source files. To solve it, just reload each file after recompilation. Dynamic loading of documentation strings from byte-compiled files is determined, at compile time, for each byte-compiled file. It can be disabled via the option byte-compiledynamic-docstrings. byte-compile-dynamic-docstrings [User Option] If this is non-nil, the byte compiler generates compiled files that are set up for dynamic loading of documentation strings. To disable the dynamic loading feature for a specific file, set this option to nil in its header line (see Section “Local Variables in Files” in The GNU Emacs Manual), like this: -*-byte-compile-dynamic-docstrings: nil;-*- This is useful mainly if you expect to change the file, and you want Emacs sessions that have already loaded it to keep working when the file changes. Internally, the dynamic loading of documentation strings is accomplished by writing compiled files with a special Lisp reader construct, ‘#@count’. This construct skips the next count characters. It also uses the ‘#$’ construct, which stands for “the name of this file, as a string”. Do not use these constructs in Lisp source files; they are not designed to be clear to humans reading the file. 16.4 Dynamic Loading of Individual Functions When you compile a file, you can optionally enable the dynamic function loading feature (also known as lazy loading). With dynamic function loading, loading the file doesn’t fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder. The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate user-callable functions, if using one of them does not imply you will probably also use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides. The dynamic loading feature has certain disadvantages: • If you delete or move the compiled file after loading it, Emacs can no longer load the remaining function definitions not already loaded. Chapter 16: Byte Compilation 243 • If you alter the compiled file (such as by compiling a new version), then trying to load any function not already loaded will usually yield nonsense results. These problems will never happen in normal circumstances with installed Emacs files. But they are quite likely to happen with Lisp files that you are changing. The easiest way to prevent these problems is to reload the new compiled file immediately after each recompilation. The byte compiler uses the dynamic function loading feature if the variable byte-compile-dynamic is non-nil at compilation time. Do not set this variable globally, since dynamic loading is desirable only for certain files. Instead, enable the feature for specific source files with file-local variable bindings. For example, you could do it by writing this text in the source file’s first line: -*-byte-compile-dynamic: t;-*- byte-compile-dynamic [Variable] If this is non-nil, the byte compiler generates compiled files that are set up for dynamic function loading. fetch-bytecode function [Function] If function is a byte-code function object, this immediately finishes loading the byte code of function from its byte-compiled file, if it is not fully loaded already. Otherwise, it does nothing. It always returns function. 16.5 Evaluation During Compilation These features permit you to write code to be evaluated during compilation of a program. eval-and-compile body. . . [Special Form] This form marks body to be evaluated both when you compile the containing code and when you run it (whether compiled or not). You can get a similar result by putting body in a separate file and referring to that file with require. That method is preferable when body is large. Effectively require is automatically eval-and-compile, the package is loaded both when compiling and executing. autoload is also effectively eval-and-compile too. It’s recognized when compiling, so uses of such a function don’t produce “not known to be defined” warnings. Most uses of eval-and-compile are fairly sophisticated. If a macro has a helper function to build its result, and that macro is used both locally and outside the package, then eval-and-compile should be used to get the helper both when compiling and then later when running. If functions are defined programmatically (with fset say), then eval-and-compile can be used to have that done at compile-time as well as run-time, so calls to those functions are checked (and warnings about “not known to be defined” suppressed). eval-when-compile body. . . [Special Form] This form marks body to be evaluated at compile time but not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which Chapter 16: Byte Compilation 244 appears in the compiled program. If you load the source file, rather than compiling it, body is evaluated normally. If you have a constant that needs some calculation to produce, eval-when-compile can do that at compile-time. For example, (defvar my-regexp (eval-when-compile (regexp-opt ’("aaa" "aba" "abb")))) If you’re using another package, but only need macros from it (the byte compiler will expand those), then eval-when-compile can be used to load it for compiling, but not executing. For example, (eval-when-compile (require ’my-macro-package)) The same sort of thing goes for macros and defsubst functions defined locally and only for use within the file. They are needed for compiling the file, but in most cases they are not needed for execution of the compiled file. For example, (eval-when-compile (unless (fboundp ’some-new-thing) (defmacro ’some-new-thing () (compatibility code)))) This is often good for code that’s only a fallback for compatibility with other versions of Emacs. Common Lisp Note: At top level, eval-when-compile is analogous to the Common Lisp idiom (eval-when (compile eval) ...). Elsewhere, the Common Lisp ‘#.’ reader macro (but not when interpreting) is closer to what eval-when-compile does. 16.6 Compiler Errors Error and warning messages from byte compilation are printed in a buffer named *Compile-Log*. These messages include file names and line numbers identifying the location of the problem. The usual Emacs commands for operating on compiler output can be used on these messages. When an error is due to invalid syntax in the program, the byte compiler might get confused about the errors’ exact location. One way to investigate is to switch to the buffer *Compiler Input*. (This buffer name starts with a space, so it does not show up in the Buffer Menu.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read; the cause of the error might be nearby. See Section 17.3 [Syntax Errors], page 277, for some tips for locating syntax errors. A common type of warning issued by the byte compiler is for functions and variables that were used but not defined. Such warnings report the line number for the end of the file, not the locations where the missing functions or variables were used; to find these, you must search the file manually. If you are sure that a warning message about a missing function or variable is unjustified, there are several ways to suppress it: • You can suppress the warning for a specific call to a function func by conditionalizing it on an fboundp test, like this: Chapter 16: Byte Compilation 245 (if (fboundp ’func) ...(func ...)...) The call to func must be in the then-form of the if, and func must appear quoted in the call to fboundp. (This feature operates for cond as well.) • Likewise, you can suppress the warning for a specific use of a variable variable by conditionalizing it on a boundp test: (if (boundp ’variable) ...variable...) The reference to variable must be in the then-form of the if, and variable must appear quoted in the call to boundp. • You can tell the compiler that a function is defined using declare-function. See Section 12.14 [Declaring Functions], page 195. • Likewise, you can tell the compiler that a variable is defined using defvar with no initial value. (Note that this marks the variable as special.) See Section 11.5 [Defining Variables], page 147. You can also suppress any and all compiler warnings within a certain expression using the construct with-no-warnings: with-no-warnings body. . . [Special Form] In execution, this is equivalent to (progn body...), but the compiler does not issue warnings for anything that occurs inside body. We recommend that you use this construct around the smallest possible piece of code, to avoid missing possible warnings other than one you intend to suppress. Byte compiler warnings can be controlled more precisely by setting the variable byte-compile-warnings. See its documentation string for details. 16.7 Byte-Code Function Objects Byte-compiled functions have a special data type: they are byte-code function objects. Whenever such an object appears as a function to be called, Emacs uses the byte-code interpreter to execute the byte-code. Internally, a byte-code function object is much like a vector; its elements can be accessed using aref. Its printed representation is like that for a vector, with an additional ‘#’ before the opening ‘[’. It must have at least four elements; there is no maximum number, but only the first six elements have any normal use. They are: arglist The list of argument symbols. byte-code The string containing the byte-code instructions. constants The vector of Lisp objects referenced by the byte code. These include symbols used as function names and variable names. stacksize The maximum stack size this function needs. docstring The documentation string (if any); otherwise, nil. The value may be a number or a list, in case the documentation string is stored in a file. Use the function documentation to get the real documentation string (see Section 23.2 [Accessing Documentation], page 462). Chapter 16: Byte Compilation 246 interactive The interactive spec (if any). This can be a string or a Lisp expression. It is nil for a function that isn’t interactive. Here’s an example of a byte-code function object, in printed representation. It is the definition of the command backward-sexp. #[(&optional arg) "^H\204^F^@\301^P\302^H[!\207" [arg 1 forward-sexp] 2 254435 "^p"] The primitive way to create a byte-code object is with make-byte-code: make-byte-code &rest elements [Function] This function constructs and returns a byte-code function object with elements as its elements. You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope). 16.8 Disassembled Byte-Code People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into human-readable form. The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function. In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack. disassemble object &optional buffer-or-name [Command] This command displays the disassembled code for object. In interactive use, or if buffer-or-name is nil or omitted, the output goes in a buffer named *Disassemble*. If buffer-or-name is non-nil, it must be a buffer or the name of an existing buffer. Then the output goes there, at point, and point is left before the output. The argument object can be a function name, a lambda expression (see Section 12.2 [Lambda Expressions], page 174), or a byte-code object (see Section 16.7 [Byte-Code Objects], page 245). If it is a lambda expression, disassemble compiles it and disassembles the resulting compiled code. Here are two examples of using the disassemble function. We have added explanatory comments to help you relate the byte-code to the Lisp source; these do not appear in the output of disassemble. Chapter 16: Byte Compilation 247 (defun factorial (integer) "Compute factorial of an integer." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) ⇒ factorial (factorial 4) ⇒ 24 (disassemble ’factorial) byte-code for factorial: doc: Compute factorial of an integer. args: (integer) 0 varref integer 1 constant 1 2 eqlsign 3 goto-if-nil 1 6 constant 1 7 return 8:1 varref integer 9 constant factorial 10 varref integer 11 sub1 12 call 1 13 mult 14 return ; Get the value of integer and ; push it onto the stack. ; Push 1 onto stack. ; Pop top two values off stack, compare ; them, and push result onto stack. ; Pop and test top of stack; ; if nil, go to 1, else continue. ; Push 1 onto top of stack. ; Return the top element of the stack. ; Push value of integer onto stack. ; Push factorial onto stack. ; Push value of integer onto stack. ; Pop integer, decrement value, ; push new value onto stack. ; Call function factorial using first ; (i.e., top) stack element as argument; ; push returned value onto stack. ; Pop top two values off stack, multiply ; them, and push result onto stack. ; Return the top element of the stack. The silly-loop function is somewhat more complex: (defun silly-loop (n) "Return time before and after N iterations of a loop." (let ((t1 (current-time-string))) (while (> (setq n (1- n)) 0)) (list t1 (current-time-string)))) ⇒ silly-loop Chapter 16: Byte Compilation 248 (disassemble ’silly-loop) byte-code for silly-loop: doc: Return time before and after N iterations of a loop. args: (n) 0 constant current-time-string ; Push current-time-string ; onto top of stack. 1 call 0 ; Call current-time-string with no ; argument, push result onto stack. 2 varbind t1 ; Pop stack and bind t1 to popped value. 3:1 varref n ; Get value of n from the environment ; and push the value on the stack. 4 sub1 ; Subtract 1 from top of stack. 5 dup ; Duplicate top of stack; i.e., copy the top ; of the stack and push copy onto stack. 6 varset n ; Pop the top of the stack, ; and bind n to the value. ;; (In effect, the sequence dup varset copies the top of the stack ;; into the value of n without popping it.) 7 constant 0 ; Push 0 onto stack. 8 gtr ; Pop top two values off stack, ; test if n is greater than 0 ; and push result onto stack. 9 goto-if-not-nil 1 ; Goto 1 if n > 0 ; (this continues the while loop) ; else continue. 12 varref t1 ; Push value of t1 onto stack. 13 constant current-time-string ; Push current-time-string ; onto the top of the stack. 14 call 0 ; Call current-time-string again. 15 unbind 1 ; Unbind t1 in local environment. 16 list2 ; Pop top two elements off stack, create a ; list of them, and push it onto stack. 17 return ; Return value of the top of stack. Chapter 17: Debugging Lisp Programs 249 17 Debugging Lisp Programs There are several ways to find and investigate problems in an Emacs Lisp program. • If a problem occurs when you run the program, you can use the built-in Emacs Lisp debugger to suspend the Lisp evaluator, and examine and/or alter its internal state. • You can use Edebug, a source-level debugger for Emacs Lisp. • If a syntactic problem is preventing Lisp from even reading the program, you can locate it using Lisp editing commands. • You can look at the error and warning messages produced by the byte compiler when it compiles the program. See Section 16.6 [Compiler Errors], page 244. • You can use the Testcover package to perform coverage testing on the program. • You can use the ERT package to write regression tests for the program. See ERT: Emacs Lisp Regression Testing. • You can profile the program to get hints about how to make it more efficient. Other useful tools for debugging input and output problems are the dribble file (see Section 38.12 [Terminal Input], page 948) and the open-termscript function (see Section 38.13 [Terminal Output], page 949). 17.1 The Lisp Debugger The ordinary Lisp debugger provides the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a break), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of Emacs are available; you can even run programs that will enter the debugger recursively. See Section 20.13 [Recursive Editing], page 362. 17.1.1 Entering the Debugger on an Error The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error. However, entry to the debugger is not a normal consequence of an error. Many commands signal Lisp errors when invoked inappropriately, and during ordinary editing it would be very inconvenient to enter the debugger each time this happens. So if you want errors to enter the debugger, set the variable debug-on-error to non-nil. (The command toggle-debugon-error provides an easy way to do this.) debug-on-error [User Option] This variable determines whether the debugger is called when an error is signaled and not handled. If debug-on-error is t, all kinds of errors call the debugger, except those listed in debug-ignored-errors (see below). If it is nil, none call the debugger. The value can also be a list of error conditions (see Section 10.5.3.1 [Signaling Errors], page 134). Then the debugger is called only for error conditions in this list (except those also listed in debug-ignored-errors). For example, if you set debug-on-error to the list (void-variable), the debugger is only called for errors about a variable that has no value. Chapter 17: Debugging Lisp Programs 250 Note that eval-expression-debug-on-error overrides this variable in some cases; see below. When this variable is non-nil, Emacs does not create an error handler around process filter functions and sentinels. Therefore, errors in these functions also invoke the debugger. See Chapter 36 [Processes], page 792. debug-ignored-errors [User Option] This variable specifies errors which should not enter the debugger, regardless of the value of debug-on-error. Its value is a list of error condition symbols and/or regular expressions. If the error has any of those condition symbols, or if the error message matches any of the regular expressions, then that error does not enter the debugger. The normal value of this variable includes user-error, as well as several errors that happen often during editing but rarely result from bugs in Lisp programs. However, “rarely” is not “never”; if your program fails with an error that matches this list, you may try changing this list to debug the error. The easiest way is usually to set debug-ignored-errors to nil. eval-expression-debug-on-error [User Option] If this variable has a non-nil value (the default), running the command eval-expression causes debug-on-error to be temporarily bound to to t. See Section “Evaluating Emacs-Lisp Expressions” in The GNU Emacs Manual. If eval-expression-debug-on-error is nil, then the value of debug-on-error is not changed during eval-expression. debug-on-signal [Variable] Normally, errors caught by condition-case never invoke the debugger. The condition-case gets a chance to handle the error before the debugger gets a chance. If you change debug-on-signal to a non-nil value, the debugger gets the first chance at every error, regardless of the presence of condition-case. (To invoke the debugger, the error must still fulfill the criteria specified by debug-on-error and debug-ignored-errors.) Warning: Setting this variable to non-nil may have annoying effects. Various parts of Emacs catch errors in the normal course of affairs, and you may not even realize that errors happen there. If you need to debug code wrapped in condition-case, consider using condition-case-unless-debug (see Section 10.5.3.3 [Handling Errors], page 136). debug-on-event [User Option] If you set debug-on-event to a special event (see Section 20.9 [Special Events], page 357), Emacs will try to enter the debugger as soon as it receives this event, bypassing special-event-map. At present, the only supported values correspond to the signals SIGUSR1 and SIGUSR2 (this is the default). This can be helpful when inhibit-quit is set and Emacs is not otherwise responding. debug-on-message [Variable] If you set debug-on-message to a regular expression, Emacs will enter the debugger if it displays a matching message in the echo area. For example, this can be useful when trying to find the cause of a particular message. Chapter 17: Debugging Lisp Programs 251 To debug an error that happens during loading of the init file, use the option ‘--debug-init’. This binds debug-on-error to t while loading the init file, and bypasses the condition-case which normally catches errors in the init file. 17.1.2 Debugging Infinite Loops When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with C-g, which causes a quit. See Section 20.11 [Quitting], page 358. Ordinary quitting gives no information about why the program was looping. To get more information, you can set the variable debug-on-quit to non-nil. Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you may get enough information to solve the problem. Quitting with C-g is not considered an error, and debug-on-error has no effect on the handling of C-g. Likewise, debug-on-quit has no effect on errors. debug-on-quit [User Option] This variable determines whether the debugger is called when quit is signaled and not handled. If debug-on-quit is non-nil, then the debugger is called whenever you quit (that is, type C-g). If debug-on-quit is nil (the default), then the debugger is not called when you quit. 17.1.3 Entering the Debugger on a Function Call To investigate a problem that happens in the middle of a program, one useful technique is to enter the debugger whenever a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller. debug-on-entry function-name [Command] This function requests function-name to invoke the debugger each time it is called. Any function or macro defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. If the function is a command, it will enter the debugger when called from Lisp and when called interactively (after the reading of the arguments). You can also set debug-on-entry for primitive functions (i.e., those written in C) this way, but it only takes effect when the primitive is called from Lisp code. Debug-on-entry is not allowed for special forms. When debug-on-entry is called interactively, it prompts for function-name in the minibuffer. If the function is already set up to invoke the debugger on entry, debug-on-entry does nothing. debug-on-entry always returns function-name. Here’s an example to illustrate use of this function: (defun fact (n) (if (zerop n) 1 (* n (fact (1- n))))) ⇒ fact (debug-on-entry ’fact) ⇒ fact Chapter 17: Debugging Lisp Programs 252 (fact 3) ------ Buffer: *Backtrace* -----Debugger entered--entering a function: * fact(3) eval((fact 3)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ------ Buffer: *Backtrace* ------ cancel-debug-on-entry &optional function-name [Command] This function undoes the effect of debug-on-entry on function-name. When called interactively, it prompts for function-name in the minibuffer. If function-name is omitted or nil, it cancels break-on-entry for all functions. Calling cancel-debug- on-entry does nothing to a function which is not currently set up to break on entry. 17.1.4 Explicit Entry to the Debugger You can cause the debugger to be called at a certain point in your program by writing the expression (debug) at that point. To do this, visit the source file, insert the text ‘(debug)’ at the proper place, and type C-M-x (eval-defun, a Lisp mode key binding). Warning: if you do this for temporary debugging purposes, be sure to undo this insertion before you save the file! The place where you insert ‘(debug)’ must be a place where an additional form can be evaluated and its value ignored. (If the value of (debug) isn’t ignored, it will alter the execution of the program!) The most common suitable places are inside a progn or an implicit progn (see Section 10.1 [Sequencing], page 124). If you don’t know exactly where in the source code you want to put the debug statement, but you want to display a backtrace when a certain message is displayed, you can set debug-on-message to a regular expression matching the desired message. 17.1.5 Using the Debugger When the debugger is entered, it displays the previously selected buffer in one window and a buffer named *Backtrace* in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error). The backtrace buffer is read-only and uses a special major mode, Debugger mode, in which letters are defined as debugger commands. The usual Emacs editing commands are available; thus, you can switch windows to examine the buffer that was being edited at the time of the error, switch buffers, visit files, or do any other sort of editing. However, the debugger is a recursive editing level (see Section 20.13 [Recursive Editing], page 362) and it is wise to go back to the backtrace buffer and exit the debugger (with the q command) when you are finished with it. Exiting the debugger gets out of the recursive edit and buries the backtrace buffer. (You can customize what the q command does with the backtrace Chapter 17: Debugging Lisp Programs 253 buffer by setting the variable debugger-bury-or-kill. For example, set it to kill if you prefer to kill the buffer rather than bury it. Consult the variable’s documentation for more possibilities.) When the debugger has been entered, the debug-on-error variable is temporarily set according to eval-expression-debug-on-error. If the latter variable is non-nil, debug-onerror will temporarily be set to t. This means that any further errors that occur while doing a debugging session will (by default) trigger another backtrace. If this is not what you want, you can either set eval-expression-debug-on-error to nil, or set debug-on-error to nil in debugger-mode-hook. The backtrace buffer shows you the functions that are executing and their argument values. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The frame whose line point is on is considered the current frame. Some of the debugger commands operate on the current frame. If a line starts with a star, that means that exiting that frame will call the debugger again. This is useful for examining the return value of a function. If a function name is underlined, that means the debugger knows where its source code is located. You can click with the mouse on that name, or move to it and type RET, to visit the source code. The debugger itself must be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted. 17.1.6 Debugger Commands The debugger buffer (in Debugger mode) provides special commands in addition to the usual Emacs commands. The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source for the function and type C-M-x on its definition.) You cannot use the Lisp debugger to step through a primitive function. Here is a list of Debugger mode commands: c Exit the debugger and continue execution. This resumes execution of the pro- gram as if the debugger had never been entered (aside from any side-effects that you caused by changing variable values or data structures while inside the debugger). d Continue execution, but enter the debugger the next time any Lisp function is called. This allows you to step through the subexpressions of an expression, seeing what values the subexpressions compute, and what else they do. The stack frame made for the function call which enters the debugger in this way will be flagged automatically so that the debugger will be called again when the frame is exited. You can use the u command to cancel this flag. b Flag the current frame so that the debugger will be entered when the frame is exited. Frames flagged in this way are marked with stars in the backtrace buffer. Chapter 17: Debugging Lisp Programs 254 u Don’t enter the debugger when the current frame is exited. This cancels a b command on that frame. The visible effect is to remove the star from the line in the backtrace buffer. j Flag the current frame like b. Then continue execution like c, but temporarily disable break-on-entry for all functions that are set up to do so by debug-on- entry. e Read a Lisp expression in the minibuffer, evaluate it (with the relevant lexical environment, if applicable), and print the value in the echo area. The debugger alters certain important variables, and the current buffer, as part of its opera- tion; e temporarily restores their values from outside the debugger, so you can examine and change them. This makes the debugger more transparent. By contrast, M-: does nothing special in the debugger; it shows you the variable values within the debugger. R Like e, but also save the result of evaluation in the buffer *Debugger-record*. q Terminate the program being debugged; return to top-level Emacs command execution. If the debugger was entered due to a C-g but you really want to quit, and not debug, use the q command. r Return a value from the debugger. The value is computed by reading an ex- pression with the minibuffer and evaluating it. The r command is useful when the debugger was invoked due to exit from a Lisp call frame (as requested with b or by entering the frame with d); then the value specified in the r command is used as the value of that frame. It is also useful if you call debug and use its return value. Otherwise, r has the same effect as c, and the specified return value does not matter. You can’t use r when the debugger was entered due to an error. l Display a list of functions that will invoke the debugger when called. This is a list of functions that are set to break on entry by means of debug-on-entry. v Toggle the display of local variables of the current stack frame. 17.1.7 Invoking the Debugger Here we describe in full detail the function debug that is used to invoke the debugger. debug &rest debugger-args [Command] This function enters the debugger. It switches buffers to a buffer named *Backtrace* (or *Backtrace*<2> if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, showing the backtrace buffer in Debugger mode. The Debugger mode c, d, j, and r commands exit the recursive edit; then debug switches back to the previous buffer and returns to whatever called debug. This is the only way the function debug can return to its caller. The use of the debugger-args is that debug displays the rest of its arguments at the top of the *Backtrace* buffer, so that the user can see them. Except as described below, this is the only way these arguments are used. Chapter 17: Debugging Lisp Programs 255 However, certain values for first argument to debug have a special significance. (Normally, these values are used only by the internals of Emacs, and not by programmers calling debug.) Here is a table of these special values: lambda A first argument of lambda means debug was called because of entry to a function when debug-on-next-call was non-nil. The debugger displays ‘Debugger entered--entering a function:’ as a line of text at the top of the buffer. debug debug as first argument means debug was called because of entry to a function that was set to debug on entry. The debugger displays the string ‘Debugger entered--entering a function:’, just as in the lambda case. It also marks the stack frame for that function so that it will invoke the debugger when exited. t When the first argument is t, this indicates a call to debug due to eval- uation of a function call form when debug-on-next-call is non-nil. The debugger displays ‘Debugger entered--beginning evaluation of function call form:’ as the top line in the buffer. exit When the first argument is exit, it indicates the exit of a stack frame previously marked to invoke the debugger on exit. The second argument given to debug in this case is the value being returned from the frame. The debugger displays ‘Debugger entered--returning value:’ in the top line of the buffer, followed by the value being returned. error When the first argument is error, the debugger indicates that it is being entered because an error or quit was signaled and not handled, by displaying ‘Debugger entered--Lisp error:’ followed by the error signaled and any arguments to signal. For example, (let ((debug-on-error t)) (/ 1 0)) ------ Buffer: *Backtrace* -----Debugger entered--Lisp error: (arith-error) /(1 0) ... ------ Buffer: *Backtrace* ------ If an error was signaled, presumably the variable debug-on-error is nonnil. If quit was signaled, then presumably the variable debug-on-quit is non-nil. nil Use nil as the first of the debugger-args when you want to enter the debugger explicitly. The rest of the debugger-args are printed on the top line of the buffer. You can use this feature to display messages—for example, to remind yourself of the conditions under which debug is called. 17.1.8 Internals of the Debugger This section describes functions and variables used internally by the debugger. Chapter 17: Debugging Lisp Programs 256 debugger [Variable] The value of this variable is the function to call to invoke the debugger. Its value must be a function of any number of arguments, or, more typically, the name of a function. This function should invoke some kind of debugger. The default value of the variable is debug. The first argument that Lisp hands to the function indicates why it was called. The convention for arguments is detailed in the description of debug (see Section 17.1.7 [Invoking the Debugger], page 254). backtrace [Command] This function prints a trace of Lisp function calls currently active. This is the function used by debug to fill up the *Backtrace* buffer. It is written in C, since it must have access to the stack to determine which function calls are active. The return value is always nil. In the following example, a Lisp expression calls backtrace explicitly. This prints the backtrace to the stream standard-output, which, in this case, is the buffer ‘backtrace-output’. Each line of the backtrace represents one function call. The line shows the values of the function’s arguments if they are all known; if they are still being computed, the line says so. The arguments of special forms are elided. (with-output-to-temp-buffer "backtrace-output" (let ((var 1)) (save-excursion (setq var (eval ’(progn (1+ var) (list ’testing (backtrace)))))))) ⇒ (testing nil) ----------- Buffer: backtrace-output -----------backtrace() (list ...computing arguments...) (progn ...) eval((progn (1+ var) (list (quote testing) (backtrace)))) (setq ...) (save-excursion ...) (let ...) (with-output-to-temp-buffer ...) eval((with-output-to-temp-buffer ...)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ----------- Buffer: backtrace-output ------------ debug-on-next-call [Variable] If this variable is non-nil, it says to call the debugger before the next eval, apply or funcall. Entering the debugger sets debug-on-next-call to nil. The d command in the debugger works by setting this variable. backtrace-debug level flag [Function] This function sets the debug-on-exit flag of the stack frame level levels down the stack, giving it the value flag. If flag is non-nil, this will cause the debugger to be entered Chapter 17: Debugging Lisp Programs 257 when that frame later exits. Even a nonlocal exit through that frame will enter the debugger. This function is used only by the debugger. command-debug-status [Variable] This variable records the debugging status of the current interactive command. Each time a command is called interactively, this variable is bound to nil. The debugger can set this variable to leave information for future debugger invocations during the same command invocation. The advantage of using this variable rather than an ordinary global variable is that the data will never carry over to a subsequent command invocation. backtrace-frame frame-number [Function] The function backtrace-frame is intended for use in Lisp debuggers. It returns information about what computation is happening in the stack frame frame-number levels down. If that frame has not evaluated the arguments yet, or is a special form, the value is (nil function arg-forms...). If that frame has evaluated its arguments and called its function already, the return value is (t function arg-values...). In the return value, function is whatever was supplied as the car of the evaluated list, or a lambda expression in the case of a macro call. If the function has a &rest argument, that is represented as the tail of the list arg-values. If frame-number is out of range, backtrace-frame returns nil. 17.2 Edebug Edebug is a source-level debugger for Emacs Lisp programs, with which you can: • Step through evaluation, stopping before and after each expression. • Set conditional or unconditional breakpoints. • Stop when a specified condition is true (the global break event). • Trace slow or fast, stopping briefly at each stop point, or at each breakpoint. • Display expression results and evaluate expressions as if outside of Edebug. • Automatically re-evaluate a list of expressions and display their results each time Edebug updates the display. • Output trace information on function calls and returns. • Stop when an error occurs. • Display a backtrace, omitting Edebug’s own frames. • Specify argument evaluation for macros and defining forms. • Obtain rudimentary coverage testing and frequency counts. The first three sections below should tell you enough about Edebug to start using it. Chapter 17: Debugging Lisp Programs 258 17.2.1 Using Edebug To debug a Lisp program with Edebug, you must first instrument the Lisp code that you want to debug. A simple way to do this is to first move point into the definition of a function or macro and then do C-u C-M-x (eval-defun with a prefix argument). See Section 17.2.2 [Instrumenting], page 259, for alternative ways to instrument code. Once a function is instrumented, any call to the function activates Edebug. Depending on which Edebug execution mode you have selected, activating Edebug may stop execution and let you step through the function, or it may update the display and continue execution while checking for debugging commands. The default execution mode is step, which stops execution. See Section 17.2.3 [Edebug Execution Modes], page 259. Within Edebug, you normally view an Emacs buffer showing the source of the Lisp code you are debugging. This is referred to as the source code buffer, and it is temporarily read-only. An arrow in the left fringe indicates the line where the function is executing. Point initially shows where within the line the function is executing, but this ceases to be true if you move point yourself. If you instrument the definition of fac (shown below) and then execute (fac 3), here is what you would normally see. Point is at the open-parenthesis before if. (defun fac (n) => (if (< 0 n) (* n (fac (1- n))) 1)) The places within a function where Edebug can stop execution are called stop points. These occur both before and after each subexpression that is a list, and also after each variable reference. Here we use periods to show the stop points in the function fac: (defun fac (n) .(if .(< 0 n.). .(* n. .(fac .(1- n.).).). 1).) The special commands of Edebug are available in the source code buffer in addition to the commands of Emacs Lisp mode. For example, you can type the Edebug command SPC to execute until the next stop point. If you type SPC once after entry to fac, here is the display you will see: (defun fac (n) =>(if (< 0 n) (* n (fac (1- n))) 1)) When Edebug stops execution after an expression, it displays the expression’s value in the echo area. Other frequently used commands are b to set a breakpoint at a stop point, g to execute until a breakpoint is reached, and q to exit Edebug and return to the top-level command loop. Type ? to display a list of all Edebug commands. Chapter 17: Debugging Lisp Programs 259 17.2.2 Instrumenting for Edebug In order to use Edebug to debug Lisp code, you must first instrument the code. Instrumenting code inserts additional code into it, to invoke Edebug at the proper places. When you invoke command C-M-x (eval-defun) with a prefix argument on a function definition, it instruments the definition before evaluating it. (This does not modify the source code itself.) If the variable edebug-all-defs is non-nil, that inverts the meaning of the prefix argument: in this case, C-M-x instruments the definition unless it has a prefix argument. The default value of edebug-all-defs is nil. The command M-x edebug-alldefs toggles the value of the variable edebug-all-defs. If edebug-all-defs is non-nil, then the commands eval-region, eval-currentbuffer, and eval-buffer also instrument any definitions they evaluate. Similarly, edebug-all-forms controls whether eval-region should instrument any form, even non-defining forms. This doesn’t apply to loading or evaluations in the minibuffer. The command M-x edebug-all-forms toggles this option. Another command, M-x edebug-eval-top-level-form, is available to instrument any top-level form regardless of the values of edebug-all-defs and edebug-all-forms. edebug-defun is an alias for edebug-eval-top-level-form. While Edebug is active, the command I (edebug-instrument-callee) instruments the definition of the function or macro called by the list form after point, if it is not already instrumented. This is possible only if Edebug knows where to find the source for that function; for this reason, after loading Edebug, eval-region records the position of every definition it evaluates, even if not instrumenting it. See also the i command (see Section 17.2.4 [Jumping], page 261), which steps into the call after instrumenting the function. Edebug knows how to instrument all the standard special forms, interactive forms with an expression argument, anonymous lambda expressions, and other defining forms. However, Edebug cannot determine on its own what a user-defined macro will do with the arguments of a macro call, so you must provide that information using Edebug specifications; for details, see Section 17.2.15 [Edebug and Macros], page 270. When Edebug is about to instrument code for the first time in a session, it runs the hook edebug-setup-hook, then sets it to nil. You can use this to load Edebug specifications associated with a package you are using, but only when you use Edebug. To remove instrumentation from a definition, simply re-evaluate its definition in a way that does not instrument. There are two ways of evaluating forms that never instrument them: from a file with load, and from the minibuffer with eval-expression (M-:). If Edebug detects a syntax error while instrumenting, it leaves point at the erroneous code and signals an invalid-read-syntax error. See Section 17.2.9 [Edebug Eval], page 265, for other evaluation functions available inside of Edebug. 17.2.3 Edebug Execution Modes Edebug supports several execution modes for running the program you are debugging. We call these alternatives Edebug execution modes; do not confuse them with major or minor modes. The current Edebug execution mode determines how far Edebug continues execution before stopping—whether it stops at each stop point, or continues to the next breakpoint, for example—and how much Edebug displays the progress of the evaluation before it stops. Chapter 17: Debugging Lisp Programs 260 Normally, you specify the Edebug execution mode by typing a command to continue the program in a certain mode. Here is a table of these commands; all except for S resume execution of the program, at least for a certain distance. S Stop: don’t execute any more of the program, but wait for more Edebug com- mands (edebug-stop). SPC Step: stop at the next stop point encountered (edebug-step-mode). n Next: stop at the next stop point encountered after an expression (edebug-next-mode). Also see edebug-forward-sexp in Section 17.2.4 [Jumping], page 261. t Trace: pause (normally one second) at each Edebug stop point (edebug-trace- mode). T Rapid trace: update the display at each stop point, but don’t actually pause (edebug-Trace-fast-mode). g Go: run until the next breakpoint (edebug-go-mode). See Section 17.2.6.1 [Breakpoints], page 262. c Continue: pause one second at each breakpoint, and then continue (edebug-continue-mode). C Rapid continue: move point to each breakpoint, but don’t pause (edebug-Continue-fast-mode). G Go non-stop: ignore breakpoints (edebug-Go-nonstop-mode). You can still stop the program by typing S, or any editing command. In general, the execution modes earlier in the above list run the program more slowly or stop sooner than the modes later in the list. While executing or tracing, you can interrupt the execution by typing any Edebug command. Edebug stops the program at the next stop point and then executes the command you typed. For example, typing t during execution switches to trace mode at the next stop point. You can use S to stop execution without doing anything else. If your function happens to read input, a character you type intending to interrupt execution may be read by the function instead. You can avoid such unintended results by paying attention to when your program wants input. Keyboard macros containing the commands in this section do not completely work: exiting from Edebug, to resume the program, loses track of the keyboard macro. This is not easy to fix. Also, defining or executing a keyboard macro outside of Edebug does not affect commands inside Edebug. This is usually an advantage. See also the edebug-continuekbd-macro option in Section 17.2.16 [Edebug Options], page 275. When you enter a new Edebug level, the initial execution mode comes from the value of the variable edebug-initial-mode (see Section 17.2.16 [Edebug Options], page 275). By default, this specifies step mode. Note that you may reenter the same Edebug level several times if, for example, an instrumented function is called several times from one command. edebug-sit-for-seconds [User Option] This option specifies how many seconds to wait between execution steps in trace mode or continue mode. The default is 1 second. Chapter 17: Debugging Lisp Programs 261 17.2.4 Jumping The commands described in this section execute until they reach a specified location. All except i make a temporary breakpoint to establish the place to stop, then switch to go mode. Any other breakpoint reached before the intended stop point will also stop execution. See Section 17.2.6.1 [Breakpoints], page 262, for the details on breakpoints. These commands may fail to work as expected in case of nonlocal exit, as that can bypass the temporary breakpoint where you expected the program to stop. h Proceed to the stop point near where point is (edebug-goto-here). f Run the program for one expression (edebug-forward-sexp). o Run the program until the end of the containing sexp (edebug-step-out). i Step into the function or macro called by the form after point (edebug-step- in). The h command proceeds to the stop point at or after the current location of point, using a temporary breakpoint. The f command runs the program forward over one expression. More precisely, it sets a temporary breakpoint at the position that forward-sexp would reach, then executes in go mode so that the program will stop at breakpoints. With a prefix argument n, the temporary breakpoint is placed n sexps beyond point. If the containing list ends before n more elements, then the place to stop is after the containing expression. You must check that the position forward-sexp finds is a place that the program will really get to. In cond, for example, this may not be true. For flexibility, the f command does forward-sexp starting at point, rather than at the stop point. If you want to execute one expression from the current stop point, first type w (edebug-where) to move point there, and then type f. The o command continues “out of” an expression. It places a temporary breakpoint at the end of the sexp containing point. If the containing sexp is a function definition itself, o continues until just before the last sexp in the definition. If that is where you are now, it returns from the function and then stops. In other words, this command does not exit the currently executing function unless you are positioned after the last sexp. The i command steps into the function or macro called by the list form after point, and stops at its first stop point. Note that the form need not be the one about to be evaluated. But if the form is a function call about to be evaluated, remember to use this command before any of the arguments are evaluated, since otherwise it will be too late. The i command instruments the function or macro it’s supposed to step into, if it isn’t instrumented already. This is convenient, but keep in mind that the function or macro remains instrumented unless you explicitly arrange to deinstrument it. 17.2.5 Miscellaneous Edebug Commands Some miscellaneous Edebug commands are described here. ? Display the help message for Edebug (edebug-help). C-] Abort one level back to the previous command level (abort-recursive-edit). Chapter 17: Debugging Lisp Programs 262 q Return to the top level editor command loop (top-level). This exits all re- cursive editing levels, including all levels of Edebug activity. However, instru- mented code protected with unwind-protect or condition-case forms may resume debugging. Q Like q, but don’t stop even for protected code (edebug-top-level-nonstop). r Redisplay the most recently known expression result in the echo area (edebug-previous-result). d Display a backtrace, excluding Edebug’s own functions for clarity (edebug-backtrace). You cannot use debugger commands in the backtrace buffer in Edebug as you would in the standard debugger. The backtrace buffer is killed automatically when you continue execution. You can invoke commands from Edebug that activate Edebug again recursively. Whenever Edebug is active, you can quit to the top level with q or abort one recursive edit level with C-]. You can display a backtrace of all the pending evaluations with d. 17.2.6 Breaks Edebug’s step mode stops execution when the next stop point is reached. There are three other ways to stop Edebug execution once it has started: breakpoints, the global break condition, and source breakpoints. 17.2.6.1 Edebug Breakpoints While using Edebug, you can specify breakpoints in the program you are testing: these are places where execution should stop. You can set a breakpoint at any stop point, as defined in Section 17.2.1 [Using Edebug], page 258. For setting and unsetting breakpoints, the stop point that is affected is the first one at or after point in the source code buffer. Here are the Edebug commands for breakpoints: b Set a breakpoint at the stop point at or after point (edebug-set-breakpoint). If you use a prefix argument, the breakpoint is temporary—it turns off the first time it stops the program. u Unset the breakpoint (if any) at the stop point at or after point (edebug-unset- breakpoint). x condition RET Set a conditional breakpoint which stops the program only if evaluating condition produces a non-nil value (edebug-set-conditional-breakpoint). With a prefix argument, the breakpoint is temporary. B Move point to the next breakpoint in the current definition (edebug-next- breakpoint). While in Edebug, you can set a breakpoint with b and unset one with u. First move point to the Edebug stop point of your choice, then type b or u to set or unset a breakpoint there. Unsetting a breakpoint where none has been set has no effect. Re-evaluating or reinstrumenting a definition removes all of its previous breakpoints. Chapter 17: Debugging Lisp Programs 263 A conditional breakpoint tests a condition each time the program gets there. Any errors that occur as a result of evaluating the condition are ignored, as if the result were nil. To set a conditional breakpoint, use x, and specify the condition expression in the minibuffer. Setting a conditional breakpoint at a stop point that has a previously established conditional breakpoint puts the previous condition expression in the minibuffer so you can edit it. You can make a conditional or unconditional breakpoint temporary by using a prefix argument with the command to set the breakpoint. When a temporary breakpoint stops the program, it is automatically unset. Edebug always stops or pauses at a breakpoint, except when the Edebug mode is Gononstop. In that mode, it ignores breakpoints entirely. To find out where your breakpoints are, use the B command, which moves point to the next breakpoint following point, within the same function, or to the first breakpoint if there are no following breakpoints. This command does not continue execution—it just moves point in the buffer. 17.2.6.2 Global Break Condition A global break condition stops execution when a specified condition is satisfied, no matter where that may occur. Edebug evaluates the global break condition at every stop point; if it evaluates to a non-nil value, then execution stops or pauses depending on the execution mode, as if a breakpoint had been hit. If evaluating the condition gets an error, execution does not stop. The condition expression is stored in edebug-global-break-condition. You can specify a new expression using the X command from the source code buffer while Edebug is active, or using C-x X X from any buffer at any time, as long as Edebug is loaded (edebug-setglobal-break-condition). The global break condition is the simplest way to find where in your code some event occurs, but it makes code run much more slowly. So you should reset the condition to nil when not using it. 17.2.6.3 Source Breakpoints All breakpoints in a definition are forgotten each time you reinstrument it. If you wish to make a breakpoint that won’t be forgotten, you can write a source breakpoint, which is simply a call to the function edebug in your source code. You can, of course, make such a call conditional. For example, in the fac function, you can insert the first line as shown below, to stop when the argument reaches zero: (defun fac (n) (if (= n 0) (edebug)) (if (< 0 n) (* n (fac (1- n))) 1)) When the fac definition is instrumented and the function is called, the call to edebug acts as a breakpoint. Depending on the execution mode, Edebug stops or pauses there. If no instrumented code is being executed when edebug is called, that function calls debug. Chapter 17: Debugging Lisp Programs 264 17.2.7 Trapping Errors Emacs normally displays an error message when an error is signaled and not handled with condition-case. While Edebug is active and executing instrumented code, it normally responds to all unhandled errors. You can customize this with the options edebug-onerror and edebug-on-quit; see Section 17.2.16 [Edebug Options], page 275. When Edebug responds to an error, it shows the last stop point encountered before the error. This may be the location of a call to a function which was not instrumented, and within which the error actually occurred. For an unbound variable error, the last known stop point might be quite distant from the offending variable reference. In that case, you might want to display a full backtrace (see Section 17.2.5 [Edebug Misc], page 261). If you change debug-on-error or debug-on-quit while Edebug is active, these changes will be forgotten when Edebug becomes inactive. Furthermore, during Edebug’s recursive edit, these variables are bound to the values they had outside of Edebug. 17.2.8 Edebug Views These Edebug commands let you view aspects of the buffer and window status as they were before entry to Edebug. The outside window configuration is the collection of windows and contents that were in effect outside of Edebug. v Switch to viewing the outside window configuration (edebug-view-outside). Type C-x X w to return to Edebug. p Temporarily display the outside current buffer with point at its outside position (edebug-bounce-point), pausing for one second before returning to Edebug. With a prefix argument n, pause for n seconds instead. w Move point back to the current stop point in the source code buffer (edebug-where). If you use this command in a different window displaying the same buffer, that window will be used instead to display the current definition in the future. W Toggle whether Edebug saves and restores the outside window configuration (edebug-toggle-save-windows). With a prefix argument, W only toggles saving and restoring of the selected window. To specify a window that is not displaying the source code buffer, you must use C-x X W from the global keymap. You can view the outside window configuration with v or just bounce to the point in the current buffer with p, even if it is not normally displayed. After moving point, you may wish to jump back to the stop point. You can do that with w from a source code buffer. You can jump back to the stop point in the source code buffer from any buffer using C-x X w. Each time you use W to turn saving off, Edebug forgets the saved outside window configuration—so that even if you turn saving back on, the current window configuration remains unchanged when you next exit Edebug (by continuing the program). However, the automatic redisplay of *edebug* and *edebug-trace* may conflict with the buffers you wish to see unless you have enough windows open. Chapter 17: Debugging Lisp Programs 265 17.2.9 Evaluation While within Edebug, you can evaluate expressions as if Edebug were not running. Edebug tries to be invisible to the expression’s evaluation and printing. Evaluation of expressions that cause side effects will work as expected, except for changes to data that Edebug explicitly saves and restores. See Section 17.2.14 [The Outside Context], page 269, for details on this process. e exp RET Evaluate expression exp in the context outside of Edebug (edebug-evalexpression). That is, Edebug tries to minimize its interference with the evaluation. M-: exp RET Evaluate expression exp in the context of Edebug itself (eval-expression). C-x C-e Evaluate the expression before point, in the context outside of Edebug (edebug-eval-last-sexp). Edebug supports evaluation of expressions containing references to lexically bound symbols created by the following constructs in cl.el: lexical-let, macrolet, and symbol-macrolet. 17.2.10 Evaluation List Buffer You can use the evaluation list buffer, called *edebug*, to evaluate expressions interactively. You can also set up the evaluation list of expressions to be evaluated automatically each time Edebug updates the display. E Switch to the evaluation list buffer *edebug* (edebug-visit-eval-list). In the *edebug* buffer you can use the commands of Lisp Interaction mode (see Section “Lisp Interaction” in The GNU Emacs Manual) as well as these special commands: C-j Evaluate the expression before point, in the outside context, and insert the value in the buffer (edebug-eval-print-last-sexp). C-x C-e Evaluate the expression before point, in the context outside of Edebug (edebug-eval-last-sexp). C-c C-u Build a new evaluation list from the contents of the buffer (edebug-updateeval-list). C-c C-d Delete the evaluation list group that point is in (edebug-delete-eval-item). C-c C-w Switch back to the source code buffer at the current stop point (edebug-where). You can evaluate expressions in the evaluation list window with C-j or C-x C-e, just as you would in *scratch*; but they are evaluated in the context outside of Edebug. The expressions you enter interactively (and their results) are lost when you continue execution; but you can set up an evaluation list consisting of expressions to be evaluated each time execution stops. To do this, write one or more evaluation list groups in the evaluation list buffer. An evaluation list group consists of one or more Lisp expressions. Groups are separated by comment lines. Chapter 17: Debugging Lisp Programs 266 The command C-c C-u (edebug-update-eval-list) rebuilds the evaluation list, scanning the buffer and using the first expression of each group. (The idea is that the second expression of the group is the value previously computed and displayed.) Each entry to Edebug redisplays the evaluation list by inserting each expression in the buffer, followed by its current value. It also inserts comment lines so that each expression becomes its own group. Thus, if you type C-c C-u again without changing the buffer text, the evaluation list is effectively unchanged. If an error occurs during an evaluation from the evaluation list, the error message is displayed in a string as if it were the result. Therefore, expressions using variables that are not currently valid do not interrupt your debugging. Here is an example of what the evaluation list window looks like after several expressions have been added to it: (current-buffer) # ;--------------------------------------------------------------(selected-window) # ;--------------------------------------------------------------(point) 196 ;--------------------------------------------------------------bad-var "Symbol’s value as variable is void: bad-var" ;--------------------------------------------------------------(recursion-depth) 0 ;--------------------------------------------------------------this-command eval-last-sexp ;--------------------------------------------------------------- To delete a group, move point into it and type C-c C-d, or simply delete the text for the group and update the evaluation list with C-c C-u. To add a new expression to the evaluation list, insert the expression at a suitable place, insert a new comment line, then type C-c C-u. You need not insert dashes in the comment line—its contents don’t matter. After selecting *edebug*, you can return to the source code buffer with C-c C-w. The *edebug* buffer is killed when you continue execution, and recreated next time it is needed. 17.2.11 Printing in Edebug If an expression in your program produces a value containing circular list structure, you may get an error when Edebug attempts to print it. One way to cope with circular structure is to set print-length or print-level to truncate the printing. Edebug does this for you; it binds print-length and print-level to the values of the variables edebug-print-length and edebug-print-level (so long as they have non-nil values). See Section 18.6 [Output Variables], page 288. edebug-print-length [User Option] If non-nil, Edebug binds print-length to this value while printing results. The default value is 50. Chapter 17: Debugging Lisp Programs 267 edebug-print-level [User Option] If non-nil, Edebug binds print-level to this value while printing results. The default value is 50. You can also print circular structures and structures that share elements more informatively by binding print-circle to a non-nil value. Here is an example of code that creates a circular structure: (setq a ’(x y)) (setcar a a) Custom printing prints this as ‘Result: #1=(#1# y)’. The ‘#1=’ notation labels the structure that follows it with the label ‘1’, and the ‘#1#’ notation references the previously labeled structure. This notation is used for any shared elements of lists or vectors. edebug-print-circle [User Option] If non-nil, Edebug binds print-circle to this value while printing results. The default value is t. Other programs can also use custom printing; see cust-print.el for details. 17.2.12 Trace Buffer Edebug can record an execution trace, storing it in a buffer named *edebug-trace*. This is a log of function calls and returns, showing the function names and their arguments and values. To enable trace recording, set edebug-trace to a non-nil value. Making a trace buffer is not the same thing as using trace execution mode (see Section 17.2.3 [Edebug Execution Modes], page 259). When trace recording is enabled, each function entry and exit adds lines to the trace buffer. A function entry record consists of ‘::::{’, followed by the function name and argument values. A function exit record consists of ‘::::}’, followed by the function name and result of the function. The number of ‘:’s in an entry shows its recursion depth. You can use the braces in the trace buffer to find the matching beginning or end of function calls. You can customize trace recording for function entry and exit by redefining the functions edebug-print-trace-before and edebug-print-trace-after. edebug-tracing string body. . . [Macro] This macro requests additional trace information around the execution of the body forms. The argument string specifies text to put in the trace buffer, after the ‘{’ or ‘}’. All the arguments are evaluated, and edebug-tracing returns the value of the last form in body. edebug-trace format-string &rest format-args [Function] This function inserts text in the trace buffer. It computes the text with (apply ’format format-string format-args). It also appends a newline to separate en- tries. edebug-tracing and edebug-trace insert lines in the trace buffer whenever they are called, even if Edebug is not active. Adding text to the trace buffer also scrolls its window to show the last lines inserted. Chapter 17: Debugging Lisp Programs 268 17.2.13 Coverage Testing Edebug provides rudimentary coverage testing and display of execution frequency. Coverage testing works by comparing the result of each expression with the previous result; each form in the program is considered “covered” if it has returned two different values since you began testing coverage in the current Emacs session. Thus, to do coverage testing on your program, execute it under various conditions and note whether it behaves correctly; Edebug will tell you when you have tried enough different conditions that each form has returned two different values. Coverage testing makes execution slower, so it is only done if edebug-test-coverage is non-nil. Frequency counting is performed for all executions of an instrumented function, even if the execution mode is Go-nonstop, and regardless of whether coverage testing is enabled. Use C-x X = (edebug-display-freq-count) to display both the coverage information and the frequency counts for a definition. Just = (edebug-temp-display-freq-count) displays the same information temporarily, only until you type another key. edebug-display-freq-count [Command] This command displays the frequency count data for each line of the current definition. It inserts frequency counts as comment lines after each line of code. You can undo all insertions with one undo command. The counts appear under the ‘(’ before an expression or the ‘)’ after an expression, or on the last character of a variable. To simplify the display, a count is not shown if it is equal to the count of an earlier expression on the same line. The character ‘=’ following the count for an expression says that the expression has returned the same value each time it was evaluated. In other words, it is not yet “covered” for coverage testing purposes. To clear the frequency count and coverage data for a definition, simply reinstrument it with eval-defun. For example, after evaluating (fac 5) with a source breakpoint, and setting edebug-test-coverage to t, when the breakpoint is reached, the frequency data looks like this: (defun fac (n) (if (= n 0) (edebug)) ;#6 1 = =5 (if (< 0 n) ;#5 = (* n (fac (1- n))) ;# 5 0 1)) ;# 0 The comment lines show that fac was called 6 times. The first if statement returned 5 times with the same result each time; the same is true of the condition on the second if. The recursive call of fac did not return at all. Chapter 17: Debugging Lisp Programs 269 17.2.14 The Outside Context Edebug tries to be transparent to the program you are debugging, but it does not succeed completely. Edebug also tries to be transparent when you evaluate expressions with e or with the evaluation list buffer, by temporarily restoring the outside context. This section explains precisely what context Edebug restores, and how Edebug fails to be completely transparent. 17.2.14.1 Checking Whether to Stop Whenever Edebug is entered, it needs to save and restore certain data before even deciding whether to make trace information or stop the program. • max-lisp-eval-depth and max-specpdl-size are both increased to reduce Edebug’s impact on the stack. You could, however, still run out of stack space when using Edebug. • The state of keyboard macro execution is saved and restored. While Edebug is active, executing-kbd-macro is bound to nil unless edebug-continue-kbd-macro is nonnil. 17.2.14.2 Edebug Display Update When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from “outside” Edebug (see Section 27.24 [Window Configurations], page 590). When you exit Edebug, it restores the previous window configuration. Emacs redisplays only when it pauses. Usually, when you continue execution, the program re-enters Edebug at a breakpoint or after stepping, without pausing or reading input in between. In such cases, Emacs never gets a chance to redisplay the “outside” configuration. Consequently, what you see is the same window configuration as the last time Edebug was active, with no interruption. Entry to Edebug for displaying something also saves and restores the following data (though some of them are deliberately not restored if an error or quit signal occurs). • Which buffer is current, and the positions of point and the mark in the current buffer, are saved and restored. • The outside window configuration is saved and restored if edebug-save-windows is non-nil (see Section 17.2.16 [Edebug Options], page 275). The window configuration is not restored on error or quit, but the outside selected window is reselected even on error or quit in case a save-excursion is active. If the value of edebug-save-windows is a list, only the listed windows are saved and restored. The window start and horizontal scrolling of the source code buffer are not restored, however, so that the display remains coherent within Edebug. • The value of point in each displayed buffer is saved and restored if edebug-savedisplayed-buffer-points is non-nil. • The variables overlay-arrow-position and overlay-arrow-string are saved and restored, so you can safely invoke Edebug from the recursive edit elsewhere in the same buffer. • cursor-in-echo-area is locally bound to nil so that the cursor shows up in the window. Chapter 17: Debugging Lisp Programs 270 17.2.14.3 Edebug Recursive Edit When Edebug is entered and actually reads commands from the user, it saves (and later restores) these additional data: • The current match data. See Section 33.6 [Match Data], page 760. • The variables last-command, this-command, last-command-event, last-inputevent, last-event-frame, last-nonmenu-event, and track-mouse. Commands in Edebug do not affect these variables outside of Edebug. Executing commands within Edebug can change the key sequence that would be returned by this-command-keys, and there is no way to reset the key sequence from Lisp. Edebug cannot save and restore the value of unread-command-events. Entering Edebug while this variable has a nontrivial value can interfere with execution of the program you are debugging. • Complex commands executed while in Edebug are added to the variable command-history. In rare cases this can alter execution. • Within Edebug, the recursion depth appears one deeper than the recursion depth outside Edebug. This is not true of the automatically updated evaluation list window. • standard-output and standard-input are bound to nil by the recursive-edit, but Edebug temporarily restores them during evaluations. • The state of keyboard macro definition is saved and restored. While Edebug is active, defining-kbd-macro is bound to edebug-continue-kbd-macro. 17.2.15 Edebug and Macros To make Edebug properly instrument expressions that call macros, some extra care is needed. This subsection explains the details. 17.2.15.1 Instrumenting Macro Calls When Edebug instruments an expression that calls a Lisp macro, it needs additional information about the macro to do the job properly. This is because there is no a-priori way to tell which subexpressions of the macro call are forms to be evaluated. (Evaluation may occur explicitly in the macro body, or when the resulting expansion is evaluated, or any time later.) Therefore, you must define an Edebug specification for each macro that Edebug will encounter, to explain the format of calls to that macro. To do this, add a debug declaration to the macro definition. Here is a simple example that shows the specification for the for example macro (see Section 13.5.2 [Argument Evaluation], page 201). (defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." (declare (debug (symbolp "from" form "to" form "do" &rest form))) ...) The Edebug specification says which parts of a call to the macro are forms to be evaluated. For simple macros, the specification often looks very similar to the formal argument list of the macro definition, but specifications are much more general than macro arguments. See Section 13.4 [Defining Macros], page 200, for more explanation of the declare form. Chapter 17: Debugging Lisp Programs 271 Take care to ensure that the specifications are known to Edebug when you instrument code. If you are instrumenting a function from a file that uses eval-when-compile to require another file containing macro definitions, you may need to explicitly load that file. You can also define an edebug specification for a macro separately from the macro definition with def-edebug-spec. Adding debug declarations is preferred, and more convenient, for macro definitions in Lisp, but def-edebug-spec makes it possible to define Edebug specifications for special forms implemented in C. def-edebug-spec macro specification [Macro] Specify which expressions of a call to macro macro are forms to be evaluated. speci- fication should be the edebug specification. Neither argument is evaluated. The macro argument can actually be any symbol, not just a macro name. Here is a table of the possibilities for specification and how each directs processing of arguments. t All arguments are instrumented for evaluation. 0 None of the arguments is instrumented. a symbol The symbol must have an Edebug specification, which is used instead. This indirection is repeated until another kind of specification is found. This allows you to inherit the specification from another macro. a list The elements of the list describe the types of the arguments of a calling form. The possible elements of a specification list are described in the following sections. If a macro has no Edebug specification, neither through a debug declaration nor through a def-edebug-spec call, the variable edebug-eval-macro-args comes into play. edebug-eval-macro-args [User Option] This controls the way Edebug treats macro arguments with no explicit Edebug specifi- cation. If it is nil (the default), none of the arguments is instrumented for evaluation. Otherwise, all arguments are instrumented. 17.2.15.2 Specification List A specification list is required for an Edebug specification if some arguments of a macro call are evaluated while others are not. Some elements in a specification list match one or more arguments, but others modify the processing of all following elements. The latter, called specification keywords, are symbols beginning with ‘&’ (such as &optional). A specification list may contain sublists, which match arguments that are themselves lists, or it may contain vectors used for grouping. Sublists and groups thus subdivide the specification list into a hierarchy of levels. Specification keywords apply only to the remainder of the sublist or group they are contained in. When a specification list involves alternatives or repetition, matching it against an actual macro call may require backtracking. For more details, see Section 17.2.15.3 [Backtracking], page 274. Chapter 17: Debugging Lisp Programs 272 Edebug specifications provide the power of regular expression matching, plus some context-free grammar constructs: the matching of sublists with balanced parentheses, recursive processing of forms, and recursion via indirect specifications. Here’s a table of the possible elements of a specification list, with their meanings (see Section 17.2.15.4 [Specification Examples], page 275, for the referenced examples): sexp A single unevaluated Lisp object, which is not instrumented. form A single evaluated expression, which is instrumented. place A generalized variable. See Section 11.15 [Generalized Variables], page 169. body Short for &rest form. See &rest below. function-form A function form: either a quoted function symbol, a quoted lambda expression, or a form (that should evaluate to a function symbol or lambda expression). This is useful when an argument that’s a lambda expression might be quoted with quote rather than function, since it instruments the body of the lambda expression either way. lambda-expr A lambda expression with no quoting. &optional All following elements in the specification list are optional; as soon as one does not match, Edebug stops matching at this level. To make just a few elements optional, followed by non-optional elements, use [&optional specs...]. To specify that several elements must all match or none, use &optional [specs...]. See the defun example. &rest All following elements in the specification list are repeated zero or more times. In the last repetition, however, it is not a problem if the expression runs out before matching all of the elements of the specification list. To repeat only a few elements, use [&rest specs...]. To specify several elements that must all match on every repetition, use &rest [specs...]. &or Each of the following elements in the specification list is an alternative. One of the alternatives must match, or the &or specification fails. Each list element following &or is a single alternative. To group two or more list elements as a single alternative, enclose them in [...]. ¬ Each of the following elements is matched as alternatives as if by using &or, but if any of them match, the specification fails. If none of them match, nothing is matched, but the ¬ specification succeeds. &define Indicates that the specification is for a defining form. The defining form itself is not instrumented (that is, Edebug does not stop before and after the defining form), but forms inside it typically will be instrumented. The &define keyword should be the first element in a list specification. nil This is successful when there are no more arguments to match at the current ar- gument list level; otherwise it fails. See sublist specifications and the backquote example. Chapter 17: Debugging Lisp Programs 273 gate No argument is matched but backtracking through the gate is disabled while matching the remainder of the specifications at this level. This is primarily used to generate more specific syntax error messages. See Section 17.2.15.3 [Backtracking], page 274, for more details. Also see the let example. other-symbol Any other symbol in a specification list may be a predicate or an indirect specification. If the symbol has an Edebug specification, this indirect specification should be either a list specification that is used in place of the symbol, or a function that is called to process the arguments. The specification may be defined with def-edebug-spec just as for macros. See the defun example. Otherwise, the symbol should be a predicate. The predicate is called with the argument, and if the predicate returns nil, the specification fails and the argument is not instrumented. Some suitable predicates include symbolp, integerp, stringp, vectorp, and atom. [elements...] A vector of elements groups the elements into a single group specification. Its meaning has nothing to do with vectors. "string" The argument should be a symbol named string. This specification is equivalent to the quoted symbol, ’symbol, where the name of symbol is the string, but the string form is preferred. (vector elements...) The argument should be a vector whose elements must match the elements in the specification. See the backquote example. (elements...) Any other list is a sublist specification and the argument must be a list whose elements match the specification elements. A sublist specification may be a dotted list and the corresponding list argument may then be a dotted list. Alternatively, the last cdr of a dotted list specification may be another sublist specification (via a grouping or an indirect specification, e.g., (spec . [(more specs...)])) whose elements match the non-dotted list arguments. This is useful in recursive specifications such as in the backquote example. Also see the description of a nil specification above for terminating such recursion. Note that a sublist specification written as (specs . nil) is equivalent to (specs), and (specs . (sublist-elements...)) is equivalent to (specs sublist-elements...). Here is a list of additional specifications that may appear only after &define. See the defun example. name The argument, a symbol, is the name of the defining form. A defining form is not required to have a name field; and it may have multiple name fields. Chapter 17: Debugging Lisp Programs 274 :name This construct does not actually match an argument. The element following :name should be a symbol; it is used as an additional name component for the definition. You can use this to add a unique, static component to the name of the definition. It may be used more than once. arg The argument, a symbol, is the name of an argument of the defining form. However, lambda-list keywords (symbols starting with ‘&’) are not allowed. lambda-list This matches a lambda list—the argument list of a lambda expression. def-body The argument is the body of code in a definition. This is like body, described above, but a definition body must be instrumented with a different Edebug call that looks up information associated with the definition. Use def-body for the highest level list of forms within the definition. def-form The argument is a single, highest-level form in a definition. This is like def-body, except it is used to match a single form rather than a list of forms. As a special case, def-form also means that tracing information is not output when the form is executed. See the interactive example. 17.2.15.3 Backtracking in Specifications If a specification fails to match at some point, this does not necessarily mean a syntax error will be signaled; instead, backtracking will take place until all alternatives have been exhausted. Eventually every element of the argument list must be matched by some element in the specification, and every required element in the specification must match some argument. When a syntax error is detected, it might not be reported until much later, after higherlevel alternatives have been exhausted, and with the point positioned further from the real error. But if backtracking is disabled when an error occurs, it can be reported immediately. Note that backtracking is also reenabled automatically in several situations; when a new alternative is established by &optional, &rest, or &or, or at the start of processing a sublist, group, or indirect specification. The effect of enabling or disabling backtracking is limited to the remainder of the level currently being processed and lower levels. Backtracking is disabled while matching any of the form specifications (that is, form, body, def-form, and def-body). These specifications will match any form so any error must be in the form itself rather than at a higher level. Backtracking is also disabled after successfully matching a quoted symbol or string specification, since this usually indicates a recognized construct. But if you have a set of alternative constructs that all begin with the same symbol, you can usually work around this constraint by factoring the symbol out of the alternatives, e.g., ["foo" &or [first case] [second case] ...]. Most needs are satisfied by these two ways that backtracking is automatically disabled, but occasionally it is useful to explicitly disable backtracking by using the gate specification. This is useful when you know that no higher alternatives could apply. See the example of the let specification. Chapter 17: Debugging Lisp Programs 275 17.2.15.4 Specification Examples It may be easier to understand Edebug specifications by studying the examples provided here. A let special form has a sequence of bindings and a body. Each of the bindings is either a symbol or a sublist with a symbol and optional expression. In the specification below, notice the gate inside of the sublist to prevent backtracking once a sublist is found. (def-edebug-spec let ((&rest &or symbolp (gate symbolp &optional form)) body)) Edebug uses the following specifications for defun and the associated argument list and interactive specifications. It is necessary to handle interactive forms specially since an expression argument is actually evaluated outside of the function body. (The specification for defmacro is very similar to that for defun, but allows for the declare statement.) (def-edebug-spec defun (&define name lambda-list [&optional stringp] ; Match the doc string, if present. [&optional ("interactive" interactive)] def-body)) (def-edebug-spec lambda-list (([&rest arg] [&optional ["&optional" arg &rest arg]] &optional ["&rest" arg] ))) (def-edebug-spec interactive (&optional &or stringp def-form)) ; Notice: def-form The specification for backquote below illustrates how to match dotted lists and use nil to terminate recursion. It also illustrates how components of a vector may be matched. (The actual specification defined by Edebug is a little different, and does not support dotted lists because doing so causes very deep recursion that could fail.) (def-edebug-spec \‘ (backquote-form)) ; Alias just for clarity. (def-edebug-spec backquote-form (&or ([&or "," ",@"] &or ("quote" backquote-form) form) (backquote-form . [&or nil backquote-form]) (vector &rest backquote-form) sexp)) 17.2.16 Edebug Options These options affect the behavior of Edebug: edebug-setup-hook [User Option] Functions to call before Edebug is used. Each time it is set to a new value, Edebug will call those functions once and then reset edebug-setup-hook to nil. You could use this to load up Edebug specifications associated with a package you are using, but only when you also use Edebug. See Section 17.2.2 [Instrumenting], page 259. Chapter 17: Debugging Lisp Programs 276 edebug-all-defs [User Option] If this is non-nil, normal evaluation of defining forms such as defun and defmacro instruments them for Edebug. This applies to eval-defun, eval-region, eval-buffer, and eval-current-buffer. Use the command M-x edebug-all-defs to toggle the value of this option. See Section 17.2.2 [Instrumenting], page 259. edebug-all-forms [User Option] If this is non-nil, the commands eval-defun, eval-region, eval-buffer, and eval-current-buffer instrument all forms, even those that don’t define anything. This doesn’t apply to loading or evaluations in the minibuffer. Use the command M-x edebug-all-forms to toggle the value of this option. See Section 17.2.2 [Instrumenting], page 259. edebug-save-windows [User Option] If this is non-nil, Edebug saves and restores the window configuration. That takes some time, so if your program does not care what happens to the window configura- tions, it is better to set this variable to nil. If the value is a list, only the listed windows are saved and restored. You can use the W command in Edebug to change this variable interactively. See Section 17.2.14.2 [Edebug Display Update], page 269. edebug-save-displayed-buffer-points [User Option] If this is non-nil, Edebug saves and restores point in all displayed buffers. Saving and restoring point in other buffers is necessary if you are debugging code that changes the point of a buffer that is displayed in a non-selected window. If Edebug or the user then selects the window, point in that buffer will move to the window’s value of point. Saving and restoring point in all buffers is expensive, since it requires selecting each window twice, so enable this only if you need it. See Section 17.2.14.2 [Edebug Display Update], page 269. edebug-initial-mode [User Option] If this variable is non-nil, it specifies the initial execution mode for Edebug when it is first activated. Possible values are step, next, go, Go-nonstop, trace, Trace-fast, continue, and Continue-fast. The default value is step. See Section 17.2.3 [Edebug Execution Modes], page 259. edebug-trace [User Option] If this is non-nil, trace each function entry and exit. Tracing output is displayed in a buffer named *edebug-trace*, one function entry or exit per line, indented by the recursion level. Also see edebug-tracing, in Section 17.2.12 [Trace Buffer], page 267. edebug-test-coverage [User Option] If non-nil, Edebug tests coverage of all expressions debugged. See Section 17.2.13 [Coverage Testing], page 268. Chapter 17: Debugging Lisp Programs 277 edebug-continue-kbd-macro [User Option] If non-nil, continue defining or executing any keyboard macro that is executing outside of Edebug. Use this with caution since it is not debugged. See Section 17.2.3 [Edebug Execution Modes], page 259. edebug-unwrap-results [User Option] If non-nil, Edebug tries to remove any of its own instrumentation when showing the results of expressions. This is relevant when debugging macros where the results of expressions are themselves instrumented expressions. As a very artificial example, suppose that the example function fac has been instrumented, and consider a macro of the form: (defmacro test () "Edebug example." (if (symbol-function ’fac) ...)) If you instrument the test macro and step through it, then by default the result of the symbol-function call has numerous edebug-after and edebug-before forms, which can make it difficult to see the “actual” result. If edebug-unwrap-results is non-nil, Edebug tries to remove these forms from the result. edebug-on-error [User Option] Edebug binds debug-on-error to this value, if debug-on-error was previously nil. See Section 17.2.7 [Trapping Errors], page 264. edebug-on-quit [User Option] Edebug binds debug-on-quit to this value, if debug-on-quit was previously nil. See Section 17.2.7 [Trapping Errors], page 264. If you change the values of edebug-on-error or edebug-on-quit while Edebug is active, their values won’t be used until the next time Edebug is invoked via a new command. edebug-global-break-condition [User Option] If non-nil, an expression to test for at every stop point. If the result is non-nil, then break. Errors are ignored. See Section 17.2.6.2 [Global Break Condition], page 263. 17.3 Debugging Invalid Lisp Syntax The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error “End of file during parsing” in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, “Invalid read syntax: ")"” indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change? If the problem is not simply an imbalance of parentheses, a useful technique is to try C-M-e at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun. However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases. (In addition, just moving point through the code with Show Paren mode enabled might find the mismatch.) Chapter 17: Debugging Lisp Programs 278 17.3.1 Excess Open Parentheses The first step is to find the defun that is unbalanced. If there is an excess open parenthesis, the way to do this is to go to the end of the file and type C-u C-M-u. This will move you to the beginning of the first defun that is unbalanced. The next step is to determine precisely what is wrong. There is no way to be sure of this except by studying the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with C-M-q and see what moves. But don’t do this yet! Keep reading, first. Before you do this, make sure the defun has enough close parentheses. Otherwise, C-M-q will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don’t use C-M-e to move there, since that too will fail to work until the defun is balanced. Now you can go to the beginning of the defun and type C-M-q. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don’t assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses. After you think you have fixed the problem, use C-M-q again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything. 17.3.2 Excess Close Parentheses To deal with an excess close parenthesis, first go to the beginning of the file, then type C-u -1 C-M-u to find the end of the first unbalanced defun. Then find the actual matching close parenthesis by typing C-M-f at the beginning of that defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity. If you don’t see a problem at that point, the next thing to do is to type C-M-q at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don’t assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses. After you think you have fixed the problem, use C-M-q again. If the old indentation actually fits the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything. 17.4 Test Coverage You can do coverage testing for a file of Lisp code by loading the testcover library and using the command M-x testcover-start RET file RET to instrument the code. Then test your code by calling it one or more times. Then use the command M-x testcover-markall to display colored highlights on the code to show where coverage is insufficient. The command M-x testcover-next-mark will move point forward to the next highlighted spot. Chapter 17: Debugging Lisp Programs 279 Normally, a red highlight indicates the form was never completely evaluated; a brown highlight means it always evaluated to the same value (meaning there has been little testing of what is done with the result). However, the red highlight is skipped for forms that can’t possibly complete their evaluation, such as error. The brown highlight is skipped for forms that are expected to always evaluate to the same value, such as (setq x 14). For difficult cases, you can add do-nothing macros to your code to give advice to the test coverage tool. 1value form [Macro] Evaluate form and return its value, but inform coverage testing that form’s value should always be the same. noreturn form [Macro] Evaluate form, informing coverage testing that form should never return. If it ever does return, you get a run-time error. Edebug also has a coverage testing feature (see Section 17.2.13 [Coverage Testing], page 268). These features partly duplicate each other, and it would be cleaner to combine them. 17.5 Profiling If your program is working correctly, but you want to make it run more quickly or efficiently, the first thing to do is profile your code so that you know how it is using resources. If you find that one particular function is responsible for a significant portion of the runtime, you can start looking for ways to optimize that piece. Emacs has built-in support for this. To begin profiling, type M-x profiler-start. You can choose to profile by processor usage, memory usage, or both. After doing some work, type M-x profiler-report to display a summary buffer for each resource that you chose to profile. The names of the report buffers include the times at which the reports were generated, so you can generate another report later on without erasing previous results. When you have finished profiling, type M-x profiler-stop (there is a small overhead associated with profiling). The profiler report buffer shows, on each line, a function that was called, followed by how much resource (processor or memory) it used in absolute and percentage times since profiling started. If a given line has a ‘+’ symbol at the left-hand side, you can expand that line by typing RET, in order to see the function(s) called by the higher-level function. Pressing RET again will collapse back to the original state. Press j or mouse-2 to jump to the definition of a function. Press d to view a function’s documentation. You can save a profile to a file using C-x C-w. You can compare two profiles using =. The elp library offers an alternative approach. See the file elp.el for instructions. You can check the speed of individual Emacs Lisp forms using the benchmark library. See the functions benchmark-run and benchmark-run-compiled in benchmark.el. Chapter 18: Reading and Printing Lisp Objects 280 18 Reading and Printing Lisp Objects Printing and reading are the operations of converting Lisp objects to textual form and vice versa. They use the printed representations and read syntax described in Chapter 2 [Lisp Data Types], page 8. This chapter describes the Lisp functions for reading and printing. It also describes streams, which specify where to get the text (if reading) or where to put it (if printing). 18.1 Introduction to Reading and Printing Reading a Lisp object means parsing a Lisp expression in textual form and producing a corresponding Lisp object. This is how Lisp programs get into Lisp from files of Lisp code. We call the text the read syntax of the object. For example, the text ‘(a . 5)’ is the read syntax for a cons cell whose car is a and whose cdr is the number 5. Printing a Lisp object means producing text that represents that object—converting the object to its printed representation (see Section 2.1 [Printed Representation], page 8). Printing the cons cell described above produces the text ‘(a . 5)’. Reading and printing are more or less inverse operations: printing the object that results from reading a given piece of text often produces the same text, and reading the text that results from printing an object usually produces a similar-looking object. For example, printing the symbol foo produces the text ‘foo’, and reading that text returns the symbol foo. Printing a list whose elements are a and b produces the text ‘(a b)’, and reading that text produces a list (but not the same list) with elements a and b. However, these two operations are not precisely inverse to each other. There are three kinds of exceptions: • Printing can produce text that cannot be read. For example, buffers, windows, frames, subprocesses and markers print as text that starts with ‘#’; if you try to read this text, you get an error. There is no way to read those data types. • One object can have multiple textual representations. For example, ‘1’ and ‘01’ represent the same integer, and ‘(a b)’ and ‘(a . (b))’ represent the same list. Reading will accept any of the alternatives, but printing must choose one of them. • Comments can appear at certain points in the middle of an object’s read sequence without affecting the result of reading it. 18.2 Input Streams Most of the Lisp functions for reading text take an input stream as an argument. The input stream specifies where or how to get the characters of the text to be read. Here are the possible types of input stream: buffer The input characters are read from buffer, starting with the character directly after point. Point advances as characters are read. marker The input characters are read from the buffer that marker is in, starting with the character directly after the marker. The marker position advances as characters are read. The value of point in the buffer has no effect when the stream is a marker. Chapter 18: Reading and Printing Lisp Objects 281 string The input characters are taken from string, starting at the first character in the string and using as many characters as required. function The input characters are generated by function, which must support two kinds of calls: • When it is called with no arguments, it should return the next character. • When it is called with one argument (always a character), function should save the argument and arrange to return it on the next call. This is called unreading the character; it happens when the Lisp reader reads one character too many and wants to “put it back where it came from”. In this case, it makes no difference what value function returns. t t used as a stream means that the input is read from the minibuffer. In fact, the minibuffer is invoked once and the text given by the user is made into a string that is then used as the input stream. If Emacs is running in batch mode, standard input is used instead of the minibuffer. For example, (message "%s" (read t)) will read a Lisp expression from standard input and print the result to standard output. nil nil supplied as an input stream means to use the value of standard-input instead; that value is the default input stream, and must be a non-nil input stream. symbol A symbol as input stream is equivalent to the symbol’s function definition (if any). Here is an example of reading from a stream that is a buffer, showing where point is located before and after: ---------- Buffer: foo ---------This is the contents of foo. ---------- Buffer: foo ---------- (read (get-buffer "foo")) ⇒ is (read (get-buffer "foo")) ⇒ the ---------- Buffer: foo ---------This is the contents of foo. ---------- Buffer: foo ---------- Note that the first read skips a space. Reading skips any amount of whitespace preceding the significant text. Here is an example of reading from a stream that is a marker, initially positioned at the beginning of the buffer shown. The value read is the symbol This. ---------- Buffer: foo ---------This is the contents of foo. ---------- Buffer: foo ---------- Chapter 18: Reading and Printing Lisp Objects 282 (setq m (set-marker (make-marker) 1 (get-buffer "foo"))) ⇒ # (read m) ⇒ This m ⇒ # ;; Before the first space. Here we read from the contents of a string: (read "(When in) the course") ⇒ (When in) The following example reads from the minibuffer. The prompt is: ‘Lisp expression: ’. (That is always the prompt used when you read from the stream t.) The user’s input is shown following the prompt. (read t) ⇒ 23 ---------- Buffer: Minibuffer ---------Lisp expression: 23 RET ---------- Buffer: Minibuffer ---------- Finally, here is an example of a stream that is a function, named useless-stream. Before we use the stream, we initialize the variable useless-list to a list of characters. Then each call to the function useless-stream obtains the next character in the list or unreads a character by adding it to the front of the list. (setq useless-list (append "XY()" nil)) ⇒ (88 89 40 41) (defun useless-stream (&optional unread) (if unread (setq useless-list (cons unread useless-list)) (prog1 (car useless-list) (setq useless-list (cdr useless-list))))) ⇒ useless-stream Now we read using the stream thus constructed: (read ’useless-stream) ⇒ XY useless-list ⇒ (40 41) Note that the open and close parentheses remain in the list. The Lisp reader encountered the open parenthesis, decided that it ended the input, and unread it. Another attempt to read from the stream at this point would read ‘()’ and return nil. 18.3 Input Functions This section describes the Lisp functions and variables that pertain to reading. Chapter 18: Reading and Printing Lisp Objects 283 In the functions below, stream stands for an input stream (see the previous section). If stream is nil or omitted, it defaults to the value of standard-input. An end-of-file error is signaled if reading encounters an unterminated list, vector, or string. read &optional stream [Function] This function reads one textual Lisp expression from stream, returning it as a Lisp object. This is the basic Lisp input function. read-from-string string &optional start end [Function] This function reads the first textual Lisp expression from the text in string. It returns a cons cell whose car is that expression, and whose cdr is an integer giving the position of the next remaining character in the string (i.e., the first one not read). If start is supplied, then reading begins at index start in the string (where the first character is at index 0). If you specify end, then reading is forced to stop just before that index, as if the rest of the string were not there. For example: (read-from-string "(setq x 55) (setq y 5)") ⇒ ((setq x 55) . 11) (read-from-string "\"A short string\"") ⇒ ("A short string" . 16) ;; Read starting at the first character. (read-from-string "(list 112)" 0) ⇒ ((list 112) . 10) ;; Read starting at the second character. (read-from-string "(list 112)" 1) ⇒ (list . 5) ;; Read starting at the seventh character, ;; and stopping at the ninth. (read-from-string "(list 112)" 6 8) ⇒ (11 . 8) standard-input [Variable] This variable holds the default input stream—the stream that read uses when the stream argument is nil. The default is t, meaning use the minibuffer. read-circle [Variable] If non-nil, this variable enables the reading of circular and shared structures. See Section 2.5 [Circular Objects], page 27. Its default value is t. 18.4 Output Streams An output stream specifies what to do with the characters produced by printing. Most print functions accept an output stream as an optional argument. Here are the possible types of output stream: buffer The output characters are inserted into buffer at point. Point advances as characters are inserted. Chapter 18: Reading and Printing Lisp Objects 284 marker The output characters are inserted into the buffer that marker points into, at the marker position. The marker position advances as characters are inserted. The value of point in the buffer has no effect on printing when the stream is a marker, and this kind of printing does not move point (except that if the marker points at or before the position of point, point advances with the surrounding text, as usual). function The output characters are passed to function, which is responsible for storing them away. It is called with a single character as argument, as many times as there are characters to be output, and is responsible for storing the characters wherever you want to put them. t The output characters are displayed in the echo area. nil nil specified as an output stream means to use the value of standard-output instead; that value is the default output stream, and must not be nil. symbol A symbol as output stream is equivalent to the symbol’s function definition (if any). Many of the valid output streams are also valid as input streams. The difference between input and output streams is therefore more a matter of how you use a Lisp object, than of different types of object. Here is an example of a buffer used as an output stream. Point is initially located as shown immediately before the ‘h’ in ‘the’. At the end, point is located directly before that same ‘h’. ---------- Buffer: foo ---------This is t he contents of foo. ---------- Buffer: foo ---------- (print "This is the output" (get-buffer "foo")) ⇒ "This is the output" ---------- Buffer: foo ---------This is t "This is the output" he contents of foo. ---------- Buffer: foo ---------- Now we show a use of a marker as an output stream. Initially, the marker is in buffer foo, between the ‘t’ and the ‘h’ in the word ‘the’. At the end, the marker has advanced over the inserted text so that it remains positioned before the same ‘h’. Note that the location of point, shown in the usual fashion, has no effect. ---------- Buffer: foo ---------This is the output ---------- Buffer: foo ---------- (setq m (copy-marker 10)) ⇒ # Chapter 18: Reading and Printing Lisp Objects 285 (print "More output for foo." m) ⇒ "More output for foo." ---------- Buffer: foo ---------This is t "More output for foo." he output ---------- Buffer: foo ---------- m ⇒ # The following example shows output to the echo area: (print "Echo Area output" t) ⇒ "Echo Area output" ---------- Echo Area ---------"Echo Area output" ---------- Echo Area ---------Finally, we show the use of a function as an output stream. The function eat-output takes each character that it is given and conses it onto the front of the list last-output (see Section 5.4 [Building Lists], page 68). At the end, the list contains all the characters output, but in reverse order. (setq last-output nil) ⇒ nil (defun eat-output (c) (setq last-output (cons c last-output))) ⇒ eat-output (print "This is the output" ’eat-output) ⇒ "This is the output" last-output ⇒ (10 34 116 117 112 116 117 111 32 101 104 116 32 115 105 32 115 105 104 84 34 10) Now we can put the output in the proper order by reversing the list: (concat (nreverse last-output)) ⇒" \"This is the output\" " Calling concat converts the list to a string so you can see its contents more clearly. 18.5 Output Functions This section describes the Lisp functions for printing Lisp objects—converting objects into their printed representation. Chapter 18: Reading and Printing Lisp Objects 286 Some of the Emacs printing functions add quoting characters to the output when necessary so that it can be read properly. The quoting characters used are ‘"’ and ‘\’; they distinguish strings from symbols, and prevent punctuation characters in strings and symbols from being taken as delimiters when reading. See Section 2.1 [Printed Representation], page 8, for full details. You specify quoting or no quoting by the choice of printing function. If the text is to be read back into Lisp, then you should print with quoting characters to avoid ambiguity. Likewise, if the purpose is to describe a Lisp object clearly for a Lisp programmer. However, if the purpose of the output is to look nice for humans, then it is usually better to print without quoting. Lisp objects can refer to themselves. Printing a self-referential object in the normal way would require an infinite amount of text, and the attempt could cause infinite recursion. Emacs detects such recursion and prints ‘#level’ instead of recursively printing an object already being printed. For example, here ‘#0’ indicates a recursive reference to the object at level 0 of the current print operation: (setq foo (list nil)) ⇒ (nil) (setcar foo foo) ⇒ (#0) In the functions below, stream stands for an output stream. (See the previous section for a description of output streams.) If stream is nil or omitted, it defaults to the value of standard-output. print object &optional stream [Function] The print function is a convenient way of printing. It outputs the printed represen- tation of object to stream, printing in addition one newline before object and another after it. Quoting characters are used. print returns object. For example: (progn (print ’The\ cat\ in) (print "the hat") (print " came back")) The\ cat\ in "the hat" " came back" ⇒ " came back" prin1 object &optional stream [Function] This function outputs the printed representation of object to stream. It does not print newlines to separate output as print does, but it does use quoting characters just like print. It returns object. (progn (prin1 ’The\ cat\ in) (prin1 "the hat") (prin1 " came back")) The\ cat\ in"the hat"" came back" ⇒ " came back" Chapter 18: Reading and Printing Lisp Objects 287 princ object &optional stream [Function] This function outputs the printed representation of object to stream. It returns object. This function is intended to produce output that is readable by people, not by read, so it doesn’t insert quoting characters and doesn’t put double-quotes around the contents of strings. It does not add any spacing between calls. (progn (princ ’The\ cat) (princ " in the \"hat\"")) The cat in the "hat" ⇒ " in the \"hat\"" terpri &optional stream [Function] This function outputs a newline to stream. The name stands for “terminate print”. write-char character &optional stream This function outputs character to stream. It returns character. [Function] prin1-to-string object &optional noescape [Function] This function returns a string containing the text that prin1 would have printed for the same argument. (prin1-to-string ’foo) ⇒ "foo" (prin1-to-string (mark-marker)) ⇒ "#" If noescape is non-nil, that inhibits use of quoting characters in the output. (This argument is supported in Emacs versions 19 and later.) (prin1-to-string "foo") ⇒ "\"foo\"" (prin1-to-string "foo" t) ⇒ "foo" See format, in Section 4.7 [Formatting Strings], page 57, for other ways to obtain the printed representation of a Lisp object as a string. with-output-to-string body. . . [Macro] This macro executes the body forms with standard-output set up to feed output into a string. Then it returns that string. For example, if the current buffer name is ‘foo’, (with-output-to-string (princ "The buffer is ") (princ (buffer-name))) returns "The buffer is foo". pp object &optional stream [Function] This function outputs object to stream, just like prin1, but does it in a more “pretty” way. That is, it’ll indent and fill the object to make it more readable for humans. Chapter 18: Reading and Printing Lisp Objects 288 18.6 Variables Affecting Output standard-output [Variable] The value of this variable is the default output stream—the stream that print func- tions use when the stream argument is nil. The default is t, meaning display in the echo area. print-quoted [Variable] If this is non-nil, that means to print quoted forms using abbreviated reader syntax, e.g., (quote foo) prints as ’foo, and (function foo) as #’foo. print-escape-newlines [Variable] If this variable is non-nil, then newline characters in strings are printed as ‘\n’ and formfeeds are printed as ‘\f’. Normally these characters are printed as actual newlines and formfeeds. This variable affects the print functions prin1 and print that print with quoting. It does not affect princ. Here is an example using prin1: (prin1 "a\nb") "a b" ⇒ "a b" (let ((print-escape-newlines t)) (prin1 "a\nb")) "a\nb" ⇒ "a b" In the second expression, the local binding of print-escape-newlines is in effect during the call to prin1, but not during the printing of the result. print-escape-nonascii [Variable] If this variable is non-nil, then unibyte non-ASCII characters in strings are uncondi- tionally printed as backslash sequences by the print functions prin1 and print that print with quoting. Those functions also use backslash sequences for unibyte non-ASCII characters, regardless of the value of this variable, when the output stream is a multibyte buffer or a marker pointing into one. print-escape-multibyte [Variable] If this variable is non-nil, then multibyte non-ASCII characters in strings are un- conditionally printed as backslash sequences by the print functions prin1 and print that print with quoting. Those functions also use backslash sequences for multibyte non-ASCII characters, regardless of the value of this variable, when the output stream is a unibyte buffer or a marker pointing into one. Chapter 18: Reading and Printing Lisp Objects 289 print-length [Variable] The value of this variable is the maximum number of elements to print in any list, vector or bool-vector. If an object being printed has more than this many elements, it is abbreviated with an ellipsis. If the value is nil (the default), then there is no limit. (setq print-length 2) ⇒2 (print ’(1 2 3 4 5)) (1 2 ...) ⇒ (1 2 ...) print-level [Variable] The value of this variable is the maximum depth of nesting of parentheses and brackets when printed. Any list or vector at a depth exceeding this limit is abbreviated with an ellipsis. A value of nil (which is the default) means no limit. eval-expression-print-length [User Option] eval-expression-print-level [User Option] These are the values for print-length and print-level used by eval-expression, and thus, indirectly, by many interactive evaluation commands (see Section “Evalu- ating Emacs-Lisp Expressions” in The GNU Emacs Manual). These variables are used for detecting and reporting circular and shared structure: print-circle [Variable] If non-nil, this variable enables detection of circular and shared structure in printing. See Section 2.5 [Circular Objects], page 27. print-gensym [Variable] If non-nil, this variable enables detection of uninterned symbols (see Section 8.3 [Creating Symbols], page 107) in printing. When this is enabled, uninterned symbols print with the prefix ‘#:’, which tells the Lisp reader to produce an uninterned symbol. print-continuous-numbering [Variable] If non-nil, that means number continuously across print calls. This affects the num- bers printed for ‘#n=’ labels and ‘#m#’ references. Don’t set this variable with setq; you should only bind it temporarily to t with let. When you do that, you should also bind print-number-table to nil. print-number-table [Variable] This variable holds a vector used internally by printing to implement the print-circle feature. You should not use it except to bind it to nil when you bind print-continuous-numbering. float-output-format [Variable] This variable specifies how to print floating-point numbers. The default is nil, meaning use the shortest output that represents the number without losing information. To control output format more precisely, you can put a string in this variable. The string should hold a ‘%’-specification to be used in the C function sprintf. For further restrictions on what you can use, see the variable’s documentation string. Chapter 19: Minibuffers 290 19 Minibuffers A minibuffer is a special buffer that Emacs commands use to read arguments more complicated than the single numeric prefix argument. These arguments include file names, buffer names, and command names (as in M-x). The minibuffer is displayed on the bottom line of the frame, in the same place as the echo area (see Section 37.4 [The Echo Area], page 836), but only while it is in use for reading an argument. 19.1 Introduction to Minibuffers In most ways, a minibuffer is a normal Emacs buffer. Most operations within a buffer, such as editing commands, work normally in a minibuffer. However, many operations for managing buffers do not apply to minibuffers. The name of a minibuffer always has the form ‘ *Minibuf-number*’, and it cannot be changed. Minibuffers are displayed only in special windows used only for minibuffers; these windows always appear at the bottom of a frame. (Sometimes frames have no minibuffer window, and sometimes a special kind of frame contains nothing but a minibuffer window; see Section 28.8 [Minibuffers and Frames], page 617.) The text in the minibuffer always starts with the prompt string, the text that was specified by the program that is using the minibuffer to tell the user what sort of input to type. This text is marked read-only so you won’t accidentally delete or change it. It is also marked as a field (see Section 31.19.9 [Fields], page 706), so that certain motion functions, including beginning-of-line, forward-word, forward-sentence, and forward-paragraph, stop at the boundary between the prompt and the actual text. The minibuffer’s window is normally a single line; it grows automatically if the contents require more space. Whilst it is active, you can explicitly resize it temporarily with the window sizing commands; it reverts to its normal size when the minibuffer is exited. When the minibuffer is not active, you can resize it permanently by using the window sizing commands in the frame’s other window, or dragging the mode line with the mouse. (Due to details of the current implementation, for this to work resize-mini-windows must be nil.) If the frame contains just a minibuffer, you can change the minibuffer’s size by changing the frame’s size. Use of the minibuffer reads input events, and that alters the values of variables such as this-command and last-command (see Section 20.5 [Command Loop Info], page 331). Your program should bind them around the code that uses the minibuffer, if you do not want that to change them. Under some circumstances, a command can use a minibuffer even if there is an active minibuffer; such a minibuffer is called a recursive minibuffer. The first minibuffer is named ‘ *Minibuf-1*’. Recursive minibuffers are named by incrementing the number at the end of the name. (The names begin with a space so that they won’t show up in normal buffer lists.) Of several recursive minibuffers, the innermost (or most recently entered) is the active minibuffer. We usually call this “the” minibuffer. You can permit or forbid recursive minibuffers by setting the variable enable-recursive-minibuffers, or by putting properties of that name on command symbols (See Section 19.13 [Recursive Mini], page 319.) Like other buffers, a minibuffer uses a local keymap (see Chapter 21 [Keymaps], page 367) to specify special key bindings. The function that invokes the minibuffer also sets up its Chapter 19: Minibuffers 291 local map according to the job to be done. See Section 19.2 [Text from Minibuffer], page 291, for the non-completion minibuffer local maps. See Section 19.6.3 [Completion Commands], page 303, for the minibuffer local maps for completion. When a minibuffer is inactive, its major mode is minibuffer-inactive-mode, with keymap minibuffer-inactive-mode-map. This is only really useful if the minibuffer is in a separate frame. See Section 28.8 [Minibuffers and Frames], page 617. When Emacs is running in batch mode, any request to read from the minibuffer actually reads a line from the standard input descriptor that was supplied when Emacs was started. This supports only basic input: none of the special minibuffer features (history, completion, password hiding, etc.) are available in batch mode. 19.2 Reading Text Strings with the Minibuffer The most basic primitive for minibuffer input is read-from-minibuffer, which can be used to read either a string or a Lisp object in textual form. The function read-regexp is used for reading regular expressions (see Section 33.3 [Regular Expressions], page 747), which are a special kind of string. There are also specialized functions for reading commands, variables, file names, etc. (see Section 19.6 [Completion], page 298). In most cases, you should not call minibuffer input functions in the middle of a Lisp function. Instead, do all minibuffer input as part of reading the arguments for a command, in the interactive specification. See Section 20.2 [Defining Commands], page 322. read-from-minibuffer prompt &optional initial keymap read history [Function] default inherit-input-method This function is the most general way to get input from the minibuffer. By default, it accepts arbitrary text and returns it as a string; however, if read is non-nil, then it uses read to convert the text into a Lisp object (see Section 18.3 [Input Functions], page 282). The first thing this function does is to activate a minibuffer and display it with prompt (which must be a string) as the prompt. Then the user can edit text in the minibuffer. When the user types a command to exit the minibuffer, read-from-minibuffer constructs the return value from the text in the minibuffer. Normally it returns a string containing that text. However, if read is non-nil, read-from-minibuffer reads the text and returns the resulting Lisp object, unevaluated. (See Section 18.3 [Input Functions], page 282, for information about reading.) The argument default specifies default values to make available through the history commands. It should be a string, a list of strings, or nil. The string or strings become the minibuffer’s “future history”, available to the user with M-n. If read is non-nil, then default is also used as the input to read, if the user enters empty input. If default is a list of strings, the first string is used as the input. If default is nil, empty input results in an end-of-file error. However, in the usual case (where read is nil), read-from-minibuffer ignores default when the user enters empty input and returns an empty string, "". In this respect, it differs from all the other minibuffer input functions in this chapter. If keymap is non-nil, that keymap is the local keymap to use in the minibuffer. If keymap is omitted or nil, the value of minibuffer-local-map is used as the Chapter 19: Minibuffers 292 keymap. Specifying a keymap is the most important way to customize the minibuffer for various applications such as completion. The argument history specifies a history list variable to use for saving the input and for history commands used in the minibuffer. It defaults to minibuffer-history. You can optionally specify a starting position in the history list as well. See Section 19.4 [Minibuffer History], page 295. If the variable minibuffer-allow-text-properties is non-nil, then the string that is returned includes whatever text properties were present in the minibuffer. Otherwise all the text properties are stripped when the value is returned. If the argument inherit-input-method is non-nil, then the minibuffer inherits the current input method (see Section 32.11 [Input Methods], page 741) and the setting of enable-multibyte-characters (see Section 32.1 [Text Representations], page 717) from whichever buffer was current before entering the minibuffer. Use of initial is mostly deprecated; we recommend using a non-nil value only in conjunction with specifying a cons cell for history. See Section 19.5 [Initial Input], page 297. read-string prompt &optional initial history default [Function] inherit-input-method This function reads a string from the minibuffer and returns it. The ar- guments prompt, initial, history and inherit-input-method are used as in read-from-minibuffer. The keymap used is minibuffer-local-map. The optional argument default is used as in read-from-minibuffer, except that, if non-nil, it also specifies a default value to return if the user enters null input. As in read-from-minibuffer it should be a string, a list of strings, or nil, which is equivalent to an empty string. When default is a string, that string is the default value. When it is a list of strings, the first string is the default value. (All these strings are available to the user in the “future minibuffer history”.) This function works by calling the read-from-minibuffer function: (read-string prompt initial history default inherit) ≡ (let ((value (read-from-minibuffer prompt initial nil nil history default inherit))) (if (and (equal value "") default) (if (consp default) (car default) default) value)) read-regexp prompt &optional defaults history [Function] This function reads a regular expression as a string from the minibuffer and returns it. If the minibuffer prompt string prompt does not end in ‘:’ (followed by optional whitespace), the function adds ‘: ’ to the end, preceded by the default return value (see below), if that is non-empty. The optional argument defaults controls the default value to return if the user enters null input, and should be one of: a string; nil, which is equivalent to an empty string; a list of strings; or a symbol. Chapter 19: Minibuffers 293 If defaults is a symbol, read-regexp consults the value of the variable read-regexpdefaults-function (see below), and if that is non-nil uses it in preference to defaults. The value in this case should be either: − regexp-history-last, which means to use the first element of the appropriate minibuffer history list (see below). − A function of no arguments, whose return value (which should be nil, a string, or a list of strings) becomes the value of defaults. read-regexp now ensures that the result of processing defaults is a list (i.e., if the value is nil or a string, it converts it to a list of one element). To this list, read-regexp then appends a few potentially useful candidates for input. These are: − The word or symbol at point. − The last regexp used in an incremental search. − The last string used in an incremental search. − The last string or pattern used in query-replace commands. The function now has a list of regular expressions that it passes to read-fromminibuffer to obtain the user’s input. The first element of the list is the default result in case of empty input. All elements of the list are available to the user as the “future minibuffer history list” (see Section “Minibuffer History” in The GNU Emacs Manual). The optional argument history, if non-nil, is a symbol specifying a minibuffer history list to use (see Section 19.4 [Minibuffer History], page 295). If it is omitted or nil, the history list defaults to regexp-history. read-regexp-defaults-function [Variable] The function read-regexp may use the value of this variable to determine its list of default regular expressions. If non-nil, the value of this variable should be either: − The symbol regexp-history-last. − A function of no arguments that returns either nil, a string, or a list of strings. See read-regexp above for details of how these values are used. minibuffer-allow-text-properties [Variable] If this variable is nil, then read-from-minibuffer and read-string strip all text properties from the minibuffer input before returning it. However, read-no-blanks- input (see below), as well as read-minibuffer and related functions (see Section 19.3 [Reading Lisp Objects With the Minibuffer], page 294), and all functions that do minibuffer input with completion, discard text properties unconditionally, regardless of the value of this variable. minibuffer-local-map [Variable] This is the default local keymap for reading from the minibuffer. By default, it makes the following bindings: C-j exit-minibuffer RET exit-minibuffer Chapter 19: Minibuffers 294 C-g abort-recursive-edit M-n DOWN M-p UP M-s M-r next-history-element previous-history-element next-matching-history-element previous-matching-history-element read-no-blanks-input prompt &optional initial inherit-input-method [Function] This function reads a string from the minibuffer, but does not allow whitespace characters as part of the input: instead, those characters terminate the input. The arguments prompt, initial, and inherit-input-method are used as in read-fromminibuffer. This is a simplified interface to the read-from-minibuffer function, and passes the value of the minibuffer-local-ns-map keymap as the keymap argument for that function. Since the keymap minibuffer-local-ns-map does not rebind C-q, it is possible to put a space into the string, by quoting it. This function discards text properties, regardless of the value of minibuffer-allowtext-properties. (read-no-blanks-input prompt initial) ≡ (let (minibuffer-allow-text-properties) (read-from-minibuffer prompt initial minibuffer-local-ns-map)) minibuffer-local-ns-map [Variable] This built-in variable is the keymap used as the minibuffer local keymap in the func- tion read-no-blanks-input. By default, it makes the following bindings, in addition to those of minibuffer-local-map: SPC exit-minibuffer TAB exit-minibuffer ? self-insert-and-exit 19.3 Reading Lisp Objects with the Minibuffer This section describes functions for reading Lisp objects with the minibuffer. read-minibuffer prompt &optional initial [Function] This function reads a Lisp object using the minibuffer, and returns it without evalu- ating it. The arguments prompt and initial are used as in read-from-minibuffer. This is a simplified interface to the read-from-minibuffer function: (read-minibuffer prompt initial) ≡ (let (minibuffer-allow-text-properties) (read-from-minibuffer prompt initial nil t)) Here is an example in which we supply the string "(testing)" as initial input: Chapter 19: Minibuffers 295 (read-minibuffer "Enter an expression: " (format "%s" ’(testing))) ;; Here is how the minibuffer is displayed: ---------- Buffer: Minibuffer ---------Enter an expression: (testing) ---------- Buffer: Minibuffer ---------- The user can type RET immediately to use the initial input as a default, or can edit the input. eval-minibuffer prompt &optional initial [Function] This function reads a Lisp expression using the minibuffer, evaluates it, then returns the result. The arguments prompt and initial are used as in read-from-minibuffer. This function simply evaluates the result of a call to read-minibuffer: (eval-minibuffer prompt initial) ≡ (eval (read-minibuffer prompt initial)) edit-and-eval-command prompt form [Function] This function reads a Lisp expression in the minibuffer, evaluates it, then returns the result. The difference between this command and eval-minibuffer is that here the initial form is not optional and it is treated as a Lisp object to be converted to printed representation rather than as a string of text. It is printed with prin1, so if it is a string, double-quote characters (‘"’) appear in the initial text. See Section 18.5 [Output Functions], page 285. In the following example, we offer the user an expression with initial text that is already a valid form: (edit-and-eval-command "Please edit: " ’(forward-word 1)) ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------Please edit: (forward-word 1) ---------- Buffer: Minibuffer ---------- Typing RET right away would exit the minibuffer and evaluate the expression, thus moving point forward one word. 19.4 Minibuffer History A minibuffer history list records previous minibuffer inputs so the user can reuse them conveniently. It is a variable whose value is a list of strings (previous inputs), most recent first. There are many separate minibuffer history lists, used for different kinds of inputs. It’s the Lisp programmer’s job to specify the right history list for each use of the minibuffer. You specify a minibuffer history list with the optional history argument to read-fromminibuffer or completing-read. Here are the possible values for it: variable Use variable (a symbol) as the history list. Chapter 19: Minibuffers 296 (variable . startpos) Use variable (a symbol) as the history list, and assume that the initial history position is startpos (a nonnegative integer). Specifying 0 for startpos is equivalent to just specifying the symbol variable. previous-history-element will display the most recent element of the history list in the minibuffer. If you specify a positive startpos, the minibuffer history functions behave as if (elt variable (1- startpos)) were the history element currently shown in the minibuffer. For consistency, you should also specify that element of the history as the initial minibuffer contents, using the initial argument to the minibuffer input function (see Section 19.5 [Initial Input], page 297). If you don’t specify history, then the default history list minibuffer-history is used. For other standard history lists, see below. You can also create your own history list variable; just initialize it to nil before the first use. Both read-from-minibuffer and completing-read add new elements to the history list automatically, and provide commands to allow the user to reuse items on the list. The only thing your program needs to do to use a history list is to initialize it and to pass its name to the input functions when you wish. But it is safe to modify the list by hand when the minibuffer input functions are not using it. Emacs functions that add a new element to a history list can also delete old elements if the list gets too long. The variable history-length specifies the maximum length for most history lists. To specify a different maximum length for a particular history list, put the length in the history-length property of the history list symbol. The variable history-delete-duplicates specifies whether to delete duplicates in history. add-to-history history-var newelt &optional maxelt keep-all [Function] This function adds a new element newelt, if it isn’t the empty string, to the history list stored in the variable history-var, and returns the updated history list. It limits the list length to the value of maxelt (if non-nil) or history-length (described below). The possible values of maxelt have the same meaning as the values of history-length. Normally, add-to-history removes duplicate members from the history list if history-delete-duplicates is non-nil. However, if keep-all is non-nil, that says not to remove duplicates, and to add newelt to the list even if it is empty. history-add-new-input [Variable] If the value of this variable is nil, standard functions that read from the minibuffer don’t add new elements to the history list. This lets Lisp programs explicitly manage input history by using add-to-history. The default value is t. history-length [User Option] The value of this variable specifies the maximum length for all history lists that don’t specify their own maximum lengths. If the value is t, that means there is no maximum (don’t delete old elements). If a history list variable’s symbol has a non-nil history-length property, it overrides this variable for that particular history list. history-delete-duplicates [User Option] If the value of this variable is t, that means when adding a new history element, all previous identical elements are deleted. Chapter 19: Minibuffers 297 Here are some of the standard minibuffer history list variables: minibuffer-history The default history list for minibuffer history input. [Variable] query-replace-history [Variable] A history list for arguments to query-replace (and similar arguments to other com- mands). file-name-history A history list for file-name arguments. [Variable] buffer-name-history A history list for buffer-name arguments. [Variable] regexp-history A history list for regular expression arguments. [Variable] extended-command-history A history list for arguments that are names of extended commands. [Variable] shell-command-history A history list for arguments that are shell commands. [Variable] read-expression-history A history list for arguments that are Lisp expressions to evaluate. [Variable] face-name-history A history list for arguments that are faces. [Variable] 19.5 Initial Input Several of the functions for minibuffer input have an argument called initial. This is a mostly-deprecated feature for specifying that the minibuffer should start out with certain text, instead of empty as usual. If initial is a string, the minibuffer starts out containing the text of the string, with point at the end, when the user starts to edit the text. If the user simply types RET to exit the minibuffer, it will use the initial input string to determine the value to return. We discourage use of a non-nil value for initial, because initial input is an intrusive interface. History lists and default values provide a much more convenient method to offer useful default inputs to the user. There is just one situation where you should specify a string for an initial argument. This is when you specify a cons cell for the history argument. See Section 19.4 [Minibuffer History], page 295. initial can also be a cons cell of the form (string . position). This means to insert string in the minibuffer but put point at position within the string’s text. As a historical accident, position was implemented inconsistently in different functions. In completing-read, position’s value is interpreted as origin-zero; that is, a value of 0 means the beginning of the string, 1 means after the first character, etc. In read-minibuffer, and Chapter 19: Minibuffers 298 the other non-completion minibuffer input functions that support this argument, 1 means the beginning of the string, 2 means after the first character, etc. Use of a cons cell as the value for initial arguments is deprecated. 19.6 Completion Completion is a feature that fills in the rest of a name starting from an abbreviation for it. Completion works by comparing the user’s input against a list of valid names and determining how much of the name is determined uniquely by what the user has typed. For example, when you type C-x b (switch-to-buffer) and then type the first few letters of the name of the buffer to which you wish to switch, and then type TAB (minibuffer-complete), Emacs extends the name as far as it can. Standard Emacs commands offer completion for names of symbols, files, buffers, and processes; with the functions in this section, you can implement completion for other kinds of names. The try-completion function is the basic primitive for completion: it returns the longest determined completion of a given initial string, with a given set of strings to match against. The function completing-read provides a higher-level interface for completion. A call to completing-read specifies how to determine the list of valid names. The function then activates the minibuffer with a local keymap that binds a few keys to commands useful for completion. Other functions provide convenient simple interfaces for reading certain kinds of names with completion. 19.6.1 Basic Completion Functions The following completion functions have nothing in themselves to do with minibuffers. We describe them here to keep them near the higher-level completion features that do use the minibuffer. try-completion string collection &optional predicate [Function] This function returns the longest common substring of all possible completions of string in collection. collection is called the completion table. Its value must be a list of strings or cons cells, an obarray, a hash table, or a completion function. try-completion compares string against each of the permissible completions specified by the completion table. If no permissible completions match, it returns nil. If there is just one matching completion, and the match is exact, it returns t. Otherwise, it returns the longest initial sequence common to all possible matching completions. If collection is an list, the permissible completions are specified by the elements of the list, each of which should be either a string, or a cons cell whose car is either a string or a symbol (a symbol is converted to a string using symbol-name). If the list contains elements of any other type, those are ignored. If collection is an obarray (see Section 8.3 [Creating Symbols], page 107), the names of all symbols in the obarray form the set of permissible completions. If collection is a hash table, then the keys that are strings are the possible completions. Other keys are ignored. Chapter 19: Minibuffers 299 You can also use a function as collection. Then the function is solely responsible for performing completion; try-completion returns whatever this function returns. The function is called with three arguments: string, predicate and nil (the third argument is so that the same function can be used in all-completions and do the appropriate thing in either case). See Section 19.6.7 [Programmed Completion], page 311. If the argument predicate is non-nil, then it must be a function of one argument, unless collection is a hash table, in which case it should be a function of two arguments. It is used to test each possible match, and the match is accepted only if predicate returns non-nil. The argument given to predicate is either a string or a cons cell (the car of which is a string) from the alist, or a symbol (not a symbol name) from the obarray. If collection is a hash table, predicate is called with two arguments, the string key and the associated value. In addition, to be acceptable, a completion must also match all the regular expressions in completion-regexp-list. (Unless collection is a function, in which case that function has to handle completion-regexp-list itself.) In the first of the following examples, the string ‘foo’ is matched by three of the alist cars. All of the matches begin with the characters ‘fooba’, so that is the result. In the second example, there is only one possible match, and it is exact, so the return value is t. (try-completion "foo" ’(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))) ⇒ "fooba" (try-completion "foo" ’(("barfoo" 2) ("foo" 3))) ⇒t In the following example, numerous symbols begin with the characters ‘forw’, and all of them begin with the word ‘forward’. In most of the symbols, this is followed with a ‘-’, but not in all, so no more than ‘forward’ can be completed. (try-completion "forw" obarray) ⇒ "forward" Finally, in the following example, only two of the three possible matches pass the predicate test (the string ‘foobaz’ is too short). Both of those begin with the string ‘foobar’. (defun test (s) (> (length (car s)) 6)) ⇒ test (try-completion "foo" ’(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) ’test) ⇒ "foobar" all-completions string collection &optional predicate [Function] This function returns a list of all possible completions of string. The arguments to this function are the same as those of try-completion, and it uses completion-regexp- list in the same way that try-completion does. Chapter 19: Minibuffers 300 If collection is a function, it is called with three arguments: string, predicate and t; then all-completions returns whatever the function returns. See Section 19.6.7 [Programmed Completion], page 311. Here is an example, using the function test shown in the example for try-completion: (defun test (s) (> (length (car s)) 6)) ⇒ test (all-completions "foo" ’(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) ’test) ⇒ ("foobar1" "foobar2") test-completion string collection &optional predicate [Function] This function returns non-nil if string is a valid completion alternative specified by collection and predicate. The arguments are the same as in try-completion. For instance, if collection is a list of strings, this is true if string appears in the list and predicate is satisfied. This function uses completion-regexp-list in the same way that try-completion does. If predicate is non-nil and if collection contains several strings that are equal to each other, as determined by compare-strings according to completion-ignore-case, then predicate should accept either all or none of them. Otherwise, the return value of test-completion is essentially unpredictable. If collection is a function, it is called with three arguments, the values string, predicate and lambda; whatever it returns, test-completion returns in turn. completion-boundaries string collection predicate suffix [Function] This function returns the boundaries of the field on which collection will operate, assuming that string holds the text before point and suffix holds the text after point. Normally completion operates on the whole string, so for all normal collections, this will always return (0 . (length suffix)). But more complex completion such as completion on files is done one field at a time. For example, completion of "/usr/sh" will include "/usr/share/" but not "/usr/share/doc" even if "/usr/share/doc" exists. Also all-completions on "/usr/sh" will not include "/usr/share/" but only "share/". So if string is "/usr/sh" and suffix is "e/doc", completion-boundaries will return (5 . 1) which tells us that the collection will only return completion information that pertains to the area after "/usr/" and before "/doc". If you store a completion alist in a variable, you should mark the variable as “risky” by giving it a non-nil risky-local-variable property. See Section 11.11 [File Local Variables], page 163. completion-ignore-case [Variable] If the value of this variable is non-nil, case is not considered significant in completion. Within read-file-name, this variable is overridden by read-file- name-completion-ignore-case (see Section 19.6.5 [Reading File Names], page 307); Chapter 19: Minibuffers 301 within read-buffer, it is overridden by read-buffer-completion-ignore-case (see Section 19.6.4 [High-Level Completion], page 305). completion-regexp-list [Variable] This is a list of regular expressions. The completion functions only consider a com- pletion acceptable if it matches all regular expressions in this list, with case-fold- search (see Section 33.2 [Searching and Case], page 746) bound to the value of completion-ignore-case. lazy-completion-table var fun [Macro] This macro provides a way to initialize the variable var as a collection for completion in a lazy way, not computing its actual contents until they are first needed. You use this macro to produce a value that you store in var. The actual computation of the proper value is done the first time you do completion using var. It is done by calling fun with no arguments. The value fun returns becomes the permanent value of var. Here is an example: (defvar foo (lazy-completion-table foo make-my-alist)) There are several functions that take an existing completion table and return a modified version. completion-table-case-fold returns a case-insensitive table. completion-table-in-turn and completion-table-merge combine multiple input tables in different ways. completion-table-subvert alters a table to use a different initial prefix. completion-table-with-quoting returns a table suitable for operating on quoted text. completion-table-with-predicate filters a table with a predicate function. completion-table-with-terminator adds a terminating string. 19.6.2 Completion and the Minibuffer This section describes the basic interface for reading from the minibuffer with completion. completing-read prompt collection &optional predicate require-match [Function] initial history default inherit-input-method This function reads a string in the minibuffer, assisting the user by providing completion. It activates the minibuffer with prompt prompt, which must be a string. The actual completion is done by passing the completion table collection and the completion predicate predicate to the function try-completion (see Section 19.6.1 [Basic Completion], page 298). This happens in certain commands bound in the local keymaps used for completion. Some of these commands also call test-completion. Thus, if predicate is non-nil, it should be compatible with collection and completion-ignore-case. See [Definition of test-completion], page 300. The value of the optional argument require-match determines how the user may exit the minibuffer: • If nil, the usual minibuffer exit commands work regardless of the input in the minibuffer. • If t, the usual minibuffer exit commands won’t exit unless the input completes to an element of collection. • If confirm, the user can exit with any input, but is asked for confirmation if the input is not an element of collection. Chapter 19: Minibuffers 302 • If confirm-after-completion, the user can exit with any input, but is asked for confirmation if the preceding command was a completion command (i.e., one of the commands in minibuffer-confirm-exit-commands) and the resulting input is not an element of collection. See Section 19.6.3 [Completion Commands], page 303. • Any other value of require-match behaves like t, except that the exit commands won’t exit if it performs completion. However, empty input is always permitted, regardless of the value of require-match; in that case, completing-read returns the first element of default, if it is a list; "", if default is nil; or default. The string or strings in default are also available to the user through the history commands. The function completing-read uses minibuffer-local-completion-map as the keymap if require-match is nil, and uses minibuffer-local-must-match-map if require-match is non-nil. See Section 19.6.3 [Completion Commands], page 303. The argument history specifies which history list variable to use for saving the input and for minibuffer history commands. It defaults to minibuffer-history. See Section 19.4 [Minibuffer History], page 295. The argument initial is mostly deprecated; we recommend using a non-nil value only in conjunction with specifying a cons cell for history. See Section 19.5 [Initial Input], page 297. For default input, use default instead. If the argument inherit-input-method is non-nil, then the minibuffer inherits the current input method (see Section 32.11 [Input Methods], page 741) and the setting of enable-multibyte-characters (see Section 32.1 [Text Representations], page 717) from whichever buffer was current before entering the minibuffer. If the variable completion-ignore-case is non-nil, completion ignores case when comparing the input against the possible matches. See Section 19.6.1 [Basic Completion], page 298. In this mode of operation, predicate must also ignore case, or you will get surprising results. Here’s an example of using completing-read: (completing-read "Complete a foo: " ’(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) nil t "fo") ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------Complete a foo: fo ---------- Buffer: Minibuffer ---------- If the user then types DEL DEL b RET, completing-read returns barfoo. The completing-read function binds variables to pass information to the commands that actually do completion. They are described in the following section. completing-read-function [Variable] The value of this variable must be a function, which is called by completing-read to actually do its work. It should accept the same arguments as completing-read. Chapter 19: Minibuffers 303 This can be bound to a different function to completely override the normal behavior of completing-read. 19.6.3 Minibuffer Commands that Do Completion This section describes the keymaps, commands and user options used in the minibuffer to do completion. minibuffer-completion-table [Variable] The value of this variable is the completion table used for completion in the minibuffer. This is the global variable that contains what completing-read passes to try-completion. It is used by minibuffer completion commands such as minibuffer-complete-word. minibuffer-completion-predicate [Variable] This variable’s value is the predicate that completing-read passes to try-completion. The variable is also used by the other minibuffer completion functions. minibuffer-completion-confirm [Variable] This variable determines whether Emacs asks for confirmation before exit- ing the minibuffer; completing-read binds this variable, and the function minibuffer-complete-and-exit checks the value before exiting. If the value is nil, confirmation is not required. If the value is confirm, the user may exit with an input that is not a valid completion alternative, but Emacs asks for confirmation. If the value is confirm-after-completion, the user may exit with an input that is not a valid completion alternative, but Emacs asks for confirmation if the user submitted the input right after any of the completion commands in minibuffer-confirm-exit-commands. minibuffer-confirm-exit-commands [Variable] This variable holds a list of commands that cause Emacs to ask for confirmation before exiting the minibuffer, if the require-match argument to completing-read is confirm-after-completion. The confirmation is requested if the user attempts to exit the minibuffer immediately after calling any command in this list. minibuffer-complete-word [Command] This function completes the minibuffer contents by at most a single word. Even if the minibuffer contents have only one completion, minibuffer-complete-word does not add any characters beyond the first character that is not a word constituent. See Chapter 34 [Syntax Tables], page 769. minibuffer-complete This function completes the minibuffer contents as far as possible. [Command] minibuffer-complete-and-exit [Command] This function completes the minibuffer contents, and exits if confirmation is not required, i.e., if minibuffer-completion-confirm is nil. If confirmation is required, it is given by repeating this command immediately—the command is programmed to work without confirmation when run twice in succession. Chapter 19: Minibuffers 304 minibuffer-completion-help [Command] This function creates a list of the possible completions of the current minibuffer contents. It works by calling all-completions using the value of the variable minibuffer-completion-table as the collection argument, and the value of minibuffer-completion-predicate as the predicate argument. The list of completions is displayed as text in a buffer named *Completions*. display-completion-list completions [Function] This function displays completions to the stream in standard-output, usually a buffer. (See Chapter 18 [Read and Print], page 280, for more information about streams.) The argument completions is normally a list of completions just returned by all-completions, but it does not have to be. Each element may be a symbol or a string, either of which is simply printed. It can also be a list of two strings, which is printed as if the strings were concatenated. The first of the two strings is the actual completion, the second string serves as annotation. This function is called by minibuffer-completion-help. A common way to use it is together with with-output-to-temp-buffer, like this: (with-output-to-temp-buffer "*Completions*" (display-completion-list (all-completions (buffer-string) my-alist))) completion-auto-help [User Option] If this variable is non-nil, the completion commands automatically display a list of possible completions whenever nothing can be completed because the next character is not uniquely determined. minibuffer-local-completion-map [Variable] completing-read uses this value as the local keymap when an exact match of one of the completions is not required. By default, this keymap makes the following bindings: ? minibuffer-completion-help SPC minibuffer-complete-word TAB minibuffer-complete and uses minibuffer-local-map as its parent keymap (see [Definition of minibufferlocal-map], page 293). minibuffer-local-must-match-map [Variable] completing-read uses this value as the local keymap when an exact match of one of the completions is required. Therefore, no keys are bound to exit-minibuffer, the command that exits the minibuffer unconditionally. By default, this keymap makes the following bindings: C-j minibuffer-complete-and-exit RET minibuffer-complete-and-exit and uses minibuffer-local-completion-map as its parent keymap. Chapter 19: Minibuffers 305 minibuffer-local-filename-completion-map [Variable] This is a sparse keymap that simply unbinds SPC; because filenames can contain spaces. The function read-file-name combines this keymap with either minibuffer-local-completion-map or minibuffer-local-must-match-map. 19.6.4 High-Level Completion Functions This section describes the higher-level convenience functions for reading certain sorts of names with completion. In most cases, you should not call these functions in the middle of a Lisp function. When possible, do all minibuffer input as part of reading the arguments for a command, in the interactive specification. See Section 20.2 [Defining Commands], page 322. read-buffer prompt &optional default require-match [Function] This function reads the name of a buffer and returns it as a string. The argument default is the default name to use, the value to return if the user exits with an empty minibuffer. If non-nil, it should be a string, a list of strings, or a buffer. If it is a list, the default value is the first element of this list. It is mentioned in the prompt, but is not inserted in the minibuffer as initial input. The argument prompt should be a string ending with a colon and a space. If default is non-nil, the function inserts it in prompt before the colon to follow the convention for reading from the minibuffer with a default value (see Section D.3 [Programming Tips], page 990). The optional argument require-match has the same meaning as in completing-read. See Section 19.6.2 [Minibuffer Completion], page 301. In the following example, the user enters ‘minibuffer.t’, and then types RET. The argument require-match is t, and the only buffer name starting with the given input is ‘minibuffer.texi’, so that name is the value. (read-buffer "Buffer name: " "foo" t) ;; After evaluation of the preceding expression, ;; the following prompt appears, ;; with an empty minibuffer: ---------- Buffer: Minibuffer ---------Buffer name (default foo): ---------- Buffer: Minibuffer ---------- ;; The user types minibuffer.t RET. ⇒ "minibuffer.texi" read-buffer-function [User Option] This variable, if non-nil, specifies a function for reading buffer names. read-buffer calls this function instead of doing its usual work, with the same arguments passed to read-buffer. read-buffer-completion-ignore-case [User Option] If this variable is non-nil, read-buffer ignores case when performing completion. Chapter 19: Minibuffers 306 read-command prompt &optional default [Function] This function reads the name of a command and returns it as a Lisp symbol. The argument prompt is used as in read-from-minibuffer. Recall that a command is anything for which commandp returns t, and a command name is a symbol for which commandp returns t. See Section 20.3 [Interactive Call], page 328. The argument default specifies what to return if the user enters null input. It can be a symbol, a string or a list of strings. If it is a string, read-command interns it before returning it. If it is a list, read-command interns the first element of this list. If default is nil, that means no default has been specified; then if the user enters null input, the return value is (intern ""), that is, a symbol whose name is an empty string. (read-command "Command name? ") ;; After evaluation of the preceding expression, ;; the following prompt appears with an empty minibuffer: ---------- Buffer: Minibuffer ---------Command name? ---------- Buffer: Minibuffer ---------- If the user types forward-c RET, then this function returns forward-char. The read-command function is a simplified interface to completing-read. It uses the variable obarray so as to complete in the set of extant Lisp symbols, and it uses the commandp predicate so as to accept only command names: (read-command prompt) ≡ (intern (completing-read prompt obarray ’commandp t nil)) read-variable prompt &optional default [Function] This function reads the name of a customizable variable and returns it as a sym- bol. Its arguments have the same form as those of read-command. It behaves just like read-command, except that it uses the predicate custom-variable-p instead of commandp. read-color &optional prompt convert allow-empty display [Command] This function reads a string that is a color specification, either the color’s name or an RGB hex value such as #RRRGGGBBB. It prompts with prompt (default: "Color (name or #RGB triplet):") and provides completion for color names, but not for hex RGB values. In addition to names of standard colors, completion candidates include the foreground and background colors at point. Valid RGB values are described in Section 28.20 [Color Names], page 626. The function’s return value is the string typed by the user in the minibuffer. However, when called interactively or if the optional argument convert is non-nil, it converts any input color name into the corresponding RGB value string and instead returns that. This function requires a valid color specification to be input. Empty color names are allowed when allow-empty is non-nil and the user enters null input. Chapter 19: Minibuffers 307 Interactively, or when display is non-nil, the return value is also displayed in the echo area. See also the functions read-coding-system and read-non-nil-coding-system, in Section 32.10.4 [User-Chosen Coding Systems], page 734, and read-input-method-name, in Section 32.11 [Input Methods], page 741. 19.6.5 Reading File Names The high-level completion functions read-file-name, read-directory-name, and read-shell-command are designed to read file names, directory names, and shell commands, respectively. They provide special features, including automatic insertion of the default directory. read-file-name prompt &optional directory default require-match [Function] initial predicate This function reads a file name, prompting with prompt and providing completion. As an exception, this function reads a file name using a graphical file dialog instead of the minibuffer, if all of the following are true: 1. It is invoked via a mouse command. 2. The selected frame is on a graphical display supporting such dialogs. 3. The variable use-dialog-box is non-nil. See Section “Dialog Boxes” in The GNU Emacs Manual. 4. The directory argument, described below, does not specify a remote file. See Section “Remote Files” in The GNU Emacs Manual. The exact behavior when using a graphical file dialog is platform-dependent. Here, we simply document the behavior when using the minibuffer. read-file-name does not automatically expand the returned file name. You must call expand-file-name yourself if an absolute file name is required. The optional argument require-match has the same meaning as in completing-read. See Section 19.6.2 [Minibuffer Completion], page 301. The argument directory specifies the directory to use for completing relative file names. It should be an absolute directory name. If the variable insert-defaultdirectory is non-nil, directory is also inserted in the minibuffer as initial input. It defaults to the current buffer’s value of default-directory. If you specify initial, that is an initial file name to insert in the buffer (after directory, if that is inserted). In this case, point goes at the beginning of initial. The default for initial is nil—don’t insert any file name. To see what initial does, try the command C-x C-v in a buffer visiting a file. Please note: we recommend using default rather than initial in most cases. If default is non-nil, then the function returns default if the user exits the minibuffer with the same non-empty contents that read-file-name inserted initially. The initial minibuffer contents are always non-empty if insert-default-directory is non-nil, as it is by default. default is not checked for validity, regardless of the value of requirematch. However, if require-match is non-nil, the initial minibuffer contents should be a valid file (or directory) name. Otherwise read-file-name attempts completion Chapter 19: Minibuffers 308 if the user exits without any editing, and does not return default. default is also available through the history commands. If default is nil, read-file-name tries to find a substitute default to use in its place, which it treats in exactly the same way as if it had been specified explicitly. If default is nil, but initial is non-nil, then the default is the absolute file name obtained from directory and initial. If both default and initial are nil and the buffer is visiting a file, read-file-name uses the absolute file name of that file as default. If the buffer is not visiting a file, then there is no default. In that case, if the user types RET without any editing, read-file-name simply returns the pre-inserted contents of the minibuffer. If the user types RET in an empty minibuffer, this function returns an empty string, regardless of the value of require-match. This is, for instance, how the user can make the current buffer visit no file using M-x set-visited-file-name. If predicate is non-nil, it specifies a function of one argument that decides which file names are acceptable completion alternatives. A file name is an acceptable value if predicate returns non-nil for it. Here is an example of using read-file-name: (read-file-name "The file is ") ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------The file is /gp/gnu/elisp/ ---------- Buffer: Minibuffer ---------- Typing manual TAB results in the following: ---------- Buffer: Minibuffer ---------The file is /gp/gnu/elisp/manual.texi ---------- Buffer: Minibuffer ---------- If the user types RET, read-file-name returns the file name as the string "/gp/gnu/elisp/manual.texi". read-file-name-function [Variable] If non-nil, this should be a function that accepts the same arguments as read-file- name. When read-file-name is called, it calls this function with the supplied argu- ments instead of doing its usual work. read-file-name-completion-ignore-case [User Option] If this variable is non-nil, read-file-name ignores case when performing completion. read-directory-name prompt &optional directory default [Function] require-match initial This function is like read-file-name but allows only directory names as completion alternatives. If default is nil and initial is non-nil, read-directory-name constructs a substitute default by combining directory (or the current buffer’s default directory if directory Chapter 19: Minibuffers 309 is nil) and initial. If both default and initial are nil, this function uses directory as substitute default, or the current buffer’s default directory if directory is nil. insert-default-directory [User Option] This variable is used by read-file-name, and thus, indirectly, by most commands reading file names. (This includes all commands that use the code letters ‘f’ or ‘F’ in their interactive form. See Section 20.2.2 [Code Characters for interactive], page 324.) Its value controls whether read-file-name starts by placing the name of the default directory in the minibuffer, plus the initial file name, if any. If the value of this variable is nil, then read-file-name does not place any initial input in the minibuffer (unless you specify initial input with the initial argument). In that case, the default directory is still used for completion of relative file names, but is not displayed. If this variable is nil and the initial minibuffer contents are empty, the user may have to explicitly fetch the next history element to access a default value. If the variable is non-nil, the initial minibuffer contents are always non-empty and the user can always request a default value by immediately typing RET in an unedited minibuffer. (See above.) For example: ;; Here the minibuffer starts out with the default directory. (let ((insert-default-directory t)) (read-file-name "The file is ")) ---------- Buffer: Minibuffer ---------The file is ~lewis/manual/ ---------- Buffer: Minibuffer ---------- ;; Here the minibuffer is empty and only the prompt ;; appears on its line. (let ((insert-default-directory nil)) (read-file-name "The file is ")) ---------- Buffer: Minibuffer ---------The file is ---------- Buffer: Minibuffer ---------- read-shell-command prompt &optional initial history &rest args [Function] This function reads a shell command from the minibuffer, prompting with prompt and providing intelligent completion. It completes the first word of the command using candidates that are appropriate for command names, and the rest of the command words as file names. This function uses minibuffer-local-shell-command-map as the keymap for minibuffer input. The history argument specifies the history list to use; if is omitted or nil, it defaults to shell-command-history (see Section 19.4 [Minibuffer History], page 295). The optional argument initial specifies the initial content of the minibuffer (see Section 19.5 [Initial Input], page 297). The rest of args, if present, are used Chapter 19: Minibuffers 310 as the default and inherit-input-method arguments in read-from-minibuffer (see Section 19.2 [Text from Minibuffer], page 291). minibuffer-local-shell-command-map [Variable] This keymap is used by read-shell-command for completing command and file names that are part of a shell command. It uses minibuffer-local-map as its parent keymap, and binds TAB to completion-at-point. 19.6.6 Completion Variables Here are some variables that can be used to alter the default completion behavior. completion-styles [User Option] The value of this variable is a list of completion style (symbols) to use for performing completion. A completion style is a set of rules for generating completions. Each symbol occurring this list must have a corresponding entry in completion-styles- alist. completion-styles-alist [Variable] This variable stores a list of available completion styles. Each element in the list has the form (style try-completion all-completions doc) Here, style is the name of the completion style (a symbol), which may be used in the completion-styles variable to refer to this style; try-completion is the function that does the completion; all-completions is the function that lists the completions; and doc is a string describing the completion style. The try-completion and all-completions functions should each accept four arguments: string, collection, predicate, and point. The string, collection, and predicate arguments have the same meanings as in try-completion (see Section 19.6.1 [Basic Completion], page 298), and the point argument is the position of point within string. Each function should return a non-nil value if it performed its job, and nil if it did not (e.g., if there is no way to complete string according to the completion style). When the user calls a completion command like minibuffer-complete (see Section 19.6.3 [Completion Commands], page 303), Emacs looks for the first style listed in completion-styles and calls its try-completion function. If this function returns nil, Emacs moves to the next listed completion style and calls its try-completion function, and so on until one of the try-completion functions successfully performs completion and returns a non-nil value. A similar procedure is used for listing completions, via the all-completions functions. See Section “Completion Styles” in The GNU Emacs Manual, for a description of the available completion styles. completion-category-overrides [User Option] This variable specifies special completion styles and other completion behaviors to use when completing certain types of text. Its value should be an alist with elements of the form (category . alist). category is a symbol describing what is being completed; currently, the buffer, file, and unicode-name categories are defined, but others can be defined via specialized completion functions (see Section 19.6.7 [Programmed Chapter 19: Minibuffers 311 Completion], page 311). alist is an association list describing how completion should behave for the corresponding category. The following alist keys are supported: styles The value should be a list of completion styles (symbols). cycle The value should be a value for completion-cycle-threshold (see Section “Completion Options” in The GNU Emacs Manual) for this category. Additional alist entries may be defined in the future. completion-extra-properties [Variable] This variable is used to specify extra properties of the current completion command. It is intended to be let-bound by specialized completion commands. Its value should be a list of property and value pairs. The following properties are supported: :annotation-function The value should be a function to add annotations in the completions buffer. This function must accept one argument, a completion, and should either return nil or a string to be displayed next to the completion. :exit-function The value should be a function to run after performing completion. The function should accept two arguments, string and status, where string is the text to which the field was completed, and status indicates what kind of operation happened: finished if text is now complete, sole if the text cannot be further completed but completion is not finished, or exact if the text is a valid completion but may be further completed. 19.6.7 Programmed Completion Sometimes it is not possible or convenient to create an alist or an obarray containing all the intended possible completions ahead of time. In such a case, you can supply your own function to compute the completion of a given string. This is called programmed completion. Emacs uses programmed completion when completing file names (see Section 24.8.6 [File Name Completion], page 499), among many other cases. To use this feature, pass a function as the collection argument to completing-read. The function completing-read arranges to pass your completion function along to try-completion, all-completions, and other basic completion functions, which will then let your function do all the work. The completion function should accept three arguments: • The string to be completed. • A predicate function with which to filter possible matches, or nil if none. The function should call the predicate for each possible match, and ignore the match if the predicate returns nil. • A flag specifying the type of completion operation to perform. This is one of the following four values: nil This specifies a try-completion operation. The function should return t if the specified string is a unique and exact match; if there is more than one Chapter 19: Minibuffers 312 match, it should return the common substring of all matches (if the string is an exact match for one completion alternative but also matches other longer alternatives, the return value is the string); if there are no matches, it should return nil. t This specifies an all-completions operation. The function should return a list of all possible completions of the specified string. lambda This specifies a test-completion operation. The function should return t if the specified string is an exact match for some completion alternative; nil otherwise. (boundaries . suffix) This specifies a completion-boundaries operation. The function should return (boundaries start . end), where start is the position of the beginning boundary in the specified string, and end is the position of the end boundary in suffix. metadata This specifies a request for information about the state of the current completion. The return value should have the form (metadata . alist), where alist is an alist whose elements are described below. If the flag has any other value, the completion function should return nil. The following is a list of metadata entries that a completion function may return in response to a metadata flag argument: category The value should be a symbol describing what kind of text the completion function is trying to complete. If the symbol matches one of the keys in completion-category-overrides, the usual completion behavior is overridden. See Section 19.6.6 [Completion Variables], page 310. annotation-function The value should be a function for annotating completions. The function should take one argument, string, which is a possible completion. It should return a string, which is displayed after the completion string in the *Completions* buffer. display-sort-function The value should be a function for sorting completions. The function should take one argument, a list of completion strings, and return a sorted list of completion strings. It is allowed to alter the input list destructively. cycle-sort-function The value should be a function for sorting completions, when completion-cycle-threshold is non-nil and the user is cycling through completion alternatives. See Section “Completion Options” in The GNU Emacs Manual. Its argument list and return value are the same as for display-sort-function. completion-table-dynamic function [Function] This function is a convenient way to write a function that can act as a programmed completion function. The argument function should be a function that takes one Chapter 19: Minibuffers 313 argument, a string, and returns an alist of possible completions of it. You can think of completion-table-dynamic as a transducer between that interface and the interface for programmed completion functions. completion-table-with-cache function &optional ignore-case [Function] This is a wrapper for completion-table-dynamic that saves the last argument-result pair. This means that multiple lookups with the same argument only need to call function once. This can be useful when a slow operation is involved, such as calling an external process. 19.6.8 Completion in Ordinary Buffers Although completion is usually done in the minibuffer, the completion facility can also be used on the text in ordinary Emacs buffers. In many major modes, in-buffer completion is performed by the C-M-i or M-TAB command, bound to completion-at-point. See Section “Symbol Completion” in The GNU Emacs Manual. This command uses the abnormal hook variable completion-at-point-functions: completion-at-point-functions [Variable] The value of this abnormal hook should be a list of functions, which are used to compute a completion table for completing the text at point. It can be used by major modes to provide mode-specific completion tables (see Section 22.2.1 [Major Mode Conventions], page 409). When the command completion-at-point runs, it calls the functions in the list one by one, without any argument. Each function should return nil if it is unable to produce a completion table for the text at point. Otherwise it should return a list of the form (start end collection . props) start and end delimit the text to complete (which should enclose point). collection is a completion table for completing that text, in a form suitable for passing as the second argument to try-completion (see Section 19.6.1 [Basic Completion], page 298); completion alternatives will be generated from this completion table in the usual way, via the completion styles defined in completion-styles (see Section 19.6.6 [Completion Variables], page 310). props is a property list for additional information; any of the properties in completion-extra-properties are recognized (see Section 19.6.6 [Completion Variables], page 310), as well as the following additional ones: :predicate The value should be a predicate that completion candidates need to satisfy. :exclusive If the value is no, then if the completion table fails to match the text at point, completion-at-point moves on to the next function in completion-at-point-functions instead of reporting a completion failure. A function in completion-at-point-functions may also return a function. In that case, that returned function is called, with no argument, and it is entirely responsible Chapter 19: Minibuffers 314 for performing the completion. We discourage this usage; it is intended to help convert old code to using completion-at-point. The first function in completion-at-point-functions to return a non-nil value is used by completion-at-point. The remaining functions are not called. The exception to this is when there is an :exclusive specification, as described above. The following function provides a convenient way to perform completion on an arbitrary stretch of text in an Emacs buffer: completion-in-region start end collection &optional predicate [Function] This function completes the text in the current buffer between the positions start and end, using collection. The argument collection has the same meaning as in try-completion (see Section 19.6.1 [Basic Completion], page 298). This function inserts the completion text directly into the current buffer. Unlike completing-read (see Section 19.6.2 [Minibuffer Completion], page 301), it does not activate the minibuffer. For this function to work, point must be somewhere between start and end. 19.7 Yes-or-No Queries This section describes functions used to ask the user a yes-or-no question. The function y-or-n-p can be answered with a single character; it is useful for questions where an inadvertent wrong answer will not have serious consequences. yes-or-no-p is suitable for more momentous questions, since it requires three or four characters to answer. If either of these functions is called in a command that was invoked using the mouse— more precisely, if last-nonmenu-event (see Section 20.5 [Command Loop Info], page 331) is either nil or a list—then it uses a dialog box or pop-up menu to ask the question. Otherwise, it uses keyboard input. You can force use either of the mouse or of keyboard input by binding last-nonmenu-event to a suitable value around the call. Strictly speaking, yes-or-no-p uses the minibuffer and y-or-n-p does not; but it seems best to describe them together. y-or-n-p prompt [Function] This function asks the user a question, expecting input in the echo area. It returns t if the user types y, nil if the user types n. This function also accepts SPC to mean yes and DEL to mean no. It accepts C-] to mean “quit”, like C-g, because the question might look like a minibuffer and for that reason the user might try to use C-] to get out. The answer is a single character, with no RET needed to terminate it. Upper and lower case are equivalent. “Asking the question” means printing prompt in the echo area, followed by the string ‘(y or n) ’. If the input is not one of the expected answers (y, n, SPC, DEL, or something that quits), the function responds ‘Please answer y or n.’, and repeats the request. This function does not actually use the minibuffer, since it does not allow editing of the answer. It actually uses the echo area (see Section 37.4 [The Echo Area], page 836), which uses the same screen space as the minibuffer. The cursor moves to the echo area while the question is being asked. Chapter 19: Minibuffers 315 The answers and their meanings, even ‘y’ and ‘n’, are not hardwired, and are specified by the keymap query-replace-map (see Section 33.7 [Search and Replace], page 765). In particular, if the user enters the special responses recenter, scroll-up, scroll-down, scroll-other-window, or scroll-other-window-down (respectively bound to C-l, C-v, M-v, C-M-v and C-M-S-v in query-replace-map), this function performs the specified window recentering or scrolling operation, and poses the question again. We show successive lines of echo area messages, but only one actually appears on the screen at a time. y-or-n-p-with-timeout prompt seconds default [Function] Like y-or-n-p, except that if the user fails to answer within seconds seconds, this function stops waiting and returns default. It works by setting up a timer; see Section 38.10 [Timers], page 944. The argument seconds should be a number. yes-or-no-p prompt [Function] This function asks the user a question, expecting input in the minibuffer. It returns t if the user enters ‘yes’, nil if the user types ‘no’. The user must type RET to finalize the response. Upper and lower case are equivalent. yes-or-no-p starts by displaying prompt in the echo area, followed by ‘(yes or no) ’. The user must type one of the expected responses; otherwise, the function responds ‘Please answer yes or no.’, waits about two seconds and repeats the request. yes-or-no-p requires more work from the user than y-or-n-p and is appropriate for more crucial decisions. Here is an example: (yes-or-no-p "Do you really want to remove everything? ") ;; After evaluation of the preceding expression, ;; the following prompt appears, ;; with an empty minibuffer: ---------- Buffer: minibuffer ---------Do you really want to remove everything? (yes or no) ---------- Buffer: minibuffer ---------- If the user first types y RET, which is invalid because this function demands the entire word ‘yes’, it responds by displaying these prompts, with a brief pause between them: ---------- Buffer: minibuffer ---------Please answer yes or no. Do you really want to remove everything? (yes or no) ---------- Buffer: minibuffer ---------- 19.8 Asking Multiple Y-or-N Questions When you have a series of similar questions to ask, such as “Do you want to save this buffer” for each buffer in turn, you should use map-y-or-n-p to ask the collection of questions, rather than asking each question individually. This gives the user certain convenient facilities such as the ability to answer the whole series at once. Chapter 19: Minibuffers 316 map-y-or-n-p prompter actor list &optional help action-alist [Function] no-cursor-in-echo-area This function asks the user a series of questions, reading a single-character answer in the echo area for each one. The value of list specifies the objects to ask questions about. It should be either a list of objects or a generator function. If it is a function, it should expect no arguments, and should return either the next object to ask about, or nil, meaning to stop asking questions. The argument prompter specifies how to ask each question. If prompter is a string, the question text is computed like this: (format prompter object) where object is the next object to ask about (as obtained from list). If not a string, prompter should be a function of one argument (the next object to ask about) and should return the question text. If the value is a string, that is the question to ask the user. The function can also return t, meaning do act on this object (and don’t ask the user), or nil, meaning ignore this object (and don’t ask the user). The argument actor says how to act on the answers that the user gives. It should be a function of one argument, and it is called with each object that the user says yes for. Its argument is always an object obtained from list. If the argument help is given, it should be a list of this form: (singular plural action) where singular is a string containing a singular noun that describes the objects conceptually being acted on, plural is the corresponding plural noun, and action is a transitive verb describing what actor does. If you don’t specify help, the default is ("object" "objects" "act on"). Each time a question is asked, the user may enter y, Y, or SPC to act on that object; n, N, or DEL to skip that object; ! to act on all following objects; ESC or q to exit (skip all following objects); . (period) to act on the current object and then exit; or C-h to get help. These are the same answers that query-replace accepts. The keymap query-replace-map defines their meaning for map-y-or-n-p as well as for query-replace; see Section 33.7 [Search and Replace], page 765. You can use action-alist to specify additional possible answers and what they mean. It is an alist of elements of the form (char function help), each of which defines one additional answer. In this element, char is a character (the answer); function is a function of one argument (an object from list); help is a string. When the user responds with char, map-y-or-n-p calls function. If it returns nonnil, the object is considered “acted upon”, and map-y-or-n-p advances to the next object in list. If it returns nil, the prompt is repeated for the same object. Normally, map-y-or-n-p binds cursor-in-echo-area while prompting. But if nocursor-in-echo-area is non-nil, it does not do that. If map-y-or-n-p is called in a command that was invoked using the mouse—more precisely, if last-nonmenu-event (see Section 20.5 [Command Loop Info], page 331) Chapter 19: Minibuffers 317 is either nil or a list—then it uses a dialog box or pop-up menu to ask the question. In this case, it does not use keyboard input or the echo area. You can force use either of the mouse or of keyboard input by binding last-nonmenu-event to a suitable value around the call. The return value of map-y-or-n-p is the number of objects acted on. 19.9 Reading a Password To read a password to pass to another program, you can use the function read-passwd. read-passwd prompt &optional confirm default [Function] This function reads a password, prompting with prompt. It does not echo the pass- word as the user types it; instead, it echoes ‘.’ for each character in the password. (Note that in batch mode, the input is not hidden.) The optional argument confirm, if non-nil, says to read the password twice and insist it must be the same both times. If it isn’t the same, the user has to type it over and over until the last two times match. The optional argument default specifies the default password to return if the user enters empty input. If default is nil, then read-passwd returns the null string in that case. 19.10 Minibuffer Commands This section describes some commands meant for use in the minibuffer. exit-minibuffer [Command] This command exits the active minibuffer. It is normally bound to keys in minibuffer local keymaps. self-insert-and-exit [Command] This command exits the active minibuffer after inserting the last character typed on the keyboard (found in last-command-event; see Section 20.5 [Command Loop Info], page 331). previous-history-element n [Command] This command replaces the minibuffer contents with the value of the nth previous (older) history element. next-history-element n [Command] This command replaces the minibuffer contents with the value of the nth more recent history element. previous-matching-history-element pattern n [Command] This command replaces the minibuffer contents with the value of the nth previous (older) history element that matches pattern (a regular expression). next-matching-history-element pattern n [Command] This command replaces the minibuffer contents with the value of the nth next (newer) history element that matches pattern (a regular expression). Chapter 19: Minibuffers 318 previous-complete-history-element n [Command] This command replaces the minibuffer contents with the value of the nth previous (older) history element that completes the current contents of the minibuffer before the point. next-complete-history-element n [Command] This command replaces the minibuffer contents with the value of the nth next (newer) history element that completes the current contents of the minibuffer before the point. 19.11 Minibuffer Windows These functions access and select minibuffer windows and test whether they are active. active-minibuffer-window [Function] This function returns the currently active minibuffer window, or nil if there is none. minibuffer-window &optional frame [Function] This function returns the minibuffer window used for frame frame. If frame is nil, that stands for the current frame. Note that the minibuffer window used by a frame need not be part of that frame—a frame that has no minibuffer of its own necessarily uses some other frame’s minibuffer window. set-minibuffer-window window [Function] This function specifies window as the minibuffer window to use. This affects where the minibuffer is displayed if you put text in it without invoking the usual minibuffer commands. It has no effect on the usual minibuffer input functions because they all start by choosing the minibuffer window according to the current frame. window-minibuffer-p &optional window [Function] This function returns non-nil if window is a minibuffer window. window defaults to the selected window. It is not correct to determine whether a given window is a minibuffer by comparing it with the result of (minibuffer-window), because there can be more than one minibuffer window if there is more than one frame. minibuffer-window-active-p window [Function] This function returns non-nil if window is the currently active minibuffer window. 19.12 Minibuffer Contents These functions access the minibuffer prompt and contents. minibuffer-prompt [Function] This function returns the prompt string of the currently active minibuffer. If no minibuffer is active, it returns nil. minibuffer-prompt-end [Function] This function returns the current position of the end of the minibuffer prompt, if a minibuffer is current. Otherwise, it returns the minimum valid buffer position. Chapter 19: Minibuffers 319 minibuffer-prompt-width [Function] This function returns the current display-width of the minibuffer prompt, if a mini- buffer is current. Otherwise, it returns zero. minibuffer-contents [Function] This function returns the editable contents of the minibuffer (that is, everything except the prompt) as a string, if a minibuffer is current. Otherwise, it returns the entire contents of the current buffer. minibuffer-contents-no-properties [Function] This is like minibuffer-contents, except that it does not copy text properties, just the characters themselves. See Section 31.19 [Text Properties], page 690. delete-minibuffer-contents [Function] This function erases the editable contents of the minibuffer (that is, everything except the prompt), if a minibuffer is current. Otherwise, it erases the entire current buffer. 19.13 Recursive Minibuffers These functions and variables deal with recursive minibuffers (see Section 20.13 [Recursive Editing], page 362): minibuffer-depth [Function] This function returns the current depth of activations of the minibuffer, a nonnegative integer. If no minibuffers are active, it returns zero. enable-recursive-minibuffers [User Option] If this variable is non-nil, you can invoke commands (such as find-file) that use minibuffers even while the minibuffer window is active. Such invocation produces a recursive editing level for a new minibuffer. The outer-level minibuffer is invisible while you are editing the inner one. If this variable is nil, you cannot invoke minibuffer commands when the minibuffer window is active, not even if you switch to another window to do it. If a command name has a property enable-recursive-minibuffers that is non-nil, then the command can use the minibuffer to read arguments even if it is invoked from the minibuffer. A command can also achieve this by binding enable-recursive-minibuffers to t in the interactive declaration (see Section 20.2.1 [Using Interactive], page 322). The minibuffer command next-matching-history-element (normally M-s in the minibuffer) does the latter. 19.14 Minibuffer Miscellany minibufferp &optional buffer-or-name [Function] This function returns non-nil if buffer-or-name is a minibuffer. If buffer-or-name is omitted, it tests the current buffer. minibuffer-setup-hook [Variable] This is a normal hook that is run whenever the minibuffer is entered. See Section 22.1 [Hooks], page 406. Chapter 19: Minibuffers 320 minibuffer-exit-hook [Variable] This is a normal hook that is run whenever the minibuffer is exited. See Section 22.1 [Hooks], page 406. minibuffer-help-form [Variable] The current value of this variable is used to rebind help-form locally inside the minibuffer (see Section 23.5 [Help Functions], page 467). minibuffer-scroll-window [Variable] If the value of this variable is non-nil, it should be a window object. When the function scroll-other-window is called in the minibuffer, it scrolls this window. minibuffer-selected-window [Function] This function returns the window that was selected when the minibuffer was entered. If selected window is not a minibuffer window, it returns nil. max-mini-window-height [User Option] This variable specifies the maximum height for resizing minibuffer windows. If a float, it specifies a fraction of the height of the frame. If an integer, it specifies a number of lines. minibuffer-message string &rest args [Function] This function displays string temporarily at the end of the minibuffer text, for a few seconds, or until the next input event arrives, whichever comes first. The variable minibuffer-message-timeout specifies the number of seconds to wait in the absence of input. It defaults to 2. If args is non-nil, the actual message is obtained by passing string and args through format. See Section 4.7 [Formatting Strings], page 57. minibuffer-inactive-mode [Command] This is the major mode used in inactive minibuffers. It uses keymap minibuffer-inactive-mode-map. This can be useful if the minibuffer is in a separate frame. See Section 28.8 [Minibuffers and Frames], page 617. Chapter 20: Command Loop 321 20 Command Loop When you run Emacs, it enters the editor command loop almost immediately. This loop reads key sequences, executes their definitions, and displays the results. In this chapter, we describe how these things are done, and the subroutines that allow Lisp programs to do them. 20.1 Command Loop Overview The first thing the command loop must do is read a key sequence, which is a sequence of input events that translates into a command. It does this by calling the function read-key-sequence. Lisp programs can also call this function (see Section 20.8.1 [Key Sequence Input], page 349). They can also read input at a lower level with read-key or read-event (see Section 20.8.2 [Reading One Event], page 351), or discard pending input with discard-input (see Section 20.8.6 [Event Input Misc], page 355). The key sequence is translated into a command through the currently active keymaps. See Section 21.10 [Key Lookup], page 379, for information on how this is done. The result should be a keyboard macro or an interactively callable function. If the key is M-x, then it reads the name of another command, which it then calls. This is done by the command execute-extended-command (see Section 20.3 [Interactive Call], page 328). Prior to executing the command, Emacs runs undo-boundary to create an undo boundary. See Section 31.10 [Maintaining Undo], page 673. To execute a command, Emacs first reads its arguments by calling command-execute (see Section 20.3 [Interactive Call], page 328). For commands written in Lisp, the interactive specification says how to read the arguments. This may use the prefix argument (see Section 20.12 [Prefix Command Arguments], page 360) or may read with prompting in the minibuffer (see Chapter 19 [Minibuffers], page 290). For example, the command find-file has an interactive specification which says to read a file name using the minibuffer. The function body of find-file does not use the minibuffer, so if you call find-file as a function from Lisp code, you must supply the file name string as an ordinary Lisp function argument. If the command is a keyboard macro (i.e., a string or vector), Emacs executes it using execute-kbd-macro (see Section 20.16 [Keyboard Macros], page 365). pre-command-hook [Variable] This normal hook is run by the editor command loop before it executes each com- mand. At that time, this-command contains the command that is about to run, and last-command describes the previous command. See Section 20.5 [Command Loop Info], page 331. post-command-hook [Variable] This normal hook is run by the editor command loop after it executes each command (including commands terminated prematurely by quitting or by errors). At that time, this-command refers to the command that just ran, and last-command refers to the command before that. This hook is also run when Emacs first enters the command loop (at which point this-command and last-command are both nil). Chapter 20: Command Loop 322 Quitting is suppressed while running pre-command-hook and post-command-hook. If an error happens while executing one of these hooks, it does not terminate execution of the hook; instead the error is silenced and the function in which the error occurred is removed from the hook. A request coming into the Emacs server (see Section “Emacs Server” in The GNU Emacs Manual) runs these two hooks just as a keyboard command does. 20.2 Defining Commands The special form interactive turns a Lisp function into a command. The interactive form must be located at top-level in the function body, usually as the first form in the body; this applies to both lambda expressions (see Section 12.2 [Lambda Expressions], page 174) and defun forms (see Section 12.4 [Defining Functions], page 178). This form does nothing during the actual execution of the function; its presence serves as a flag, telling the Emacs command loop that the function can be called interactively. The argument of the interactive form specifies how the arguments for an interactive call should be read. Alternatively, an interactive form may be specified in a function symbol’s interactive-form property. A non-nil value for this property takes precedence over any interactive form in the function body itself. This feature is seldom used. Sometimes, a function is only intended to be called interactively, never directly from Lisp. In that case, give the function a non-nil interactive-only property. This causes the byte compiler to warn if the command is called from Lisp. The value of the property can be: a string, which the byte-compiler will use directly in its warning (it should end with a period, and not start with a capital, e.g. “use . . . instead.”); t; any other symbol, which should be an alternative function to use in Lisp code. 20.2.1 Using interactive This section describes how to write the interactive form that makes a Lisp function an interactively-callable command, and how to examine a command’s interactive form. interactive arg-descriptor [Special Form] This special form declares that a function is a command, and that it may therefore be called interactively (via M-x or by entering a key sequence bound to it). The argument arg-descriptor declares how to compute the arguments to the command when the command is called interactively. A command may be called from Lisp programs like any other function, but then the caller supplies the arguments and arg-descriptor has no effect. The interactive form must be located at top-level in the function body, or in the function symbol’s interactive-form property (see Section 8.4 [Symbol Properties], page 109). It has its effect because the command loop looks for it before calling the function (see Section 20.3 [Interactive Call], page 328). Once the function is called, all its body forms are executed; at this time, if the interactive form occurs within the body, the form simply returns nil without even evaluating its argument. By convention, you should put the interactive form in the function body, as the first top-level form. If there is an interactive form in both the interactive-form symbol property and the function body, the former takes precedence. The Chapter 20: Command Loop 323 interactive-form symbol property can be used to add an interactive form to an existing function, or change how its arguments are processed interactively, without redefining the function. There are three possibilities for the argument arg-descriptor: • It may be omitted or nil; then the command is called with no arguments. This leads quickly to an error if the command requires one or more arguments. • It may be a string; its contents are a sequence of elements separated by newlines, one for each argument1. Each element consists of a code character (see Section 20.2.2 [Interactive Codes], page 324) optionally followed by a prompt (which some code characters use and some ignore). Here is an example: (interactive "P\nbFrobnicate buffer: ") The code letter ‘P’ sets the command’s first argument to the raw command prefix (see Section 20.12 [Prefix Command Arguments], page 360). ‘bFrobnicate buffer: ’ prompts the user with ‘Frobnicate buffer: ’ to enter the name of an existing buffer, which becomes the second and final argument. The prompt string can use ‘%’ to include previous argument values (starting with the first argument) in the prompt. This is done using format (see Section 4.7 [Formatting Strings], page 57). For example, here is how you could read the name of an existing buffer followed by a new name to give to that buffer: (interactive "bBuffer to rename: \nsRename buffer %s to: ") If ‘*’ appears at the beginning of the string, then an error is signaled if the buffer is read-only. If ‘@’ appears at the beginning of the string, and if the key sequence used to invoke the command includes any mouse events, then the window associated with the first of those events is selected before the command is run. If ‘^’ appears at the beginning of the string, and if the command was invoked through shift-translation, set the mark and activate the region temporarily, or extend an already active region, before the command is run. If the command was invoked without shift-translation, and the region is temporarily active, deactivate the region before the command is run. Shift-translation is controlled on the user level by shift-selectmode; see Section “Shift Selection” in The GNU Emacs Manual. You can use ‘*’, ‘@’, and ^ together; the order does not matter. Actual reading of arguments is controlled by the rest of the prompt string (starting with the first character that is not ‘*’, ‘@’, or ‘^’). • It may be a Lisp expression that is not a string; then it should be a form that is evaluated to get a list of arguments to pass to the command. Usually this form will call various functions to read input from the user, most often through the minibuffer (see Chapter 19 [Minibuffers], page 290) or directly from the keyboard (see Section 20.8 [Reading Input], page 349). Providing point or the mark as an argument value is also common, but if you do this and read input (whether using the minibuffer or not), be sure to get the integer values of point or the mark after reading. The current buffer may be receiving subprocess 1 Some elements actually supply two arguments. Chapter 20: Command Loop 324 output; if subprocess output arrives while the command is waiting for input, it could relocate point and the mark. Here’s an example of what not to do: (interactive (list (region-beginning) (region-end) (read-string "Foo: " nil ’my-history))) Here’s how to avoid the problem, by examining point and the mark after reading the keyboard input: (interactive (let ((string (read-string "Foo: " nil ’my-history))) (list (region-beginning) (region-end) string))) Warning: the argument values should not include any data types that can’t be printed and then read. Some facilities save command-history in a file to be read in the subsequent sessions; if a command’s arguments contain a data type that prints using ‘#<...>’ syntax, those facilities won’t work. There are, however, a few exceptions: it is ok to use a limited set of expressions such as (point), (mark), (region-beginning), and (region-end), because Emacs recognizes them specially and puts the expression (rather than its value) into the command history. To see whether the expression you wrote is one of these exceptions, run the command, then examine (car command-history). interactive-form function [Function] This function returns the interactive form of function. If function is an interac- tively callable function (see Section 20.3 [Interactive Call], page 328), the value is the command’s interactive form (interactive spec), which specifies how to compute its arguments. Otherwise, the value is nil. If function is a symbol, its function definition is used. 20.2.2 Code Characters for interactive The code character descriptions below contain a number of key words, defined here as follows: Completion Provide completion. TAB, SPC, and RET perform name completion because the argument is read using completing-read (see Section 19.6 [Completion], page 298). ? displays a list of possible completions. Existing Require the name of an existing object. An invalid name is not accepted; the commands to exit the minibuffer do not exit if the current input is not valid. Default A default value of some sort is used if the user enters no text in the minibuffer. The default depends on the code character. No I/O This code letter computes an argument without reading any input. Therefore, it does not use a prompt string, and any prompt string you supply is ignored. Even though the code letter doesn’t use a prompt string, you must follow it with a newline if it is not the last code character in the string. Prompt A prompt immediately follows the code character. The prompt ends either with the end of the string or with a newline. Chapter 20: Command Loop 325 Special This code character is meaningful only at the beginning of the interactive string, and it does not look for a prompt or a newline. It is a single, isolated character. Here are the code character descriptions for use with interactive: ‘*’ Signal an error if the current buffer is read-only. Special. ‘@’ Select the window mentioned in the first mouse event in the key sequence that invoked this command. Special. ‘^’ If the command was invoked through shift-translation, set the mark and activate the region temporarily, or extend an already active region, before the command is run. If the command was invoked without shift-translation, and the region is temporarily active, deactivate the region before the command is run. Special. ‘a’ A function name (i.e., a symbol satisfying fboundp). Existing, Completion, Prompt. ‘b’ The name of an existing buffer. By default, uses the name of the current buffer (see Chapter 26 [Buffers], page 523). Existing, Completion, Default, Prompt. ‘B’ A buffer name. The buffer need not exist. By default, uses the name of a re- cently used buffer other than the current buffer. Completion, Default, Prompt. ‘c’ A character. The cursor does not move into the echo area. Prompt. ‘C’ A command name (i.e., a symbol satisfying commandp). Existing, Completion, Prompt. ‘d’ The position of point, as an integer (see Section 29.1 [Point], page 633). No I/O. ‘D’ A directory name. The default is the current default directory of the cur- rent buffer, default-directory (see Section 24.8.4 [File Name Expansion], page 496). Existing, Completion, Default, Prompt. ‘e’ The first or next non-keyboard event in the key sequence that invoked the command. More precisely, ‘e’ gets events that are lists, so you can look at the data in the lists. See Section 20.7 [Input Events], page 333. No I/O. You use ‘e’ for mouse events and for special system events (see Section 20.7.10 [Misc Events], page 341). The event list that the command receives depends on the event. See Section 20.7 [Input Events], page 333, which describes the forms of the list for each event in the corresponding subsections. You can use ‘e’ more than once in a single command’s interactive specification. If the key sequence that invoked the command has n events that are lists, the nth ‘e’ provides the nth such event. Events that are not lists, such as function keys and ASCII characters, do not count where ‘e’ is concerned. ‘f’ A file name of an existing file (see Section 24.8 [File Names], page 492). The de- fault directory is default-directory. Existing, Completion, Default, Prompt. ‘F’ A file name. The file need not exist. Completion, Default, Prompt. ‘G’ A file name. The file need not exist. If the user enters just a directory name, then the value is just that directory name, with no file name within the directory added. Completion, Default, Prompt. Chapter 20: Command Loop 326 ‘i’ An irrelevant argument. This code always supplies nil as the argument’s value. No I/O. ‘k’ A key sequence (see Section 21.1 [Key Sequences], page 367). This keeps reading events until a command (or undefined command) is found in the current key maps. The key sequence argument is represented as a string or vector. The cursor does not move into the echo area. Prompt. If ‘k’ reads a key sequence that ends with a down-event, it also reads and discards the following up-event. You can get access to that up-event with the ‘U’ code character. This kind of input is used by commands such as describe-key and global-set-key. ‘K’ A key sequence, whose definition you intend to change. This works like ‘k’, except that it suppresses, for the last input event in the key sequence, the conversions that are normally used (when necessary) to convert an undefined key into a defined one. ‘m’ The position of the mark, as an integer. No I/O. ‘M’ Arbitrary text, read in the minibuffer using the current buffer’s input method, and returned as a string (see Section “Input Methods” in The GNU Emacs Manual). Prompt. ‘n’ A number, read with the minibuffer. If the input is not a number, the user has to try again. ‘n’ never uses the prefix argument. Prompt. ‘N’ The numeric prefix argument; but if there is no prefix argument, read a number as with n. The value is always a number. See Section 20.12 [Prefix Command Arguments], page 360. Prompt. ‘p’ The numeric prefix argument. (Note that this ‘p’ is lower case.) No I/O. ‘P’ The raw prefix argument. (Note that this ‘P’ is upper case.) No I/O. ‘r’ Point and the mark, as two numeric arguments, smallest first. This is the only code letter that specifies two successive arguments rather than one. No I/O. ‘s’ Arbitrary text, read in the minibuffer and returned as a string (see Section 19.2 [Text from Minibuffer], page 291). Terminate the input with either C-j or RET. (C-q may be used to include either of these characters in the input.) Prompt. ‘S’ An interned symbol whose name is read in the minibuffer. Terminate the input with either C-j or RET. Other characters that normally terminate a symbol (e.g., whitespace, parentheses and brackets) do not do so here. Prompt. ‘U’ A key sequence or nil. Can be used after a ‘k’ or ‘K’ argument to get the up-event that was discarded (if any) after ‘k’ or ‘K’ read a down-event. If no up-event has been discarded, ‘U’ provides nil as the argument. No I/O. ‘v’ A variable declared to be a user option (i.e., satisfying the predicate custom-variable-p). This reads the variable using read-variable. See [Definition of read-variable], page 306. Existing, Completion, Prompt. Chapter 20: Command Loop 327 ‘x’ A Lisp object, specified with its read syntax, terminated with a C-j or RET. The object is not evaluated. See Section 19.3 [Object from Minibuffer], page 294. Prompt. ‘X’ A Lisp form’s value. ‘X’ reads as ‘x’ does, then evaluates the form so that its value becomes the argument for the command. Prompt. ‘z’ A coding system name (a symbol). If the user enters null input, the argu- ment value is nil. See Section 32.10 [Coding Systems], page 728. Completion, Existing, Prompt. ‘Z’ A coding system name (a symbol)—but only if this command has a prefix argument. With no prefix argument, ‘Z’ provides nil as the argument value. Completion, Existing, Prompt. 20.2.3 Examples of Using interactive Here are some examples of interactive: (defun foo1 () (interactive) (forward-word 2)) ⇒ foo1 ; foo1 takes no arguments, ; just moves forward two words. (defun foo2 (n) ; foo2 takes one argument, (interactive "^p") ; which is the numeric prefix. ; under shift-select-mode, ; will activate or extend region. (forward-word (* 2 n))) ⇒ foo2 (defun foo3 (n) ; foo3 takes one argument, (interactive "nCount:") ; which is read with the Minibuffer. (forward-word (* 2 n))) ⇒ foo3 (defun three-b (b1 b2 b3) "Select three existing buffers. Put them into three windows, selecting the last one." (interactive "bBuffer1:\nbBuffer2:\nbBuffer3:") (delete-other-windows) (split-window (selected-window) 8) (switch-to-buffer b1) (other-window 1) (split-window (selected-window) 8) (switch-to-buffer b2) (other-window 1) (switch-to-buffer b3)) ⇒ three-b (three-b "*scratch*" "declarations.texi" "*mail*") ⇒ nil Chapter 20: Command Loop 328 20.2.4 Select among Command Alternatives The macro define-alternatives can be used to define generic commands. These are interactive functions whose implementation can be selected from several alternatives, as a matter of user preference. define-alternatives command &rest customizations Define the new command command, a symbol. [Macro] When a user runs M-x command RET for the first time, Emacs prompts for which real form of the command to use, and records the selection by way of a custom variable. Using a prefix argument repeats this process of choosing an alternative. The variable command-alternatives should contain an alist with alternative implementations of command. Until this variable is set, define-alternatives has no effect. If customizations is non-nil, it should consist of alternating defcustom keywords (typically :group and :version) and values to add to the declaration of command-alternatives. 20.3 Interactive Call After the command loop has translated a key sequence into a command, it invokes that command using the function command-execute. If the command is a function, command-execute calls call-interactively, which reads the arguments and calls the command. You can also call these functions yourself. Note that the term “command”, in this context, refers to an interactively callable function (or function-like object), or a keyboard macro. It does not refer to the key sequence used to invoke a command (see Chapter 21 [Keymaps], page 367). commandp object &optional for-call-interactively [Function] This function returns t if object is a command. Otherwise, it returns nil. Commands include strings and vectors (which are treated as keyboard macros), lambda expressions that contain a top-level interactive form (see Section 20.2.1 [Using Interactive], page 322), byte-code function objects made from such lambda expressions, autoload objects that are declared as interactive (non-nil fourth argument to autoload), and some primitive functions. Also, a symbol is considered a command if it has a non-nil interactive-form property, or if its function definition satisfies commandp. If for-call-interactively is non-nil, then commandp returns t only for objects that call-interactively could call—thus, not for keyboard macros. See documentation in Section 23.2 [Accessing Documentation], page 462, for a realistic example of using commandp. call-interactively command &optional record-flag keys [Function] This function calls the interactively callable function command, providing arguments according to its interactive calling specifications. It returns whatever command re- turns. If, for instance, you have a function with the following signature: Chapter 20: Command Loop 329 (defun foo (begin end) (interactive "r") ...) then saying (call-interactively ’foo) will call foo with the region (point and mark) as the arguments. An error is signaled if command is not a function or if it cannot be called interactively (i.e., is not a command). Note that keyboard macros (strings and vectors) are not accepted, even though they are considered commands, because they are not functions. If command is a symbol, then call-interactively uses its function definition. If record-flag is non-nil, then this command and its arguments are unconditionally added to the list command-history. Otherwise, the command is added only if it uses the minibuffer to read an argument. See Section 20.15 [Command History], page 364. The argument keys, if given, should be a vector which specifies the sequence of events to supply if the command inquires which events were used to invoke it. If keys is omitted or nil, the default is the return value of this-command-keys-vector. See [Definition of this-command-keys-vector], page 332. command-execute command &optional record-flag keys special [Function] This function executes command. The argument command must satisfy the commandp predicate; i.e., it must be an interactively callable function or a keyboard macro. A string or vector as command is executed with execute-kbd-macro. A function is passed to call-interactively (see above), along with the record-flag and keys arguments. If command is a symbol, its function definition is used in its place. A symbol with an autoload definition counts as a command if it was declared to stand for an interactively callable function. Such a definition is handled by loading the specified library and then rechecking the definition of the symbol. The argument special, if given, means to ignore the prefix argument and not clear it. This is used for executing special events (see Section 20.9 [Special Events], page 357). execute-extended-command prefix-argument [Command] This function reads a command name from the minibuffer using completing-read (see Section 19.6 [Completion], page 298). Then it uses command-execute to call the specified command. Whatever that command returns becomes the value of execute-extended-command. If the command asks for a prefix argument, it receives the value prefix-argument. If execute-extended-command is called interactively, the current raw prefix argument is used for prefix-argument, and thus passed on to whatever command is run. execute-extended-command is the normal definition of M-x, so it uses the string ‘M-x ’ as a prompt. (It would be better to take the prompt from the events used to invoke execute-extended-command, but that is painful to implement.) A description of the value of the prefix argument, if any, also becomes part of the prompt. Chapter 20: Command Loop 330 (execute-extended-command 3) ---------- Buffer: Minibuffer ---------3 M-x forward-word RET ---------- Buffer: Minibuffer ---------- ⇒t 20.4 Distinguish Interactive Calls Sometimes a command should display additional visual feedback (such as an informative message in the echo area) for interactive calls only. There are three ways to do this. The recommended way to test whether the function was called using call-interactively is to give it an optional argument print-message and use the interactive spec to make it non-nil in interactive calls. Here’s an example: (defun foo (&optional print-message) (interactive "p") (when print-message (message "foo"))) We use "p" because the numeric prefix argument is never nil. Defined in this way, the function does display the message when called from a keyboard macro. The above method with the additional argument is usually best, because it allows callers to say “treat this call as interactive”. But you can also do the job by testing called-interactively-p. called-interactively-p kind [Function] This function returns t when the calling function was called using call-interactively. The argument kind should be either the symbol interactive or the symbol any. If it is interactive, then called-interactively-p returns t only if the call was made directly by the user—e.g., if the user typed a key sequence bound to the calling function, but not if the user ran a keyboard macro that called the function (see Section 20.16 [Keyboard Macros], page 365). If kind is any, called-interactivelyp returns t for any kind of interactive call, including keyboard macros. If in doubt, use any; the only known proper use of interactive is if you need to decide whether to display a helpful message while a function is running. A function is never considered to be called interactively if it was called via Lisp evaluation (or with apply or funcall). Here is an example of using called-interactively-p: (defun foo () (interactive) (when (called-interactively-p ’any) (message "Interactive!") ’foo-called-interactively)) ;; Type M-x foo. Interactive! Chapter 20: Command Loop 331 (foo) ⇒ nil Here is another example that contrasts direct and indirect calls to called-interactivelyp. (defun bar () (interactive) (message "%s" (list (foo) (called-interactively-p ’any)))) ;; Type M-x bar. (nil t) 20.5 Information from the Command Loop The editor command loop sets several Lisp variables to keep status records for itself and for commands that are run. With the exception of this-command and last-command it’s generally a bad idea to change any of these variables in a Lisp program. last-command [Variable] This variable records the name of the previous command executed by the command loop (the one before the current command). Normally the value is a symbol with a function definition, but this is not guaranteed. The value is copied from this-command when a command returns to the command loop, except when the command has specified a prefix argument for the following command. This variable is always local to the current terminal and cannot be buffer-local. See Section 28.2 [Multiple Terminals], page 598. real-last-command [Variable] This variable is set up by Emacs just like last-command, but never altered by Lisp programs. last-repeatable-command [Variable] This variable stores the most recently executed command that was not part of an input event. This is the command repeat will try to repeat, See Section “Repeating” in The GNU Emacs Manual. this-command [Variable] This variable records the name of the command now being executed by the editor command loop. Like last-command, it is normally a symbol with a function definition. The command loop sets this variable just before running a command, and copies its value into last-command when the command finishes (unless the command specified a prefix argument for the following command). Some commands set this variable during their execution, as a flag for whatever command runs next. In particular, the functions for killing text set this-command to kill-region so that any kill commands immediately following will know to append the killed text to the previous kill. Chapter 20: Command Loop 332 If you do not want a particular command to be recognized as the previous command in the case where it got an error, you must code that command to prevent this. One way is to set this-command to t at the beginning of the command, and set this-command back to its proper value at the end, like this: (defun foo (args...) (interactive ...) (let ((old-this-command this-command)) (setq this-command t) . . . do the work. . . (setq this-command old-this-command))) We do not bind this-command with let because that would restore the old value in case of error—a feature of let which in this case does precisely what we want to avoid. this-original-command [Variable] This has the same value as this-command except when command remapping occurs (see Section 21.13 [Remapping Commands], page 385). In that case, this-command gives the command actually run (the result of remapping), and this-original- command gives the command that was specified to run but remapped into another command. this-command-keys [Function] This function returns a string or vector containing the key sequence that invoked the present command, plus any previous commands that generated the prefix argument for this command. Any events read by the command using read-event without a timeout get tacked on to the end. However, if the command has called read-key-sequence, it returns the last read key sequence. See Section 20.8.1 [Key Sequence Input], page 349. The value is a string if all events in the sequence were characters that fit in a string. See Section 20.7 [Input Events], page 333. (this-command-keys) ;; Now use C-u C-x C-e to evaluate that. ⇒ "^U^X^E" this-command-keys-vector [Function] Like this-command-keys, except that it always returns the events in a vector, so you don’t need to deal with the complexities of storing input events in a string (see Section 20.7.15 [Strings of Events], page 348). clear-this-command-keys &optional keep-record [Function] This function empties out the table of events for this-command-keys to return. Un- less keep-record is non-nil, it also empties the records that the function recent-keys (see Section 38.12.2 [Recording Input], page 949) will subsequently return. This is useful after reading a password, to prevent the password from echoing inadvertently as part of the next command in certain cases. last-nonmenu-event [Variable] This variable holds the last input event read as part of a key sequence, not counting events resulting from mouse menus. Chapter 20: Command Loop 333 One use of this variable is for telling x-popup-menu where to pop up a menu. It is also used internally by y-or-n-p (see Section 19.7 [Yes-or-No Queries], page 314). last-command-event [Variable] This variable is set to the last input event that was read by the command loop as part of a command. The principal use of this variable is in self-insert-command, which uses it to decide which character to insert. last-command-event ;; Now use C-u C-x C-e to evaluate that. ⇒5 The value is 5 because that is the ASCII code for C-e. last-event-frame [Variable] This variable records which frame the last input event was directed to. Usually this is the frame that was selected when the event was generated, but if that frame has redirected input focus to another frame, the value is the frame to which the event was redirected. See Section 28.9 [Input Focus], page 617. If the last event came from a keyboard macro, the value is macro. 20.6 Adjusting Point After Commands It is not easy to display a value of point in the middle of a sequence of text that has the display, composition or is invisible. Therefore, after a command finishes and returns to the command loop, if point is within such a sequence, the command loop normally moves point to the edge of the sequence. A command can inhibit this feature by setting the variable disable-point-adjustment: disable-point-adjustment [Variable] If this variable is non-nil when a command returns to the command loop, then the command loop does not check for those text properties, and does not move point out of sequences that have them. The command loop sets this variable to nil before each command, so if a command sets it, the effect applies only to that command. global-disable-point-adjustment [Variable] If you set this variable to a non-nil value, the feature of moving point out of these sequences is completely turned off. 20.7 Input Events The Emacs command loop reads a sequence of input events that represent keyboard or mouse activity, or system events sent to Emacs. The events for keyboard activity are characters or symbols; other events are always lists. This section describes the representation and meaning of input events in detail. eventp object [Function] This function returns non-nil if object is an input event or event type. Note that any symbol might be used as an event or an event type. eventp cannot distinguish whether a symbol is intended by Lisp code to be used as an event. Instead, Chapter 20: Command Loop 334 it distinguishes whether the symbol has actually been used in an event that has been read as input in the current Emacs session. If a symbol has not yet been so used, eventp returns nil. 20.7.1 Keyboard Events There are two kinds of input you can get from the keyboard: ordinary keys, and function keys. Ordinary keys correspond to characters; the events they generate are represented in Lisp as characters. The event type of a character event is the character itself (an integer); see Section 20.7.12 [Classifying Events], page 343. An input character event consists of a basic code between 0 and 524287, plus any or all of these modifier bits: meta The 227 bit in the character code indicates a character typed with the meta key held down. control The 226 bit in the character code indicates a non-ASCII control character. ascii control characters such as C-a have special basic codes of their own, so Emacs needs no special bit to indicate them. Thus, the code for C-a is just 1. But if you type a control combination not in ASCII, such as % with the control key, the numeric value you get is the code for % plus 226 (assuming the terminal supports non-ASCII control characters). shift The 225 bit in the character code indicates an ASCII control character typed with the shift key held down. For letters, the basic code itself indicates upper versus lower case; for digits and punctuation, the shift key selects an entirely different character with a different basic code. In order to keep within the ASCII character set whenever possible, Emacs avoids using the 225 bit for those characters. However, ASCII provides no way to distinguish C-A from C-a, so Emacs uses the 225 bit in C-A and not in C-a. hyper The 224 bit in the character code indicates a character typed with the hyper key held down. super The 223 bit in the character code indicates a character typed with the super key held down. alt The 222 bit in the character code indicates a character typed with the alt key held down. (The key labeled Alt on most keyboards is actually treated as the meta key, not this.) It is best to avoid mentioning specific bit numbers in your program. To test the modifier bits of a character, use the function event-modifiers (see Section 20.7.12 [Classifying Events], page 343). When making key bindings, you can use the read syntax for characters with modifier bits (‘\C-’, ‘\M-’, and so on). For making key bindings with define-key, you can use lists such as (control hyper ?x) to specify the characters (see Section 21.12 [Changing Key Bindings], page 382). The function event-convert-list converts such a list into an event type (see Section 20.7.12 [Classifying Events], page 343). Chapter 20: Command Loop 335 20.7.2 Function Keys Most keyboards also have function keys—keys that have names or symbols that are not characters. Function keys are represented in Emacs Lisp as symbols; the symbol’s name is the function key’s label, in lower case. For example, pressing a key labeled F1 generates an input event represented by the symbol f1. The event type of a function key event is the event symbol itself. See Section 20.7.12 [Classifying Events], page 343. Here are a few special cases in the symbol-naming convention for function keys: backspace, tab, newline, return, delete These keys correspond to common ASCII control characters that have special keys on most keyboards. In ASCII, C-i and TAB are the same character. If the terminal can distinguish between them, Emacs conveys the distinction to Lisp programs by representing the former as the integer 9, and the latter as the symbol tab. Most of the time, it’s not useful to distinguish the two. So normally local-function-key-map (see Section 21.14 [Translation Keymaps], page 386) is set up to map tab into 9. Thus, a key binding for character code 9 (the character C-i) also applies to tab. Likewise for the other symbols in this group. The function read-char likewise converts these events into characters. In ASCII, BS is really C-h. But backspace converts into the character code 127 (DEL), not into code 8 (BS). This is what most users prefer. left, up, right, down Cursor arrow keys kp-add, kp-decimal, kp-divide, . . . Keypad keys (to the right of the regular keyboard). kp-0, kp-1, . . . Keypad keys with digits. kp-f1, kp-f2, kp-f3, kp-f4 Keypad PF keys. kp-home, kp-left, kp-up, kp-right, kp-down Keypad arrow keys. Emacs normally translates these into the corresponding non-keypad keys home, left, . . . kp-prior, kp-next, kp-end, kp-begin, kp-insert, kp-delete Additional keypad duplicates of keys ordinarily found elsewhere. Emacs normally translates these into the like-named non-keypad keys. You can use the modifier keys ALT, CTRL, HYPER, META, SHIFT, and SUPER with function keys. The way to represent them is with prefixes in the symbol name: ‘A-’ The alt modifier. ‘C-’ The control modifier. ‘H-’ The hyper modifier. Chapter 20: Command Loop 336 ‘M-’ The meta modifier. ‘S-’ The shift modifier. ‘s-’ The super modifier. Thus, the symbol for the key F3 with META held down is M-f3. When you use more than one prefix, we recommend you write them in alphabetical order; but the order does not matter in arguments to the key-binding lookup and modification functions. 20.7.3 Mouse Events Emacs supports four kinds of mouse events: click events, drag events, button-down events, and motion events. All mouse events are represented as lists. The car of the list is the event type; this says which mouse button was involved, and which modifier keys were used with it. The event type can also distinguish double or triple button presses (see Section 20.7.7 [Repeat Events], page 339). The rest of the list elements give position and time information. For key lookup, only the event type matters: two events of the same type necessarily run the same command. The command can access the full values of these events using the ‘e’ interactive code. See Section 20.2.2 [Interactive Codes], page 324. A key sequence that starts with a mouse event is read using the keymaps of the buffer in the window that the mouse was in, not the current buffer. This does not imply that clicking in a window selects that window or its buffer—that is entirely under the control of the command binding of the key sequence. 20.7.4 Click Events When the user presses a mouse button and releases it at the same location, that generates a click event. All mouse click event share the same format: (event-type position click-count) event-type This is a symbol that indicates which mouse button was used. It is one of the symbols mouse-1, mouse-2, . . . , where the buttons are numbered left to right. You can also use prefixes ‘A-’, ‘C-’, ‘H-’, ‘M-’, ‘S-’ and ‘s-’ for modifiers alt, control, hyper, meta, shift and super, just as you would with function keys. This symbol also serves as the event type of the event. Key bindings describe events by their types; thus, if there is a key binding for mouse-1, that binding would apply to all events whose event-type is mouse-1. position This is a mouse position list specifying where the mouse click occurred; see below for details. click-count This is the number of rapid repeated presses so far of the same mouse button. See Section 20.7.7 [Repeat Events], page 339. To access the contents of a mouse position list in the position slot of a click event, you should typically use the functions documented in Section 20.7.13 [Accessing Mouse], page 345. The explicit format of the list depends on where the click occurred. For clicks in the text area, mode line, header line, or in the fringe or marginal areas, the mouse position list has the form Chapter 20: Command Loop 337 (window pos-or-area (x . y) timestamp object text-pos (col . row) image (dx . dy) (width . height)) The meanings of these list elements are as follows: window The window in which the click occurred. pos-or-area The buffer position of the character clicked on in the text area; or, if the click was outside the text area, the window area where it occurred. It is one of the symbols mode-line, header-line, vertical-line, left-margin, right-margin, left-fringe, or right-fringe. In one special case, pos-or-area is a list containing a symbol (one of the symbols listed above) instead of just the symbol. This happens after the imaginary prefix keys for the event are registered by Emacs. See Section 20.8.1 [Key Sequence Input], page 349. x, y The relative pixel coordinates of the click. For clicks in the text area of a window, the coordinate origin (0 . 0) is taken to be the top left corner of the text area. See Section 27.3 [Window Sizes], page 544. For clicks in a mode line or header line, the coordinate origin is the top left corner of the window itself. For fringes, margins, and the vertical border, x does not have meaningful data. For fringes and margins, y is relative to the bottom edge of the header line. In all cases, the x and y coordinates increase rightward and downward respectively. timestamp The time at which the event occurred, as an integer number of milliseconds since a system-dependent initial time. object Either nil if there is no string-type text property at the click position, or a cons cell of the form (string . string-pos) if there is one: string The string which was clicked on, including any properties. string-pos The position in the string where the click occurred. text-pos For clicks on a marginal area or on a fringe, this is the buffer position of the first visible character in the corresponding line in the window. For other events, it is the current buffer position in the window. col, row These are the actual column and row coordinate numbers of the glyph under the x, y position. If x lies beyond the last column of actual text on its line, col is reported by adding fictional extra columns that have the default character width. Row 0 is taken to be the header line if the window has one, or the topmost row of the text area otherwise. Column 0 is taken to be the leftmost column of the text area for clicks on a window text area, or the leftmost mode line or header line column for clicks there. For clicks on fringes or vertical borders, these have no meaningful data. For clicks on margins, col is measured from the left edge of the margin area and row is measured from the top of the margin area. Chapter 20: Command Loop 338 image dx, dy This is the image object on which the click occurred. It is either nil if there is no image at the position clicked on, or it is an image object as returned by find-image if click was in an image. These are the pixel coordinates of the click, relative to the top left corner of object, which is (0 . 0). If object is nil, the coordinates are relative to the top left corner of the character glyph clicked on. width, height These are the pixel width and height of object or, if this is nil, those of the character glyph clicked on. For clicks on a scroll bar, position has this form: (window area (portion . whole) timestamp part) window The window whose scroll bar was clicked on. area This is the symbol vertical-scroll-bar. portion The number of pixels from the top of the scroll bar to the click position. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always 0. whole The total length, in pixels, of the scroll bar. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always 0. timestamp The time at which the event occurred, in milliseconds. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always 0. part The part of the scroll bar on which the click occurred. It is one of the symbols handle (the scroll bar handle), above-handle (the area above the handle), below-handle (the area below the handle), up (the up arrow at one end of the scroll bar), or down (the down arrow at one end of the scroll bar). 20.7.5 Drag Events With Emacs, you can have a drag event without even changing your clothes. A drag event happens every time the user presses a mouse button and then moves the mouse to a different character position before releasing the button. Like all mouse events, drag events are represented in Lisp as lists. The lists record both the starting mouse position and the final position, like this: (event-type (window1 START-POSITION) (window2 END-POSITION)) For a drag event, the name of the symbol event-type contains the prefix ‘drag-’. For example, dragging the mouse with button 2 held down generates a drag-mouse-2 event. The second and third elements of the event give the starting and ending position of the drag, as mouse position lists (see Section 20.7.4 [Click Events], page 336). You can access the second element of any mouse event in the same way, with no need to distinguish drag events from others. The ‘drag-’ prefix follows the modifier key prefixes such as ‘C-’ and ‘M-’. Chapter 20: Command Loop 339 If read-key-sequence receives a drag event that has no key binding, and the corresponding click event does have a binding, it changes the drag event into a click event at the drag’s starting position. This means that you don’t have to distinguish between click and drag events unless you want to. 20.7.6 Button-Down Events Click and drag events happen when the user releases a mouse button. They cannot happen earlier, because there is no way to distinguish a click from a drag until the button is released. If you want to take action as soon as a button is pressed, you need to handle button-down events.2 These occur as soon as a button is pressed. They are represented by lists that look exactly like click events (see Section 20.7.4 [Click Events], page 336), except that the event-type symbol name contains the prefix ‘down-’. The ‘down-’ prefix follows modifier key prefixes such as ‘C-’ and ‘M-’. The function read-key-sequence ignores any button-down events that don’t have command bindings; therefore, the Emacs command loop ignores them too. This means that you need not worry about defining button-down events unless you want them to do something. The usual reason to define a button-down event is so that you can track mouse motion (by reading motion events) until the button is released. See Section 20.7.8 [Motion Events], page 340. 20.7.7 Repeat Events If you press the same mouse button more than once in quick succession without moving the mouse, Emacs generates special repeat mouse events for the second and subsequent presses. The most common repeat events are double-click events. Emacs generates a double-click event when you click a button twice; the event happens when you release the button (as is normal for all click events). The event type of a double-click event contains the prefix ‘double-’. Thus, a double click on the second mouse button with meta held down comes to the Lisp program as M-doublemouse-2. If a double-click event has no binding, the binding of the corresponding ordinary click event is used to execute it. Thus, you need not pay attention to the double click feature unless you really want to. When the user performs a double click, Emacs generates first an ordinary click event, and then a double-click event. Therefore, you must design the command binding of the double click event to assume that the single-click command has already run. It must produce the desired results of a double click, starting from the results of a single click. This is convenient, if the meaning of a double click somehow “builds on” the meaning of a single click—which is recommended user interface design practice for double clicks. If you click a button, then press it down again and start moving the mouse with the button held down, then you get a double-drag event when you ultimately release the button. Its event type contains ‘double-drag’ instead of just ‘drag’. If a double-drag event has no binding, Emacs looks for an alternate binding as if the event were an ordinary drag. Before the double-click or double-drag event, Emacs generates a double-down event when the user presses the button down for the second time. Its event type contains ‘double-down’ 2 Button-down is the conservative antithesis of drag. Chapter 20: Command Loop 340 instead of just ‘down’. If a double-down event has no binding, Emacs looks for an alternate binding as if the event were an ordinary button-down event. If it finds no binding that way either, the double-down event is ignored. To summarize, when you click a button and then press it again right away, Emacs generates a down event and a click event for the first click, a double-down event when you press the button again, and finally either a double-click or a double-drag event. If you click a button twice and then press it again, all in quick succession, Emacs generates a triple-down event, followed by either a triple-click or a triple-drag. The event types of these events contain ‘triple’ instead of ‘double’. If any triple event has no binding, Emacs uses the binding that it would use for the corresponding double event. If you click a button three or more times and then press it again, the events for the presses beyond the third are all triple events. Emacs does not have separate event types for quadruple, quintuple, etc. events. However, you can look at the event list to find out precisely how many times the button was pressed. event-click-count event [Function] This function returns the number of consecutive button presses that led up to event. If event is a double-down, double-click or double-drag event, the value is 2. If event is a triple event, the value is 3 or greater. If event is an ordinary mouse event (not a repeat event), the value is 1. double-click-fuzz [User Option] To generate repeat events, successive mouse button presses must be at approximately the same screen position. The value of double-click-fuzz specifies the maximum number of pixels the mouse may be moved (horizontally or vertically) between two successive clicks to make a double-click. This variable is also the threshold for motion of the mouse to count as a drag. double-click-time [User Option] To generate repeat events, the number of milliseconds between successive button presses must be less than the value of double-click-time. Setting double-click- time to nil disables multi-click detection entirely. Setting it to t removes the time limit; Emacs then detects multi-clicks by position only. 20.7.8 Motion Events Emacs sometimes generates mouse motion events to describe motion of the mouse without any button activity. Mouse motion events are represented by lists that look like this: (mouse-movement POSITION) position is a mouse position list (see Section 20.7.4 [Click Events], page 336), specifying the current position of the mouse cursor. The special form track-mouse enables generation of motion events within its body. Outside of track-mouse forms, Emacs does not generate events for mere motion of the mouse, and these events do not appear. See Section 28.13 [Mouse Tracking], page 621. Chapter 20: Command Loop 341 20.7.9 Focus Events Window systems provide general ways for the user to control which window gets keyboard input. This choice of window is called the focus. When the user does something to switch between Emacs frames, that generates a focus event. The normal definition of a focus event, in the global keymap, is to select a new frame within Emacs, as the user would expect. See Section 28.9 [Input Focus], page 617. Focus events are represented in Lisp as lists that look like this: (switch-frame new-frame) where new-frame is the frame switched to. Some X window managers are set up so that just moving the mouse into a window is enough to set the focus there. Usually, there is no need for a Lisp program to know about the focus change until some other kind of input arrives. Emacs generates a focus event only when the user actually types a keyboard key or presses a mouse button in the new frame; just moving the mouse between frames does not generate a focus event. A focus event in the middle of a key sequence would garble the sequence. So Emacs never generates a focus event in the middle of a key sequence. If the user changes focus in the middle of a key sequence—that is, after a prefix key—then Emacs reorders the events so that the focus event comes either before or after the multi-event key sequence, and not within it. 20.7.10 Miscellaneous System Events A few other event types represent occurrences within the system. (delete-frame (frame)) This kind of event indicates that the user gave the window manager a command to delete a particular window, which happens to be an Emacs frame. The standard definition of the delete-frame event is to delete frame. (iconify-frame (frame)) This kind of event indicates that the user iconified frame using the window manager. Its standard definition is ignore; since the frame has already been iconified, Emacs has no work to do. The purpose of this event type is so that you can keep track of such events if you want to. (make-frame-visible (frame)) This kind of event indicates that the user deiconified frame using the window manager. Its standard definition is ignore; since the frame has already been made visible, Emacs has no work to do. (wheel-up position) (wheel-down position) These kinds of event are generated by moving a mouse wheel. The position element is a mouse position list (see Section 20.7.4 [Click Events], page 336), specifying the position of the mouse cursor when the event occurred. This kind of event is generated only on some kinds of systems. On some systems, mouse-4 and mouse-5 are used instead. For portable code, use the variables mouse-wheel-up-event and mouse-wheel-down-event defined in mwheel.el to determine what event types to expect for the mouse wheel. Chapter 20: Command Loop 342 (drag-n-drop position files) This kind of event is generated when a group of files is selected in an application outside of Emacs, and then dragged and dropped onto an Emacs frame. The element position is a list describing the position of the event, in the same format as used in a mouse-click event (see Section 20.7.4 [Click Events], page 336), and files is the list of file names that were dragged and dropped. The usual way to handle this event is by visiting these files. This kind of event is generated, at present, only on some kinds of systems. help-echo This kind of event is generated when a mouse pointer moves onto a portion of buffer text which has a help-echo text property. The generated event has this form: (help-echo frame help window object pos) The precise meaning of the event parameters and the way these parameters are used to display the help-echo text are described in [Text help-echo], page 697. sigusr1 sigusr2 These events are generated when the Emacs process receives the signals SIGUSR1 and SIGUSR2. They contain no additional data because signals do not carry additional information. They can be useful for debugging (see Section 17.1.1 [Error Debugging], page 249). To catch a user signal, bind the corresponding event to an interactive command in the special-event-map (see Section 21.7 [Active Keymaps], page 374). The command is called with no arguments, and the specific signal event is available in last-input-event. For example: (defun sigusr-handler () (interactive) (message "Caught signal %S" last-input-event)) (define-key special-event-map [sigusr1] ’sigusr-handler) To test the signal handler, you can make Emacs send a signal to itself: (signal-process (emacs-pid) ’sigusr1) language-change This kind of event is generated on MS-Windows when the input language has changed. This typically means that the keyboard keys will send to Emacs characters from a different language. The generated event has this form: (language-change frame codepage language-id) Here frame is the frame which was current when the input language changed; codepage is the new codepage number; and language-id is the numerical ID of the new input language. The coding-system (see Section 32.10 [Coding Systems], page 728) that corresponds to codepage is cpcodepage or windows-codepage. To convert language-id to a string (e.g., to use it for various language-dependent features, such as set-language-environment), use the w32-get-locale-info function, like this: ;; Get the abbreviated language name, such as "ENU" for English (w32-get-locale-info language-id) Chapter 20: Command Loop 343 ;; Get the full English name of the language, ;; such as "English (United States)" (w32-get-locale-info language-id 4097) ;; Get the full localized name of the language (w32-get-locale-info language-id t) If one of these events arrives in the middle of a key sequence—that is, after a prefix key—then Emacs reorders the events so that this event comes either before or after the multi-event key sequence, not within it. 20.7.11 Event Examples If the user presses and releases the left mouse button over the same location, that generates a sequence of events like this: (down-mouse-1 (# 2613 (0 . 38) -864320)) (mouse-1 (# 2613 (0 . 38) -864180)) While holding the control key down, the user might hold down the second mouse button, and drag the mouse from one line to the next. That produces two events, as shown here: (C-down-mouse-2 (# 3440 (0 . 27) -731219)) (C-drag-mouse-2 (# 3440 (0 . 27) -731219) (# 3510 (0 . 28) -729648)) While holding down the meta and shift keys, the user might press the second mouse button on the window’s mode line, and then drag the mouse into another window. That produces a pair of events like these: (M-S-down-mouse-2 (# mode-line (33 . 31) -457844)) (M-S-drag-mouse-2 (# mode-line (33 . 31) -457844) (# 161 (33 . 3) -453816)) To handle a SIGUSR1 signal, define an interactive function, and bind it to the signal usr1 event sequence: (defun usr1-handler () (interactive) (message "Got USR1 signal")) (global-set-key [signal usr1] ’usr1-handler) 20.7.12 Classifying Events Every event has an event type, which classifies the event for key binding purposes. For a keyboard event, the event type equals the event value; thus, the event type for a character is the character, and the event type for a function key symbol is the symbol itself. For events that are lists, the event type is the symbol in the car of the list. Thus, the event type is always a symbol or a character. Two events of the same type are equivalent where key bindings are concerned; thus, they always run the same command. That does not necessarily mean they do the same things, however, as some commands look at the whole event to decide what to do. For example, some commands use the location of a mouse event to decide where in the buffer to act. Sometimes broader classifications of events are useful. For example, you might want to ask whether an event involved the META key, regardless of which other key or mouse button was used. The functions event-modifiers and event-basic-type are provided to get such information conveniently. Chapter 20: Command Loop 344 event-modifiers event [Function] This function returns a list of the modifiers that event has. The modifiers are symbols; they include shift, control, meta, alt, hyper and super. In addition, the modifiers list of a mouse event symbol always contains one of click, drag, and down. For double or triple events, it also contains double or triple. The argument event may be an entire event object, or just an event type. If event is a symbol that has never been used in an event that has been read as input in the current Emacs session, then event-modifiers can return nil, even when event actually has modifiers. Here are some examples: (event-modifiers ?a) ⇒ nil (event-modifiers ?A) ⇒ (shift) (event-modifiers ?\C-a) ⇒ (control) (event-modifiers ?\C-%) ⇒ (control) (event-modifiers ?\C-\S-a) ⇒ (control shift) (event-modifiers ’f5) ⇒ nil (event-modifiers ’s-f5) ⇒ (super) (event-modifiers ’M-S-f5) ⇒ (meta shift) (event-modifiers ’mouse-1) ⇒ (click) (event-modifiers ’down-mouse-1) ⇒ (down) The modifiers list for a click event explicitly contains click, but the event symbol name itself does not contain ‘click’. event-basic-type event [Function] This function returns the key or mouse button that event describes, with all modifiers removed. The event argument is as in event-modifiers. For example: (event-basic-type ?a) ⇒ 97 (event-basic-type ?A) ⇒ 97 (event-basic-type ?\C-a) ⇒ 97 (event-basic-type ?\C-\S-a) ⇒ 97 (event-basic-type ’f5) ⇒ f5 Chapter 20: Command Loop 345 (event-basic-type ’s-f5) ⇒ f5 (event-basic-type ’M-S-f5) ⇒ f5 (event-basic-type ’down-mouse-1) ⇒ mouse-1 mouse-movement-p object This function returns non-nil if object is a mouse movement event. [Function] event-convert-list list [Function] This function converts a list of modifier names and a basic event type to an event type which specifies all of them. The basic event type must be the last element of the list. For example, (event-convert-list ’(control ?a)) ⇒1 (event-convert-list ’(control meta ?a)) ⇒ -134217727 (event-convert-list ’(control super f1)) ⇒ C-s-f1 20.7.13 Accessing Mouse Events This section describes convenient functions for accessing the data in a mouse button or motion event. Keyboard event data can be accessed using the same functions, but data elements that aren’t applicable to keyboard events are zero or nil. The following two functions return a mouse position list (see Section 20.7.4 [Click Events], page 336), specifying the position of a mouse event. event-start event This returns the starting position of event. [Function] If event is a click or button-down event, this returns the location of the event. If event is a drag event, this returns the drag’s starting position. event-end event This returns the ending position of event. [Function] If event is a drag event, this returns the position where the user released the mouse button. If event is a click or button-down event, the value is actually the starting position, which is the only position such events have. posnp object [Function] This function returns non-nil if object is a mouse position list, in either of the formats documented in Section 20.7.4 [Click Events], page 336); and nil otherwise. These functions take a mouse position list as argument, and return various parts of it: posn-window position Return the window that position is in. [Function] Chapter 20: Command Loop 346 posn-area position [Function] Return the window area recorded in position. It returns nil when the event occurred in the text area of the window; otherwise, it is a symbol identifying the area in which the event occurred. posn-point position [Function] Return the buffer position in position. When the event occurred in the text area of the window, in a marginal area, or on a fringe, this is an integer specifying a buffer position. Otherwise, the value is undefined. posn-x-y position [Function] Return the pixel-based x and y coordinates in position, as a cons cell (x . y). These coordinates are relative to the window given by posn-window. This example shows how to convert the window-relative coordinates in the text area of a window into frame-relative coordinates: (defun frame-relative-coordinates (position) "Return frame-relative coordinates from POSITION. POSITION is assumed to lie in a window text area." (let* ((x-y (posn-x-y position)) (window (posn-window position)) (edges (window-inside-pixel-edges window))) (cons (+ (car x-y) (car edges)) (+ (cdr x-y) (cadr edges))))) posn-col-row position [Function] This function returns a cons cell (col . row), containing the estimated column and row corresponding to buffer position in position. The return value is given in units of the frame’s default character width and default line height (including spacing), as computed from the x and y values corresponding to position. (So, if the actual characters have non-default sizes, the actual row and column may differ from these computed values.) Note that row is counted from the top of the text area. If the window given by position possesses a header line (see Section 22.4.7 [Header Lines], page 435), it is not included in the row count. posn-actual-col-row position [Function] Return the actual row and column in position, as a cons cell (col . row). The values are the actual row and column numbers in the window given by position. See Section 20.7.4 [Click Events], page 336, for details. The function returns nil if position does not include actual position values. posn-string position [Function] Return the string object in position, either nil, or a cons cell (string . string- pos). posn-image position [Function] Return the image object in position, either nil, or an image (image ...). Chapter 20: Command Loop 347 posn-object position [Function] Return the image or string object in position, either nil, an image (image ...), or a cons cell (string . string-pos). posn-object-x-y position [Function] Return the pixel-based x and y coordinates relative to the upper left corner of the object in position as a cons cell (dx . dy). If the position is a buffer position, return the relative position in the character at that position. posn-object-width-height position [Function] Return the pixel width and height of the object in position as a cons cell (width . height). If the position is a buffer position, return the size of the character at that position. posn-timestamp position [Function] Return the timestamp in position. This is the time at which the event occurred, in milliseconds. These functions compute a position list given particular buffer position or screen position. You can access the data in this position list with the functions described above. posn-at-point &optional pos window [Function] This function returns a position list for position pos in window. pos defaults to point in window; window defaults to the selected window. posn-at-point returns nil if pos is not visible in window. posn-at-x-y x y &optional frame-or-window whole [Function] This function returns position information corresponding to pixel coordinates x and y in a specified frame or window, frame-or-window, which defaults to the selected window. The coordinates x and y are relative to the frame or window used. If whole is nil, the coordinates are relative to the window text area, otherwise they are relative to the entire window area including scroll bars, margins and fringes. 20.7.14 Accessing Scroll Bar Events These functions are useful for decoding scroll bar events. scroll-bar-event-ratio event [Function] This function returns the fractional vertical position of a scroll bar event within the scroll bar. The value is a cons cell (portion . whole) containing two integers whose ratio is the fractional position. scroll-bar-scale ratio total [Function] This function multiplies (in effect) ratio by total, rounding the result to an integer. The argument ratio is not a number, but rather a pair (num . denom)—typically a value returned by scroll-bar-event-ratio. This function is handy for scaling a position on a scroll bar into a buffer position. Here’s how to do that: Chapter 20: Command Loop 348 (+ (point-min) (scroll-bar-scale (posn-x-y (event-start event)) (- (point-max) (point-min)))) Recall that scroll bar events have two integers forming a ratio, in place of a pair of x and y coordinates. 20.7.15 Putting Keyboard Events in Strings In most of the places where strings are used, we conceptualize the string as containing text characters—the same kind of characters found in buffers or files. Occasionally Lisp programs use strings that conceptually contain keyboard characters; for example, they may be key sequences or keyboard macro definitions. However, storing keyboard characters in a string is a complex matter, for reasons of historical compatibility, and it is not always possible. We recommend that new programs avoid dealing with these complexities by not storing keyboard events in strings. Here is how to do that: • Use vectors instead of strings for key sequences, when you plan to use them for anything other than as arguments to lookup-key and define-key. For example, you can use read-key-sequence-vector instead of read-key-sequence, and this-commandkeys-vector instead of this-command-keys. • Use vectors to write key sequence constants containing meta characters, even when passing them directly to define-key. • When you have to look at the contents of a key sequence that might be a string, use listify-key-sequence (see Section 20.8.6 [Event Input Misc], page 355) first, to convert it to a list. The complexities stem from the modifier bits that keyboard input characters can include. Aside from the Meta modifier, none of these modifier bits can be included in a string, and the Meta modifier is allowed only in special cases. The earliest GNU Emacs versions represented meta characters as codes in the range of 128 to 255. At that time, the basic character codes ranged from 0 to 127, so all keyboard character codes did fit in a string. Many Lisp programs used ‘\M-’ in string constants to stand for meta characters, especially in arguments to define-key and similar functions, and key sequences and sequences of events were always represented as strings. When we added support for larger basic character codes beyond 127, and additional modifier bits, we had to change the representation of meta characters. Now the flag that represents the Meta modifier in a character is 227 and such numbers cannot be included in a string. To support programs with ‘\M-’ in string constants, there are special rules for including certain meta characters in a string. Here are the rules for interpreting a string as a sequence of input characters: • If the keyboard character value is in the range of 0 to 127, it can go in the string unchanged. • The meta variants of those characters, with codes in the range of 227 to 227 + 127, can also go in the string, but you must change their numeric values. You must set the 27 Chapter 20: Command Loop 349 bit instead of the 227 bit, resulting in a value between 128 and 255. Only a unibyte string can include these codes. • Non-ASCII characters above 256 can be included in a multibyte string. • Other keyboard character events cannot fit in a string. This includes keyboard events in the range of 128 to 255. Functions such as read-key-sequence that construct strings of keyboard input characters follow these rules: they construct vectors instead of strings, when the events won’t fit in a string. When you use the read syntax ‘\M-’ in a string, it produces a code in the range of 128 to 255—the same code that you get if you modify the corresponding keyboard event to put it in the string. Thus, meta events in strings work consistently regardless of how they get into the strings. However, most programs would do well to avoid these issues by following the recommendations at the beginning of this section. 20.8 Reading Input The editor command loop reads key sequences using the function read-key-sequence, which uses read-event. These and other functions for event input are also available for use in Lisp programs. See also momentary-string-display in Section 37.8 [Temporary Displays], page 848, and sit-for in Section 20.10 [Waiting], page 357. See Section 38.12 [Terminal Input], page 948, for functions and variables for controlling terminal input modes and debugging terminal input. For higher-level input facilities, see Chapter 19 [Minibuffers], page 290. 20.8.1 Key Sequence Input The command loop reads input a key sequence at a time, by calling read-key-sequence. Lisp programs can also call this function; for example, describe-key uses it to read the key to describe. read-key-sequence prompt &optional continue-echo dont-downcase-last [Function] switch-frame-ok command-loop This function reads a key sequence and returns it as a string or vector. It keeps reading events until it has accumulated a complete key sequence; that is, enough to specify a non-prefix command using the currently active keymaps. (Remember that a key sequence that starts with a mouse event is read using the keymaps of the buffer in the window that the mouse was in, not the current buffer.) If the events are all characters and all can fit in a string, then read-key-sequence returns a string (see Section 20.7.15 [Strings of Events], page 348). Otherwise, it returns a vector, since a vector can hold all kinds of events—characters, symbols, and lists. The elements of the string or vector are the events in the key sequence. Reading a key sequence includes translating the events in various ways. See Section 21.14 [Translation Keymaps], page 386. The argument prompt is either a string to be displayed in the echo area as a prompt, or nil, meaning not to display a prompt. The argument continue-echo, if non-nil, means to echo this key as a continuation of the previous key. Chapter 20: Command Loop 350 Normally any upper case event is converted to lower case if the original event is undefined and the lower case equivalent is defined. The argument dont-downcase-last, if non-nil, means do not convert the last event to lower case. This is appropriate for reading a key sequence to be defined. The argument switch-frame-ok, if non-nil, means that this function should process a switch-frame event if the user switches frames before typing anything. If the user switches frames in the middle of a key sequence, or at the start of the sequence but switch-frame-ok is nil, then the event will be put off until after the current key sequence. The argument command-loop, if non-nil, means that this key sequence is being read by something that will read commands one after another. It should be nil if the caller will read just one key sequence. In the following example, Emacs displays the prompt ‘?’ in the echo area, and then the user types C-x C-f. (read-key-sequence "?") ---------- Echo Area ---------?C-x C-f ---------- Echo Area ---------- ⇒ "^X^F" The function read-key-sequence suppresses quitting: C-g typed while reading with this function works like any other character, and does not set quit-flag. See Section 20.11 [Quitting], page 358. read-key-sequence-vector prompt &optional continue-echo [Function] dont-downcase-last switch-frame-ok command-loop This is like read-key-sequence except that it always returns the key sequence as a vector, never as a string. See Section 20.7.15 [Strings of Events], page 348. If an input character is upper-case (or has the shift modifier) and has no key binding, but its lower-case equivalent has one, then read-key-sequence converts the character to lower case. Note that lookup-key does not perform case conversion in this way. When reading input results in such a shift-translation, Emacs sets the variable this-command-keys-shift-translated to a non-nil value. Lisp programs can examine this variable if they need to modify their behavior when invoked by shift-translated keys. For example, the function handle-shift-selection examines the value of this variable to determine how to activate or deactivate the region (see Section 30.7 [The Mark], page 650). The function read-key-sequence also transforms some mouse events. It converts unbound drag events into click events, and discards unbound button-down events entirely. It also reshuffles focus events and miscellaneous window events so that they never appear in a key sequence with any other events. When mouse events occur in special parts of a window, such as a mode line or a scroll bar, the event type shows nothing special—it is the same symbol that would normally represent that combination of mouse button and modifier keys. The information about the window part is kept elsewhere in the event—in the coordinates. But read-key-sequence translates Chapter 20: Command Loop 351 this information into imaginary “prefix keys”, all of which are symbols: header-line, horizontal-scroll-bar, menu-bar, mode-line, vertical-line, and vertical-scrollbar. You can define meanings for mouse clicks in special window parts by defining key sequences using these imaginary prefix keys. For example, if you call read-key-sequence and then click the mouse on the window’s mode line, you get two events, like this: (read-key-sequence "Click on the mode line: ") ⇒ [mode-line (mouse-1 (# mode-line (40 . 63) 5959987))] num-input-keys [Variable] This variable’s value is the number of key sequences processed so far in this Emacs session. This includes key sequences read from the terminal and key sequences read from keyboard macros being executed. 20.8.2 Reading One Event The lowest level functions for command input are read-event, read-char, and read-charexclusive. read-event &optional prompt inherit-input-method seconds [Function] This function reads and returns the next event of command input, waiting if necessary until an event is available. The returned event may come directly from the user, or from a keyboard macro. It is not decoded by the keyboard’s input coding system (see Section 32.10.8 [Terminal I/O Encoding], page 741). If the optional argument prompt is non-nil, it should be a string to display in the echo area as a prompt. Otherwise, read-event does not display any message to indicate it is waiting for input; instead, it prompts by echoing: it displays descriptions of the events that led to or were read by the current command. See Section 37.4 [The Echo Area], page 836. If inherit-input-method is non-nil, then the current input method (if any) is employed to make it possible to enter a non-ASCII character. Otherwise, input method handling is disabled for reading this event. If cursor-in-echo-area is non-nil, then read-event moves the cursor temporarily to the echo area, to the end of any message displayed there. Otherwise read-event does not move the cursor. If seconds is non-nil, it should be a number specifying the maximum time to wait for input, in seconds. If no input arrives within that time, read-event stops waiting and returns nil. A floating point seconds means to wait for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, seconds is rounded down. If seconds is nil, read-event waits as long as necessary for input to arrive. If seconds is nil, Emacs is considered idle while waiting for user input to arrive. Idle timers—those created with run-with-idle-timer (see Section 38.11 [Idle Timers], Chapter 20: Command Loop 352 page 946)—can run during this period. However, if seconds is non-nil, the state of idleness remains unchanged. If Emacs is non-idle when read-event is called, it remains non-idle throughout the operation of read-event; if Emacs is idle (which can happen if the call happens inside an idle timer), it remains idle. If read-event gets an event that is defined as a help character, then in some cases read-event processes the event directly without returning. See Section 23.5 [Help Functions], page 467. Certain other events, called special events, are also processed directly within read-event (see Section 20.9 [Special Events], page 357). Here is what happens if you call read-event and then press the right-arrow function key: (read-event) ⇒ right read-char &optional prompt inherit-input-method seconds [Function] This function reads and returns a character of command input. If the user generates an event which is not a character (i.e., a mouse click or function key event), read-char signals an error. The arguments work as in read-event. In the first example, the user types the character 1 (ASCII code 49). The second example shows a keyboard macro definition that calls read-char from the minibuffer using eval-expression. read-char reads the keyboard macro’s very next character, which is 1. Then eval-expression displays its return value in the echo area. (read-char) ⇒ 49 ;; We assume here you use M-: to evaluate this. (symbol-function ’foo) ⇒ "^[:(read-char)^M1" (execute-kbd-macro ’foo) 49 ⇒ nil read-char-exclusive &optional prompt inherit-input-method seconds [Function] This function reads and returns a character of command input. If the user generates an event which is not a character, read-char-exclusive ignores it and reads another event, until it gets a character. The arguments work as in read-event. None of the above functions suppress quitting. num-nonmacro-input-events [Variable] This variable holds the total number of input events received so far from the terminal—not counting those generated by keyboard macros. We emphasize that, unlike read-key-sequence, the functions read-event, read-char, and read-char-exclusive do not perform the translations described in Section 21.14 [Translation Keymaps], page 386. If you wish to read a single key taking these translations into account, use the function read-key: Chapter 20: Command Loop 353 read-key &optional prompt [Function] This function reads a single key. It is “intermediate” between read-key-sequence and read-event. Unlike the former, it reads a single key, not a key sequence. Unlike the latter, it does not return a raw event, but decodes and translates the user input according to input-decode-map, local-function-key-map, and key-translation- map (see Section 21.14 [Translation Keymaps], page 386). The argument prompt is either a string to be displayed in the echo area as a prompt, or nil, meaning not to display a prompt. read-char-choice prompt chars &optional inhibit-quit [Function] This function uses read-key to read and return a single character. It ignores any input that is not a member of chars, a list of accepted characters. Optionally, it will also ignore keyboard-quit events while it is waiting for valid input. If you bind help-form (see Section 23.5 [Help Functions], page 467) to a non-nil value while calling read-char-choice, then pressing help-char causes it to evaluate help-form and display the result. It then continues to wait for a valid input character, or keyboard-quit. 20.8.3 Modifying and Translating Input Events Emacs modifies every event it reads according to extra-keyboard-modifiers, then translates it through keyboard-translate-table (if applicable), before returning it from read-event. extra-keyboard-modifiers [Variable] This variable lets Lisp programs “press” the modifier keys on the keyboard. The value is a character. Only the modifiers of the character matter. Each time the user types a keyboard key, it is altered as if those modifier keys were held down. For instance, if you bind extra-keyboard-modifiers to ?\C-\M-a, then all keyboard input characters typed during the scope of the binding will have the control and meta modifiers applied to them. The character ?\C-@, equivalent to the integer 0, does not count as a control character for this purpose, but as a character with no modifiers. Thus, setting extra-keyboard-modifiers to zero cancels any modification. When using a window system, the program can “press” any of the modifier keys in this way. Otherwise, only the CTL and META keys can be virtually pressed. Note that this variable applies only to events that really come from the keyboard, and has no effect on mouse events or any other events. keyboard-translate-table [Variable] This terminal-local variable is the translate table for keyboard characters. It lets you reshuffle the keys on the keyboard without changing any command bindings. Its value is normally a char-table, or else nil. (It can also be a string or vector, but this is considered obsolete.) If keyboard-translate-table is a char-table (see Section 6.6 [Char-Tables], page 94), then each character read from the keyboard is looked up in this char-table. If the value found there is non-nil, then it is used instead of the actual input character. Chapter 20: Command Loop 354 Note that this translation is the first thing that happens to a character after it is read from the terminal. Record-keeping features such as recent-keys and dribble files record the characters after translation. Note also that this translation is done before the characters are supplied to input methods (see Section 32.11 [Input Methods], page 741). Use translation-tablefor-input (see Section 32.9 [Translation of Characters], page 727), if you want to translate characters after input methods operate. keyboard-translate from to [Function] This function modifies keyboard-translate-table to translate character code from into character code to. It creates the keyboard translate table if necessary. Here’s an example of using the keyboard-translate-table to make C-x, C-c and C-v perform the cut, copy and paste operations: (keyboard-translate ?\C-x ’control-x) (keyboard-translate ?\C-c ’control-c) (keyboard-translate ?\C-v ’control-v) (global-set-key [control-x] ’kill-region) (global-set-key [control-c] ’kill-ring-save) (global-set-key [control-v] ’yank) On a graphical terminal that supports extended ASCII input, you can still get the standard Emacs meanings of one of those characters by typing it with the shift key. That makes it a different character as far as keyboard translation is concerned, but it has the same usual meaning. See Section 21.14 [Translation Keymaps], page 386, for mechanisms that translate event sequences at the level of read-key-sequence. 20.8.4 Invoking the Input Method The event-reading functions invoke the current input method, if any (see Section 32.11 [Input Methods], page 741). If the value of input-method-function is non-nil, it should be a function; when read-event reads a printing character (including SPC) with no modifier bits, it calls that function, passing the character as an argument. input-method-function [Variable] If this is non-nil, its value specifies the current input method function. Warning: don’t bind this variable with let. It is often buffer-local, and if you bind it around reading input (which is exactly when you would bind it), switching buffers asynchronously while Emacs is waiting will cause the value to be restored in the wrong buffer. The input method function should return a list of events which should be used as input. (If the list is nil, that means there is no input, so read-event waits for another event.) These events are processed before the events in unread-command-events (see Section 20.8.6 [Event Input Misc], page 355). Events returned by the input method function are not passed to the input method function again, even if they are printing characters with no modifier bits. Chapter 20: Command Loop 355 If the input method function calls read-event or read-key-sequence, it should bind input-method-function to nil first, to prevent recursion. The input method function is not called when reading the second and subsequent events of a key sequence. Thus, these characters are not subject to input method processing. The input method function should test the values of overriding-local-map and overriding-terminal-local-map; if either of these variables is non-nil, the input method should put its argument into a list and return that list with no further processing. 20.8.5 Quoted Character Input You can use the function read-quoted-char to ask the user to specify a character, and allow the user to specify a control or meta character conveniently, either literally or as an octal character code. The command quoted-insert uses this function. read-quoted-char &optional prompt [Function] This function is like read-char, except that if the first character read is an octal digit (0–7), it reads any number of octal digits (but stopping if a non-octal digit is found), and returns the character represented by that numeric character code. If the character that terminates the sequence of octal digits is RET, it is discarded. Any other terminating character is used as input after this function returns. Quitting is suppressed when the first character is read, so that the user can enter a C-g. See Section 20.11 [Quitting], page 358. If prompt is supplied, it specifies a string for prompting the user. The prompt string is always displayed in the echo area, followed by a single ‘-’. In the following example, the user types in the octal number 177 (which is 127 in decimal). (read-quoted-char "What character") ---------- Echo Area ---------What character 1 7 7---------- Echo Area ---------- ⇒ 127 20.8.6 Miscellaneous Event Input Features This section describes how to “peek ahead” at events without using them up, how to check for pending input, and how to discard pending input. See also the function read-passwd (see Section 19.9 [Reading a Password], page 317). unread-command-events [Variable] This variable holds a list of events waiting to be read as command input. The events are used in the order they appear in the list, and removed one by one as they are used. The variable is needed because in some cases a function reads an event and then decides not to use it. Storing the event in this variable causes it to be processed normally, by the command loop or by the functions to read command input. Chapter 20: Command Loop 356 For example, the function that implements numeric prefix arguments reads any number of digits. When it finds a non-digit event, it must unread the event so that it can be read normally by the command loop. Likewise, incremental search uses this feature to unread events with no special meaning in a search, because these events should exit the search and then execute normally. The reliable and easy way to extract events from a key sequence so as to put them in unread-command-events is to use listify-key-sequence (see below). Normally you add events to the front of this list, so that the events most recently unread will be reread first. Events read from this list are not normally added to the current command’s key sequence (as returned by, e.g., this-command-keys), as the events will already have been added once as they were read for the first time. An element of the form (t . event) forces event to be added to the current command’s key sequence. listify-key-sequence key [Function] This function converts the string or vector key to a list of individual events, which you can put in unread-command-events. input-pending-p &optional check-timers [Function] This function determines whether any command input is currently available to be read. It returns immediately, with value t if there is available input, nil otherwise. On rare occasions it may return t when no input is available. If the optional argument check-timers is non-nil, then if no input is available, Emacs runs any timers which are ready. See Section 38.10 [Timers], page 944. last-input-event [Variable] This variable records the last terminal input event read, whether as part of a command or explicitly by a Lisp program. In the example below, the Lisp program reads the character 1, ASCII code 49. It becomes the value of last-input-event, while C-e (we assume C-x C-e command is used to evaluate this expression) remains the value of last-command-event. (progn (print (read-char)) (print last-command-event) last-input-event) 49 5 ⇒ 49 while-no-input body. . . [Macro] This construct runs the body forms and returns the value of the last one—but only if no input arrives. If any input arrives during the execution of the body forms, it aborts them (working much like a quit). The while-no-input form returns nil if aborted by a real quit, and returns t if aborted by arrival of other input. If a part of body binds inhibit-quit to non-nil, arrival of input during those parts won’t cause an abort until the end of that part. If you want to be able to distinguish all possible values computed by body from both kinds of abort conditions, write the code like this: Chapter 20: Command Loop 357 (while-no-input (list (progn . body))) discard-input [Function] This function discards the contents of the terminal input buffer and cancels any keyboard macro that might be in the process of definition. It returns nil. In the following example, the user may type a number of characters right after starting the evaluation of the form. After the sleep-for finishes sleeping, discard-input discards any characters typed during the sleep. (progn (sleep-for 2) (discard-input)) ⇒ nil 20.9 Special Events Certain special events are handled at a very low level—as soon as they are read. The read-event function processes these events itself, and never returns them. Instead, it keeps waiting for the first event that is not special and returns that one. Special events do not echo, they are never grouped into key sequences, and they never appear in the value of last-command-event or (this-command-keys). They do not discard a numeric argument, they cannot be unread with unread-command-events, they may not appear in a keyboard macro, and they are not recorded in a keyboard macro while you are defining one. Special events do, however, appear in last-input-event immediately after they are read, and this is the way for the event’s definition to find the actual event. The events types iconify-frame, make-frame-visible, delete-frame, drag-n-drop, language-change, and user signals like sigusr1 are normally handled in this way. The keymap which defines how to handle special events—and which events are special—is in the variable special-event-map (see Section 21.7 [Active Keymaps], page 374). 20.10 Waiting for Elapsed Time or Input The wait functions are designed to wait for a certain amount of time to pass or until there is input. For example, you may wish to pause in the middle of a computation to allow the user time to view the display. sit-for pauses and updates the screen, and returns immediately if input comes in, while sleep-for pauses without updating the screen. sit-for seconds &optional nodisp [Function] This function performs redisplay (provided there is no pending input from the user), then waits seconds seconds, or until input is available. The usual purpose of sit-for is to give the user time to read text that you display. The value is t if sit-for waited the full time with no input arriving (see Section 20.8.6 [Event Input Misc], page 355). Otherwise, the value is nil. The argument seconds need not be an integer. If it is floating point, sit-for waits for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, seconds is rounded down. Chapter 20: Command Loop 358 The expression (sit-for 0) is equivalent to (redisplay), i.e., it requests a redisplay, without any delay, if there is no pending input. See Section 37.2 [Forcing Redisplay], page 834. If nodisp is non-nil, then sit-for does not redisplay, but it still returns as soon as input is available (or when the timeout elapses). In batch mode (see Section 38.16 [Batch Mode], page 952), sit-for cannot be interrupted, even by input from the standard input descriptor. It is thus equivalent to sleep-for, which is described below. It is also possible to call sit-for with three arguments, as (sit-for seconds millisec nodisp), but that is considered obsolete. sleep-for seconds &optional millisec [Function] This function simply pauses for seconds seconds without updating the display. It pays no attention to available input. It returns nil. The argument seconds need not be an integer. If it is floating point, sleep-for waits for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, seconds is rounded down. The optional argument millisec specifies an additional waiting period measured in milliseconds. This adds to the period specified by seconds. If the system doesn’t support waiting fractions of a second, you get an error if you specify nonzero millisec. Use sleep-for when you wish to guarantee a delay. See Section 38.5 [Time of Day], page 938, for functions to get the current time. 20.11 Quitting Typing C-g while a Lisp function is running causes Emacs to quit whatever it is doing. This means that control returns to the innermost active command loop. Typing C-g while the command loop is waiting for keyboard input does not cause a quit; it acts as an ordinary input character. In the simplest case, you cannot tell the difference, because C-g normally runs the command keyboard-quit, whose effect is to quit. However, when C-g follows a prefix key, they combine to form an undefined key. The effect is to cancel the prefix key as well as any prefix argument. In the minibuffer, C-g has a different definition: it aborts out of the minibuffer. This means, in effect, that it exits the minibuffer and then quits. (Simply quitting would return to the command loop within the minibuffer.) The reason why C-g does not quit directly when the command reader is reading input is so that its meaning can be redefined in the minibuffer in this way. C-g following a prefix key is not redefined in the minibuffer, and it has its normal effect of canceling the prefix key and prefix argument. This too would not be possible if C-g always quit directly. When C-g does directly quit, it does so by setting the variable quit-flag to t. Emacs checks this variable at appropriate times and quits if it is not nil. Setting quit-flag non-nil in any way thus causes a quit. At the level of C code, quitting cannot happen just anywhere; only at the special places that check quit-flag. The reason for this is that quitting at other places might leave Chapter 20: Command Loop 359 an inconsistency in Emacs’s internal state. Because quitting is delayed until a safe place, quitting cannot make Emacs crash. Certain functions such as read-key-sequence or read-quoted-char prevent quitting entirely even though they wait for input. Instead of quitting, C-g serves as the requested input. In the case of read-key-sequence, this serves to bring about the special behavior of C-g in the command loop. In the case of read-quoted-char, this is so that C-q can be used to quote a C-g. You can prevent quitting for a portion of a Lisp function by binding the variable inhibit-quit to a non-nil value. Then, although C-g still sets quit-flag to t as usual, the usual result of this—a quit—is prevented. Eventually, inhibit-quit will become nil again, such as when its binding is unwound at the end of a let form. At that time, if quit-flag is still non-nil, the requested quit happens immediately. This behavior is ideal when you wish to make sure that quitting does not happen within a “critical section” of the program. In some functions (such as read-quoted-char), C-g is handled in a special way that does not involve quitting. This is done by reading the input with inhibit-quit bound to t, and setting quit-flag to nil before inhibit-quit becomes nil again. This excerpt from the definition of read-quoted-char shows how this is done; it also shows that normal quitting is permitted after the first character of input. (defun read-quoted-char (&optional prompt) "...documentation..." (let ((message-log-max nil) done (first t) (code 0) char) (while (not done) (let ((inhibit-quit first) ...) (and prompt (message "%s-" prompt)) (setq char (read-event)) (if inhibit-quit (setq quit-flag nil))) . . . set the variable code. . . ) code)) quit-flag [Variable] If this variable is non-nil, then Emacs quits immediately, unless inhibit-quit is non-nil. Typing C-g ordinarily sets quit-flag non-nil, regardless of inhibit-quit. inhibit-quit [Variable] This variable determines whether Emacs should quit when quit-flag is set to a value other than nil. If inhibit-quit is non-nil, then quit-flag has no special effect. with-local-quit body. . . [Macro] This macro executes body forms in sequence, but allows quitting, at least locally, within body even if inhibit-quit was non-nil outside this construct. It returns the value of the last form in body, unless exited by quitting, in which case it returns nil. If inhibit-quit is nil on entry to with-local-quit, it only executes the body, and setting quit-flag causes a normal quit. However, if inhibit-quit is non-nil so that ordinary quitting is delayed, a non-nil quit-flag triggers a special kind of local quit. This ends the execution of body and exits the with-local-quit body Chapter 20: Command Loop 360 with quit-flag still non-nil, so that another (ordinary) quit will happen as soon as that is allowed. If quit-flag is already non-nil at the beginning of body, the local quit happens immediately and the body doesn’t execute at all. This macro is mainly useful in functions that can be called from timers, process filters, process sentinels, pre-command-hook, post-command-hook, and other places where inhibit-quit is normally bound to t. keyboard-quit [Command] This function signals the quit condition with (signal ’quit nil). This is the same thing that quitting does. (See signal in Section 10.5.3 [Errors], page 134.) You can specify a character other than C-g to use for quitting. See the function set-input-mode in Section 38.12.1 [Input Modes], page 948. 20.12 Prefix Command Arguments Most Emacs commands can use a prefix argument, a number specified before the command itself. (Don’t confuse prefix arguments with prefix keys.) The prefix argument is at all times represented by a value, which may be nil, meaning there is currently no prefix argument. Each command may use the prefix argument or ignore it. There are two representations of the prefix argument: raw and numeric. The editor command loop uses the raw representation internally, and so do the Lisp variables that store the information, but commands can request either representation. Here are the possible values of a raw prefix argument: • nil, meaning there is no prefix argument. Its numeric value is 1, but numerous commands make a distinction between nil and the integer 1. • An integer, which stands for itself. • A list of one element, which is an integer. This form of prefix argument results from one or a succession of C-us with no digits. The numeric value is the integer in the list, but some commands make a distinction between such a list and an integer alone. • The symbol -. This indicates that M-- or C-u - was typed, without following digits. The equivalent numeric value is −1, but some commands make a distinction between the integer −1 and the symbol -. We illustrate these possibilities by calling the following function with various prefixes: (defun display-prefix (arg) "Display the value of the raw prefix arg." (interactive "P") (message "%s" arg)) Here are the results of calling display-prefix with various raw prefix arguments: M-x display-prefix nil C-u M-x display-prefix (4) C-u C-u M-x display-prefix (16) Chapter 20: Command Loop 361 C-u 3 M-x display-prefix 3 M-3 M-x display-prefix 3 ; (Same as C-u 3.) C-u - M-x display-prefix - M-- M-x display-prefix - ; (Same as C-u -.) C-u - 7 M-x display-prefix -7 M-- 7 M-x display-prefix -7 ; (Same as C-u -7.) Emacs uses two variables to store the prefix argument: prefix-arg and current-prefix-arg. Commands such as universal-argument that set up prefix arguments for other commands store them in prefix-arg. In contrast, current-prefix-arg conveys the prefix argument to the current command, so setting it has no effect on the prefix arguments for future commands. Normally, commands specify which representation to use for the prefix argument, either numeric or raw, in the interactive specification. (See Section 20.2.1 [Using Interactive], page 322.) Alternatively, functions may look at the value of the prefix argument directly in the variable current-prefix-arg, but this is less clean. prefix-numeric-value arg [Function] This function returns the numeric meaning of a valid raw prefix argument value, arg. The argument may be a symbol, a number, or a list. If it is nil, the value 1 is returned; if it is -, the value −1 is returned; if it is a number, that number is returned; if it is a list, the car of that list (which should be a number) is returned. current-prefix-arg [Variable] This variable holds the raw prefix argument for the current command. Commands may examine it directly, but the usual method for accessing it is with (interactive "P"). prefix-arg [Variable] The value of this variable is the raw prefix argument for the next editing command. Commands such as universal-argument that specify prefix arguments for the fol- lowing command work by setting this variable. last-prefix-arg The raw prefix argument value used by the previous command. [Variable] The following commands exist to set up prefix arguments for the following command. Do not call them for any other reason. universal-argument [Command] This command reads input and specifies a prefix argument for the following command. Don’t call this command yourself unless you know what you are doing. digit-argument arg [Command] This command adds to the prefix argument for the following command. The argument arg is the raw prefix argument as it was before this command; it is used to compute Chapter 20: Command Loop 362 the updated prefix argument. Don’t call this command yourself unless you know what you are doing. negative-argument arg [Command] This command adds to the numeric argument for the next command. The argument arg is the raw prefix argument as it was before this command; its value is negated to form the new prefix argument. Don’t call this command yourself unless you know what you are doing. 20.13 Recursive Editing The Emacs command loop is entered automatically when Emacs starts up. This top-level invocation of the command loop never exits; it keeps running as long as Emacs does. Lisp programs can also invoke the command loop. Since this makes more than one activation of the command loop, we call it recursive editing. A recursive editing level has the effect of suspending whatever command invoked it and permitting the user to do arbitrary editing before resuming that command. The commands available during recursive editing are the same ones available in the top-level editing loop and defined in the keymaps. Only a few special commands exit the recursive editing level; the others return to the recursive editing level when they finish. (The special commands for exiting are always available, but they do nothing when recursive editing is not in progress.) All command loops, including recursive ones, set up all-purpose error handlers so that an error in a command run from the command loop will not exit the loop. Minibuffer input is a special kind of recursive editing. It has a few special wrinkles, such as enabling display of the minibuffer and the minibuffer window, but fewer than you might suppose. Certain keys behave differently in the minibuffer, but that is only because of the minibuffer’s local map; if you switch windows, you get the usual Emacs commands. To invoke a recursive editing level, call the function recursive-edit. This function contains the command loop; it also contains a call to catch with tag exit, which makes it possible to exit the recursive editing level by throwing to exit (see Section 10.5.1 [Catch and Throw], page 131). If you throw a value other than t, then recursive-edit returns normally to the function that called it. The command C-M-c (exit-recursive-edit) does this. Throwing a t value causes recursive-edit to quit, so that control returns to the command loop one level up. This is called aborting, and is done by C-] (abort-recursiveedit). Most applications should not use recursive editing, except as part of using the minibuffer. Usually it is more convenient for the user if you change the major mode of the current buffer temporarily to a special major mode, which should have a command to go back to the previous mode. (The e command in Rmail uses this technique.) Or, if you wish to give the user different text to edit “recursively”, create and select a new buffer in a special mode. In this mode, define a command to complete the processing and go back to the previous buffer. (The m command in Rmail does this.) Recursive edits are useful in debugging. You can insert a call to debug into a function definition as a sort of breakpoint, so that you can look around when the function gets there. debug invokes a recursive edit but also provides the other features of the debugger. Chapter 20: Command Loop 363 Recursive editing levels are also used when you type C-r in query-replace or use C-x q (kbd-macro-query). recursive-edit [Command] This function invokes the editor command loop. It is called automatically by the ini- tialization of Emacs, to let the user begin editing. When called from a Li