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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
fd897522 | 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1995, 1998, 1999 |
177c0ea7 | 4 | @c Free Software Foundation, Inc. |
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5 | @c See the file elisp.texi for copying conditions. |
6 | @setfilename ../info/tips | |
e23a63a5 | 7 | @node Tips, GNU Emacs Internals, GPL, Top |
2323275b | 8 | @appendix Tips and Conventions |
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9 | @cindex tips |
10 | @cindex standards of coding style | |
11 | @cindex coding standards | |
12 | ||
2323275b RS |
13 | This chapter describes no additional features of Emacs Lisp. Instead |
14 | it gives advice on making effective use of the features described in the | |
15 | previous chapters, and describes conventions Emacs Lisp programmers | |
16 | should follow. | |
7015aca4 | 17 | |
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18 | You can automatically check some of the conventions described below by |
19 | running the command @kbd{M-x checkdoc RET} when visiting a Lisp file. | |
20 | It cannot check all of the conventions, and not all the warnings it | |
21 | gives necessarily correspond to problems, but it is worth examining them | |
22 | all. | |
23 | ||
7015aca4 | 24 | @menu |
2323275b | 25 | * Coding Conventions:: Conventions for clean and robust programs. |
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26 | * Compilation Tips:: Making compiled code run fast. |
27 | * Documentation Tips:: Writing readable documentation strings. | |
28 | * Comment Tips:: Conventions for writing comments. | |
29 | * Library Headers:: Standard headers for library packages. | |
30 | @end menu | |
31 | ||
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32 | @node Coding Conventions |
33 | @section Emacs Lisp Coding Conventions | |
7015aca4 | 34 | |
2323275b RS |
35 | Here are conventions that you should follow when writing Emacs Lisp |
36 | code intended for widespread use: | |
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37 | |
38 | @itemize @bullet | |
39 | @item | |
92204c92 RS |
40 | Since all global variables share the same name space, and all |
41 | functions share another name space, you should choose a short word to | |
42 | distinguish your program from other Lisp programs.@footnote{The | |
43 | benefits of a Common Lisp-style package system are considered not to | |
44 | outweigh the costs.} Then take care to begin the names of all global | |
45 | variables, constants, and functions in your program with the chosen | |
46 | prefix. This helps avoid name conflicts. | |
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47 | |
48 | This recommendation applies even to names for traditional Lisp | |
378d0f8e | 49 | primitives that are not primitives in Emacs Lisp---such as |
969fe9b5 RS |
50 | @code{copy-list}. Believe it or not, there is more than one plausible |
51 | way to define @code{copy-list}. Play it safe; append your name prefix | |
52 | to produce a name like @code{foo-copy-list} or @code{mylib-copy-list} | |
53 | instead. | |
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54 | |
55 | If you write a function that you think ought to be added to Emacs under | |
56 | a certain name, such as @code{twiddle-files}, don't call it by that name | |
57 | in your program. Call it @code{mylib-twiddle-files} in your program, | |
a9f0a989 | 58 | and send mail to @samp{bug-gnu-emacs@@gnu.org} suggesting we add |
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59 | it to Emacs. If and when we do, we can change the name easily enough. |
60 | ||
61 | If one prefix is insufficient, your package may use two or three | |
62 | alternative common prefixes, so long as they make sense. | |
63 | ||
64 | Separate the prefix from the rest of the symbol name with a hyphen, | |
65 | @samp{-}. This will be consistent with Emacs itself and with most Emacs | |
66 | Lisp programs. | |
67 | ||
68 | @item | |
69 | It is often useful to put a call to @code{provide} in each separate | |
70 | library program, at least if there is more than one entry point to the | |
71 | program. | |
72 | ||
bfe721d1 KH |
73 | @item |
74 | If a file requires certain other library programs to be loaded | |
75 | beforehand, then the comments at the beginning of the file should say | |
76 | so. Also, use @code{require} to make sure they are loaded. | |
77 | ||
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78 | @item |
79 | If one file @var{foo} uses a macro defined in another file @var{bar}, | |
bfe721d1 KH |
80 | @var{foo} should contain this expression before the first use of the |
81 | macro: | |
82 | ||
83 | @example | |
84 | (eval-when-compile (require '@var{bar})) | |
85 | @end example | |
86 | ||
87 | @noindent | |
969fe9b5 RS |
88 | (And the library @var{bar} should contain @code{(provide '@var{bar})}, |
89 | to make the @code{require} work.) This will cause @var{bar} to be | |
90 | loaded when you byte-compile @var{foo}. Otherwise, you risk compiling | |
91 | @var{foo} without the necessary macro loaded, and that would produce | |
92 | compiled code that won't work right. @xref{Compiling Macros}. | |
bfe721d1 KH |
93 | |
94 | Using @code{eval-when-compile} avoids loading @var{bar} when | |
95 | the compiled version of @var{foo} is @emph{used}. | |
7015aca4 | 96 | |
becd5943 KH |
97 | @item |
98 | Please don't require the @code{cl} package of Common Lisp extensions at | |
99 | run time. Use of this package is optional, and it is not part of the | |
100 | standard Emacs namespace. If your package loads @code{cl} at run time, | |
101 | that could cause name clashes for users who don't use that package. | |
102 | ||
103 | However, there is no problem with using the @code{cl} package at compile | |
378d0f8e | 104 | time, with @code{(eval-when-compile (require 'cl))}. |
becd5943 | 105 | |
7015aca4 | 106 | @item |
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107 | When defining a major mode, please follow the major mode |
108 | conventions. @xref{Major Mode Conventions}. | |
109 | ||
110 | @item | |
111 | When defining a minor mode, please follow the minor mode | |
112 | conventions. @xref{Minor Mode Conventions}. | |
7015aca4 | 113 | |
6cbf476c RS |
114 | @item |
115 | If the purpose of a function is to tell you whether a certain condition | |
116 | is true or false, give the function a name that ends in @samp{p}. If | |
117 | the name is one word, add just @samp{p}; if the name is multiple words, | |
118 | add @samp{-p}. Examples are @code{framep} and @code{frame-live-p}. | |
119 | ||
120 | @item | |
121 | If a user option variable records a true-or-false condition, give it a | |
122 | name that ends in @samp{-flag}. | |
123 | ||
49247521 LK |
124 | @item |
125 | If the purpose of a variable is to store a single function, give it a | |
126 | name that ends in @samp{-function}. If the purpose of a variable is | |
127 | to store a list of functions (i.e., the variable is a hook), please | |
128 | follow the naming conventions for hooks. @xref{Hooks}. | |
129 | ||
7015aca4 | 130 | @item |
a9f0a989 RS |
131 | @cindex reserved keys |
132 | @cindex keys, reserved | |
378d0f8e RS |
133 | Please do not define @kbd{C-c @var{letter}} as a key in Lisp programs. |
134 | Sequences consisting of @kbd{C-c} and a letter (either upper or lower | |
135 | case) are reserved for users; they are the @strong{only} sequences | |
136 | reserved for users, so do not block them. | |
7015aca4 | 137 | |
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138 | Changing all the Emacs major modes to respect this convention was a |
139 | lot of work; abandoning this convention would make that work go to | |
140 | waste, and inconvenience users. Please comply with it. | |
7015aca4 | 141 | |
378d0f8e RS |
142 | @item |
143 | Function keys @key{F5} through @key{F9} without modifier keys are | |
144 | also reserved for users to define. | |
145 | ||
146 | @item | |
147 | Applications should not bind mouse events based on button 1 with the | |
148 | shift key held down. These events include @kbd{S-mouse-1}, | |
149 | @kbd{M-S-mouse-1}, @kbd{C-S-mouse-1}, and so on. They are reserved for | |
150 | users. | |
151 | ||
17d4b8ce RS |
152 | @item |
153 | Sequences consisting of @kbd{C-c} followed by a control character or a | |
154 | digit are reserved for major modes. | |
00d96ada RS |
155 | |
156 | @item | |
157 | Sequences consisting of @kbd{C-c} followed by @kbd{@{}, @kbd{@}}, | |
158 | @kbd{<}, @kbd{>}, @kbd{:} or @kbd{;} are also reserved for major modes. | |
159 | ||
160 | @item | |
161 | Sequences consisting of @kbd{C-c} followed by any other punctuation | |
162 | character are allocated for minor modes. Using them in a major mode is | |
163 | not absolutely prohibited, but if you do that, the major mode binding | |
164 | may be shadowed from time to time by minor modes. | |
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165 | |
166 | @item | |
f9f59935 | 167 | Do not bind @kbd{C-h} following any prefix character (including |
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168 | @kbd{C-c}). If you don't bind @kbd{C-h}, it is automatically available |
169 | as a help character for listing the subcommands of the prefix character. | |
170 | ||
171 | @item | |
f9f59935 | 172 | Do not bind a key sequence ending in @key{ESC} except following |
969fe9b5 | 173 | another @key{ESC}. (That is, it is OK to bind a sequence ending in |
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174 | @kbd{@key{ESC} @key{ESC}}.) |
175 | ||
176 | The reason for this rule is that a non-prefix binding for @key{ESC} in | |
177 | any context prevents recognition of escape sequences as function keys in | |
178 | that context. | |
179 | ||
52c90d84 RS |
180 | @item |
181 | Anything which acts like a temporary mode or state which the user can | |
b6ae404e | 182 | enter and leave should define @kbd{@key{ESC} @key{ESC}} or |
52c90d84 RS |
183 | @kbd{@key{ESC} @key{ESC} @key{ESC}} as a way to escape. |
184 | ||
185 | For a state which accepts ordinary Emacs commands, or more generally any | |
186 | kind of state in which @key{ESC} followed by a function key or arrow key | |
187 | is potentially meaningful, then you must not define @kbd{@key{ESC} | |
188 | @key{ESC}}, since that would preclude recognizing an escape sequence | |
189 | after @key{ESC}. In these states, you should define @kbd{@key{ESC} | |
190 | @key{ESC} @key{ESC}} as the way to escape. Otherwise, define | |
191 | @kbd{@key{ESC} @key{ESC}} instead. | |
192 | ||
4b6694ef | 193 | @item |
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194 | @cindex mouse-2 |
195 | @cindex references, following | |
f9f59935 RS |
196 | Special major modes used for read-only text should usually redefine |
197 | @kbd{mouse-2} and @key{RET} to trace some sort of reference in the text. | |
198 | Modes such as Dired, Info, Compilation, and Occur redefine it in this | |
199 | way. | |
4b6694ef | 200 | |
378d0f8e RS |
201 | In addition, they should mark the text as a kind of ``link'' so that |
202 | @kbd{mouse-1} will follow it also. @xref{Links and Mouse-1}. | |
203 | ||
7015aca4 | 204 | @item |
8414f615 | 205 | When a package provides a modification of ordinary Emacs behavior, it is |
b8fbee64 | 206 | good to include a command to enable and disable the feature, provide a |
8414f615 RS |
207 | command named @code{@var{whatever}-mode} which turns the feature on or |
208 | off, and make it autoload (@pxref{Autoload}). Design the package so | |
209 | that simply loading it has no visible effect---that should not enable | |
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210 | the feature.@footnote{Consider that the package may be loaded |
211 | arbitrarily by Custom for instance.} Users will request the feature by | |
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212 | invoking the command. It is a good idea to define this command |
213 | as a minor mode. | |
b68c6256 DL |
214 | |
215 | @cindex unloading packages | |
3ff91798 RS |
216 | If loading the file adds functions to hooks, define a function |
217 | @code{@var{feature}-unload-hook}, where @var{feature} is the name of | |
218 | the feature the package provides, and make it undo any such changes. | |
219 | Using @code{unload-feature} to unload the file will run this function. | |
220 | @xref{Unloading}. | |
8414f615 RS |
221 | |
222 | @item | |
223 | It is a bad idea to define aliases for the Emacs primitives. Use the | |
224 | standard names instead. | |
7015aca4 | 225 | |
c7a401dd DL |
226 | @item |
227 | If a package needs to define an alias or a new function for | |
bbac5699 | 228 | compatibility with some other version of Emacs, name it with the package |
c7a401dd DL |
229 | prefix, not with the raw name with which it occurs in the other version. |
230 | Here is an example from Gnus, which provides many examples of such | |
231 | compatibility issues. | |
232 | ||
233 | @example | |
234 | (defalias 'gnus-point-at-bol | |
235 | (if (fboundp 'point-at-bol) | |
236 | 'point-at-bol | |
237 | 'line-beginning-position)) | |
238 | @end example | |
239 | ||
7015aca4 | 240 | @item |
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241 | Redefining (or advising) an Emacs primitive is discouraged. It may do |
242 | the right thing for a particular program, but there is no telling what | |
243 | other programs might break as a result. | |
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244 | |
245 | @item | |
246 | If a file does replace any of the functions or library programs of | |
247 | standard Emacs, prominent comments at the beginning of the file should | |
248 | say which functions are replaced, and how the behavior of the | |
249 | replacements differs from that of the originals. | |
250 | ||
3ff91798 RS |
251 | @item |
252 | Avoid using macros that define functions and variables with names that | |
ee49c7d9 | 253 | are constructed. It is best for maintenance when the name of the |
3ff91798 RS |
254 | function or variable being defined is given explicitly in the source |
255 | code, as the second element of the list---as it is when you use | |
e8d6f886 | 256 | @code{defun}, @code{defalias}, @code{defvar} and @code{defcustom}. |
3ff91798 | 257 | |
7015aca4 RS |
258 | @item |
259 | Please keep the names of your Emacs Lisp source files to 13 characters | |
260 | or less. This way, if the files are compiled, the compiled files' names | |
261 | will be 14 characters or less, which is short enough to fit on all kinds | |
262 | of Unix systems. | |
263 | ||
264 | @item | |
265 | Don't use @code{next-line} or @code{previous-line} in programs; nearly | |
266 | always, @code{forward-line} is more convenient as well as more | |
267 | predictable and robust. @xref{Text Lines}. | |
268 | ||
269 | @item | |
574efc83 RS |
270 | Don't call functions that set the mark, unless setting the mark is one |
271 | of the intended features of your program. The mark is a user-level | |
272 | feature, so it is incorrect to change the mark except to supply a value | |
273 | for the user's benefit. @xref{The Mark}. | |
7015aca4 | 274 | |
f9f59935 | 275 | In particular, don't use any of these functions: |
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276 | |
277 | @itemize @bullet | |
278 | @item | |
279 | @code{beginning-of-buffer}, @code{end-of-buffer} | |
280 | @item | |
281 | @code{replace-string}, @code{replace-regexp} | |
282 | @end itemize | |
283 | ||
284 | If you just want to move point, or replace a certain string, without any | |
285 | of the other features intended for interactive users, you can replace | |
286 | these functions with one or two lines of simple Lisp code. | |
287 | ||
1c2b5877 RS |
288 | @item |
289 | Use lists rather than vectors, except when there is a particular reason | |
290 | to use a vector. Lisp has more facilities for manipulating lists than | |
291 | for vectors, and working with lists is usually more convenient. | |
292 | ||
293 | Vectors are advantageous for tables that are substantial in size and are | |
294 | accessed in random order (not searched front to back), provided there is | |
295 | no need to insert or delete elements (only lists allow that). | |
296 | ||
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297 | @item |
298 | The recommended way to print a message in the echo area is with | |
299 | the @code{message} function, not @code{princ}. @xref{The Echo Area}. | |
300 | ||
301 | @item | |
302 | When you encounter an error condition, call the function @code{error} | |
303 | (or @code{signal}). The function @code{error} does not return. | |
304 | @xref{Signaling Errors}. | |
305 | ||
306 | Do not use @code{message}, @code{throw}, @code{sleep-for}, | |
307 | or @code{beep} to report errors. | |
308 | ||
bfe721d1 KH |
309 | @item |
310 | An error message should start with a capital letter but should not end | |
311 | with a period. | |
312 | ||
01e3636e RS |
313 | @item |
314 | In @code{interactive}, if you use a Lisp expression to produce a list | |
315 | of arguments, don't try to provide the ``correct'' default values for | |
316 | region or position arguments. Instead, provide @code{nil} for those | |
317 | arguments if they were not specified, and have the function body | |
318 | compute the default value when the argument is @code{nil}. For | |
319 | instance, write this: | |
320 | ||
321 | @example | |
322 | (defun foo (pos) | |
323 | (interactive | |
324 | (list (if @var{specified} @var{specified-pos}))) | |
325 | (unless pos (setq pos @var{default-pos})) | |
326 | ...) | |
327 | @end example | |
328 | ||
329 | @noindent | |
330 | rather than this: | |
331 | ||
332 | @example | |
333 | (defun foo (pos) | |
334 | (interactive | |
335 | (list (if @var{specified} @var{specified-pos} | |
336 | @var{default-pos}))) | |
337 | ...) | |
338 | @end example | |
339 | ||
340 | @noindent | |
341 | This is so that repetition of the command will recompute | |
342 | these defaults based on the current circumstances. | |
343 | ||
344 | You do not need to take such precautions when you use interactive | |
345 | specs @samp{d}, @samp{m} and @samp{r}, because they make special | |
346 | arrangements to recompute the argument values on repetition of the | |
347 | command. | |
348 | ||
2089b41a RS |
349 | @item |
350 | Many commands that take a long time to execute display a message that | |
01e3636e | 351 | says something like @samp{Operating...} when they start, and change it to |
2089b41a RS |
352 | @samp{Operating...done} when they finish. Please keep the style of |
353 | these messages uniform: @emph{no} space around the ellipsis, and | |
01e3636e | 354 | @emph{no} period after @samp{done}. |
2089b41a | 355 | |
7015aca4 | 356 | @item |
4b6694ef RS |
357 | Try to avoid using recursive edits. Instead, do what the Rmail @kbd{e} |
358 | command does: use a new local keymap that contains one command defined | |
359 | to switch back to the old local keymap. Or do what the | |
360 | @code{edit-options} command does: switch to another buffer and let the | |
361 | user switch back at will. @xref{Recursive Editing}. | |
7015aca4 RS |
362 | |
363 | @item | |
364 | In some other systems there is a convention of choosing variable names | |
365 | that begin and end with @samp{*}. We don't use that convention in Emacs | |
4b6694ef | 366 | Lisp, so please don't use it in your programs. (Emacs uses such names |
969fe9b5 | 367 | only for special-purpose buffers.) The users will find Emacs more |
4b6694ef | 368 | coherent if all libraries use the same conventions. |
7015aca4 | 369 | |
6a994023 RS |
370 | @item |
371 | Try to avoid compiler warnings about undefined free variables, by adding | |
155cb2e5 | 372 | dummy @code{defvar} definitions for these variables, like this: |
6a994023 | 373 | |
155cb2e5 RS |
374 | @example |
375 | (defvar foo) | |
376 | @end example | |
377 | ||
378 | Such a definition has no effect except to tell the compiler | |
379 | not to warn about uses of the variable @code{foo} in this file. | |
380 | ||
381 | @item | |
382 | If you use many functions and variables from a certain file, you can | |
383 | add a @code{require} for that package to avoid compilation warnings | |
378d0f8e | 384 | for them. For instance, |
8241495d RS |
385 | |
386 | @example | |
387 | (eval-when-compile | |
155cb2e5 | 388 | (require 'foo)) |
8241495d RS |
389 | @end example |
390 | ||
155cb2e5 RS |
391 | @item |
392 | If you bind a variable in one function, and use it or set it in | |
393 | another function, the compiler warns about the latter function unless | |
394 | the variable has a definition. But adding a definition would be | |
8b192981 | 395 | unclean if the variable has a short name, since Lisp packages should |
155cb2e5 RS |
396 | not define short variable names. The right thing to do is to rename |
397 | this variable to start with the name prefix used for the other | |
398 | functions and variables in your package. | |
6a994023 | 399 | |
7015aca4 RS |
400 | @item |
401 | Indent each function with @kbd{C-M-q} (@code{indent-sexp}) using the | |
402 | default indentation parameters. | |
403 | ||
404 | @item | |
405 | Don't make a habit of putting close-parentheses on lines by themselves; | |
406 | Lisp programmers find this disconcerting. Once in a while, when there | |
407 | is a sequence of many consecutive close-parentheses, it may make sense | |
969fe9b5 | 408 | to split the sequence in one or two significant places. |
7015aca4 RS |
409 | |
410 | @item | |
411 | Please put a copyright notice on the file if you give copies to anyone. | |
f9f59935 RS |
412 | Use a message like this one: |
413 | ||
414 | @smallexample | |
415 | ;; Copyright (C) @var{year} @var{name} | |
416 | ||
417 | ;; This program is free software; you can redistribute it and/or | |
418 | ;; modify it under the terms of the GNU General Public License as | |
419 | ;; published by the Free Software Foundation; either version 2 of | |
420 | ;; the License, or (at your option) any later version. | |
421 | ||
422 | ;; This program is distributed in the hope that it will be | |
423 | ;; useful, but WITHOUT ANY WARRANTY; without even the implied | |
424 | ;; warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR | |
425 | ;; PURPOSE. See the GNU General Public License for more details. | |
426 | ||
427 | ;; You should have received a copy of the GNU General Public | |
428 | ;; License along with this program; if not, write to the Free | |
429 | ;; Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, | |
430 | ;; MA 02111-1307 USA | |
431 | @end smallexample | |
432 | ||
433 | If you have signed papers to assign the copyright to the Foundation, | |
434 | then use @samp{Free Software Foundation, Inc.} as @var{name}. | |
378d0f8e | 435 | Otherwise, use your name. See also @xref{Library Headers}. |
7015aca4 RS |
436 | @end itemize |
437 | ||
438 | @node Compilation Tips | |
439 | @section Tips for Making Compiled Code Fast | |
440 | @cindex execution speed | |
441 | @cindex speedups | |
442 | ||
443 | Here are ways of improving the execution speed of byte-compiled | |
4b6694ef | 444 | Lisp programs. |
7015aca4 RS |
445 | |
446 | @itemize @bullet | |
447 | @item | |
448 | @cindex profiling | |
449 | @cindex timing programs | |
a9f0a989 | 450 | @cindex @file{elp.el} |
5f7eb05d EZ |
451 | Profile your program with the @file{elp} library. See the file |
452 | @file{elp.el} for instructions. | |
7015aca4 RS |
453 | |
454 | @item | |
455 | Use iteration rather than recursion whenever possible. | |
456 | Function calls are slow in Emacs Lisp even when a compiled function | |
457 | is calling another compiled function. | |
458 | ||
459 | @item | |
bfe721d1 KH |
460 | Using the primitive list-searching functions @code{memq}, @code{member}, |
461 | @code{assq}, or @code{assoc} is even faster than explicit iteration. It | |
f9f59935 | 462 | can be worth rearranging a data structure so that one of these primitive |
bfe721d1 | 463 | search functions can be used. |
7015aca4 RS |
464 | |
465 | @item | |
177c0ea7 | 466 | Certain built-in functions are handled specially in byte-compiled code, |
7015aca4 RS |
467 | avoiding the need for an ordinary function call. It is a good idea to |
468 | use these functions rather than alternatives. To see whether a function | |
469 | is handled specially by the compiler, examine its @code{byte-compile} | |
470 | property. If the property is non-@code{nil}, then the function is | |
471 | handled specially. | |
472 | ||
473 | For example, the following input will show you that @code{aref} is | |
a9f0a989 | 474 | compiled specially (@pxref{Array Functions}): |
7015aca4 | 475 | |
4b6694ef | 476 | @example |
7015aca4 RS |
477 | @group |
478 | (get 'aref 'byte-compile) | |
479 | @result{} byte-compile-two-args | |
480 | @end group | |
4b6694ef | 481 | @end example |
7015aca4 RS |
482 | |
483 | @item | |
1911e6e5 | 484 | If calling a small function accounts for a substantial part of your |
7015aca4 RS |
485 | program's running time, make the function inline. This eliminates |
486 | the function call overhead. Since making a function inline reduces | |
487 | the flexibility of changing the program, don't do it unless it gives | |
4b6694ef | 488 | a noticeable speedup in something slow enough that users care about |
7015aca4 RS |
489 | the speed. @xref{Inline Functions}. |
490 | @end itemize | |
491 | ||
492 | @node Documentation Tips | |
493 | @section Tips for Documentation Strings | |
494 | ||
969fe9b5 RS |
495 | @findex checkdoc-minor-mode |
496 | Here are some tips and conventions for the writing of documentation | |
497 | strings. You can check many of these conventions by running the command | |
498 | @kbd{M-x checkdoc-minor-mode}. | |
7015aca4 RS |
499 | |
500 | @itemize @bullet | |
501 | @item | |
574efc83 | 502 | Every command, function, or variable intended for users to know about |
7015aca4 RS |
503 | should have a documentation string. |
504 | ||
505 | @item | |
e0d32668 RS |
506 | An internal variable or subroutine of a Lisp program might as well have |
507 | a documentation string. In earlier Emacs versions, you could save space | |
508 | by using a comment instead of a documentation string, but that is no | |
2468d0c0 DL |
509 | longer the case---documentation strings now take up very little space in |
510 | a running Emacs. | |
7015aca4 | 511 | |
b090d792 RS |
512 | @item |
513 | Format the documentation string so that it fits in an Emacs window on an | |
514 | 80-column screen. It is a good idea for most lines to be no wider than | |
515 | 60 characters. The first line should not be wider than 67 characters | |
516 | or it will look bad in the output of @code{apropos}. | |
517 | ||
518 | You can fill the text if that looks good. However, rather than blindly | |
519 | filling the entire documentation string, you can often make it much more | |
520 | readable by choosing certain line breaks with care. Use blank lines | |
521 | between topics if the documentation string is long. | |
522 | ||
7015aca4 RS |
523 | @item |
524 | The first line of the documentation string should consist of one or two | |
574efc83 | 525 | complete sentences that stand on their own as a summary. @kbd{M-x |
2468d0c0 DL |
526 | apropos} displays just the first line, and if that line's contents don't |
527 | stand on their own, the result looks bad. In particular, start the | |
528 | first line with a capital letter and end with a period. | |
7015aca4 | 529 | |
aa5dbf7b RS |
530 | For a function, the first line should briefly answer the question, |
531 | ``What does this function do?'' For a variable, the first line should | |
532 | briefly answer the question, ``What does this value mean?'' | |
533 | ||
534 | Don't limit the documentation string to one line; use as many lines as | |
535 | you need to explain the details of how to use the function or | |
536 | variable. Please use complete sentences for the rest of the text too. | |
7015aca4 | 537 | |
7878d6b6 RS |
538 | @item |
539 | The first line should mention all the important arguments of the | |
540 | function, and should mention them in the order that they are written | |
541 | in a function call. If the function has many arguments, then it is | |
542 | not feasible to mention them all in the first line; in that case, the | |
543 | first line should mention the first few arguments, including the most | |
544 | important arguments. | |
545 | ||
4b6694ef | 546 | @item |
8241495d | 547 | For consistency, phrase the verb in the first sentence of a function's |
321a3725 | 548 | documentation string as an imperative---for instance, use ``Return the |
8241495d RS |
549 | cons of A and B.'' in preference to ``Returns the cons of A and B@.'' |
550 | Usually it looks good to do likewise for the rest of the first | |
551 | paragraph. Subsequent paragraphs usually look better if each sentence | |
b090d792 | 552 | is indicative and has a proper subject. |
4b6694ef | 553 | |
7015aca4 RS |
554 | @item |
555 | Write documentation strings in the active voice, not the passive, and in | |
556 | the present tense, not the future. For instance, use ``Return a list | |
557 | containing A and B.'' instead of ``A list containing A and B will be | |
558 | returned.'' | |
559 | ||
560 | @item | |
561 | Avoid using the word ``cause'' (or its equivalents) unnecessarily. | |
562 | Instead of, ``Cause Emacs to display text in boldface,'' write just | |
563 | ``Display text in boldface.'' | |
564 | ||
2468d0c0 DL |
565 | @item |
566 | When a command is meaningful only in a certain mode or situation, | |
567 | do mention that in the documentation string. For example, | |
568 | the documentation of @code{dired-find-file} is: | |
569 | ||
570 | @example | |
571 | In Dired, visit the file or directory named on this line. | |
572 | @end example | |
573 | ||
7015aca4 RS |
574 | @item |
575 | Do not start or end a documentation string with whitespace. | |
177c0ea7 | 576 | |
7015aca4 RS |
577 | @item |
578 | @strong{Do not} indent subsequent lines of a documentation string so | |
579 | that the text is lined up in the source code with the text of the first | |
580 | line. This looks nice in the source code, but looks bizarre when users | |
581 | view the documentation. Remember that the indentation before the | |
582 | starting double-quote is not part of the string! | |
583 | ||
75d97f47 RS |
584 | @item |
585 | When the user tries to use a disabled command, Emacs displays just the | |
586 | first paragraph of its documentation string---everything through the | |
587 | first blank line. If you wish, you can choose which information to | |
588 | include before the first blank line so as to make this display useful. | |
589 | ||
7015aca4 RS |
590 | @item |
591 | A variable's documentation string should start with @samp{*} if the | |
4b6694ef | 592 | variable is one that users would often want to set interactively. If |
574efc83 RS |
593 | the value is a long list, or a function, or if the variable would be set |
594 | only in init files, then don't start the documentation string with | |
7015aca4 RS |
595 | @samp{*}. @xref{Defining Variables}. |
596 | ||
597 | @item | |
598 | The documentation string for a variable that is a yes-or-no flag should | |
4b6694ef RS |
599 | start with words such as ``Non-nil means@dots{}'', to make it clear that |
600 | all non-@code{nil} values are equivalent and indicate explicitly what | |
601 | @code{nil} and non-@code{nil} mean. | |
7015aca4 | 602 | |
5c5b7d3e RS |
603 | @item |
604 | The documentation string for a function that is a yes-or-no predicate | |
605 | should start with words such as ``Return t if @dots{}'', to indicate | |
606 | explicitly what constitutes ``truth''. The word ``return'' avoids | |
607 | starting the sentence with lower-case ``t'', which is somewhat | |
608 | distracting. | |
609 | ||
7015aca4 RS |
610 | @item |
611 | When a function's documentation string mentions the value of an argument | |
612 | of the function, use the argument name in capital letters as if it were | |
613 | a name for that value. Thus, the documentation string of the function | |
2468d0c0 DL |
614 | @code{eval} refers to its second argument as @samp{FORM}, because the |
615 | actual argument name is @code{form}: | |
616 | ||
617 | @example | |
618 | Evaluate FORM and return its value. | |
619 | @end example | |
7015aca4 | 620 | |
2468d0c0 DL |
621 | Also write metasyntactic variables in capital letters, such as when you |
622 | show the decomposition of a list or vector into subunits, some of which | |
623 | may vary. @samp{KEY} and @samp{VALUE} in the following example | |
8241495d RS |
624 | illustrate this practice: |
625 | ||
626 | @example | |
627 | The argument TABLE should be an alist whose elements | |
628 | have the form (KEY . VALUE). Here, KEY is ... | |
629 | @end example | |
7015aca4 | 630 | |
5c5b7d3e RS |
631 | @item |
632 | Never change the case of a Lisp symbol when you mention it in a doc | |
633 | string. If the symbol's name is @code{foo}, write ``foo'', not | |
634 | ``Foo'' (which is a different symbol). | |
635 | ||
636 | This might appear to contradict the policy of writing function | |
637 | argument values, but there is no real contradiction; the argument | |
638 | @emph{value} is not the same thing as the @emph{symbol} which the | |
639 | function uses to hold the value. | |
640 | ||
641 | If this puts a lower-case letter at the beginning of a sentence | |
642 | and that annoys you, rewrite the sentence so that the symbol | |
643 | is not at the start of it. | |
644 | ||
2468d0c0 DL |
645 | @item |
646 | If a line in a documentation string begins with an open-parenthesis, | |
647 | write a backslash before the open-parenthesis, like this: | |
648 | ||
649 | @example | |
650 | The argument FOO can be either a number | |
651 | \(a buffer position) or a string (a file name). | |
652 | @end example | |
653 | ||
654 | This prevents the open-parenthesis from being treated as the start of a | |
655 | defun (@pxref{Defuns,, Defuns, emacs, The GNU Emacs Manual}). | |
656 | ||
70057f39 | 657 | @anchor{Docstring hyperlinks} |
7015aca4 RS |
658 | @item |
659 | @iftex | |
660 | When a documentation string refers to a Lisp symbol, write it as it | |
661 | would be printed (which usually means in lower case), with single-quotes | |
662 | around it. For example: @samp{`lambda'}. There are two exceptions: | |
663 | write @code{t} and @code{nil} without single-quotes. | |
664 | @end iftex | |
37680279 | 665 | @ifnottex |
7015aca4 RS |
666 | When a documentation string refers to a Lisp symbol, write it as it |
667 | would be printed (which usually means in lower case), with single-quotes | |
668 | around it. For example: @samp{lambda}. There are two exceptions: write | |
969fe9b5 RS |
669 | t and nil without single-quotes. (In this manual, we use a different |
670 | convention, with single-quotes for all symbols.) | |
37680279 | 671 | @end ifnottex |
7015aca4 | 672 | |
1911e6e5 RS |
673 | Help mode automatically creates a hyperlink when a documentation string |
674 | uses a symbol name inside single quotes, if the symbol has either a | |
a9f0a989 RS |
675 | function or a variable definition. You do not need to do anything |
676 | special to make use of this feature. However, when a symbol has both a | |
677 | function definition and a variable definition, and you want to refer to | |
678 | just one of them, you can specify which one by writing one of the words | |
679 | @samp{variable}, @samp{option}, @samp{function}, or @samp{command}, | |
680 | immediately before the symbol name. (Case makes no difference in | |
681 | recognizing these indicator words.) For example, if you write | |
682 | ||
683 | @example | |
684 | This function sets the variable `buffer-file-name'. | |
685 | @end example | |
686 | ||
687 | @noindent | |
688 | then the hyperlink will refer only to the variable documentation of | |
689 | @code{buffer-file-name}, and not to its function documentation. | |
690 | ||
691 | If a symbol has a function definition and/or a variable definition, but | |
692 | those are irrelevant to the use of the symbol that you are documenting, | |
693 | you can write the word @samp{symbol} before the symbol name to prevent | |
694 | making any hyperlink. For example, | |
969fe9b5 RS |
695 | |
696 | @example | |
a9f0a989 RS |
697 | If the argument KIND-OF-RESULT is the symbol `list', |
698 | this function returns a list of all the objects | |
699 | that satisfy the criterion. | |
969fe9b5 RS |
700 | @end example |
701 | ||
a9f0a989 RS |
702 | @noindent |
703 | does not make a hyperlink to the documentation, irrelevant here, of the | |
704 | function @code{list}. | |
705 | ||
e72850d5 LT |
706 | Normally, no hyperlink is made for a variable without variable |
707 | documentation. You can force a hyperlink for such variables by | |
708 | preceding them with one of the words @samp{variable} or | |
709 | @samp{option}. | |
710 | ||
711 | Hyperlinks for faces are only made if the face name is preceded or | |
712 | followed by the word @samp{face}. In that case, only the face | |
713 | documentation will be shown, even if the symbol is also defined as a | |
714 | variable or as a function. | |
715 | ||
8241495d | 716 | To make a hyperlink to Info documentation, write the name of the Info |
727c9159 LT |
717 | node (or anchor) in single quotes, preceded by @samp{info node}, |
718 | @samp{Info node}, @samp{info anchor} or @samp{Info anchor}. The Info | |
719 | file name defaults to @samp{emacs}. For example, | |
8241495d RS |
720 | |
721 | @smallexample | |
722 | See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'. | |
723 | @end smallexample | |
724 | ||
7015aca4 RS |
725 | @item |
726 | Don't write key sequences directly in documentation strings. Instead, | |
727 | use the @samp{\\[@dots{}]} construct to stand for them. For example, | |
9e2b495b RS |
728 | instead of writing @samp{C-f}, write the construct |
729 | @samp{\\[forward-char]}. When Emacs displays the documentation string, | |
730 | it substitutes whatever key is currently bound to @code{forward-char}. | |
731 | (This is normally @samp{C-f}, but it may be some other character if the | |
732 | user has moved key bindings.) @xref{Keys in Documentation}. | |
7015aca4 RS |
733 | |
734 | @item | |
735 | In documentation strings for a major mode, you will want to refer to the | |
736 | key bindings of that mode's local map, rather than global ones. | |
737 | Therefore, use the construct @samp{\\<@dots{}>} once in the | |
738 | documentation string to specify which key map to use. Do this before | |
739 | the first use of @samp{\\[@dots{}]}. The text inside the | |
740 | @samp{\\<@dots{}>} should be the name of the variable containing the | |
741 | local keymap for the major mode. | |
742 | ||
743 | It is not practical to use @samp{\\[@dots{}]} very many times, because | |
744 | display of the documentation string will become slow. So use this to | |
745 | describe the most important commands in your major mode, and then use | |
746 | @samp{\\@{@dots{}@}} to display the rest of the mode's keymap. | |
7015aca4 RS |
747 | @end itemize |
748 | ||
749 | @node Comment Tips | |
750 | @section Tips on Writing Comments | |
751 | ||
752 | We recommend these conventions for where to put comments and how to | |
753 | indent them: | |
754 | ||
755 | @table @samp | |
756 | @item ; | |
757 | Comments that start with a single semicolon, @samp{;}, should all be | |
758 | aligned to the same column on the right of the source code. Such | |
759 | comments usually explain how the code on the same line does its job. In | |
760 | Lisp mode and related modes, the @kbd{M-;} (@code{indent-for-comment}) | |
761 | command automatically inserts such a @samp{;} in the right place, or | |
4b6694ef | 762 | aligns such a comment if it is already present. |
7015aca4 | 763 | |
574efc83 | 764 | This and following examples are taken from the Emacs sources. |
7015aca4 RS |
765 | |
766 | @smallexample | |
767 | @group | |
768 | (setq base-version-list ; there was a base | |
769 | (assoc (substring fn 0 start-vn) ; version to which | |
770 | file-version-assoc-list)) ; this looks like | |
771 | ; a subversion | |
772 | @end group | |
773 | @end smallexample | |
774 | ||
775 | @item ;; | |
776 | Comments that start with two semicolons, @samp{;;}, should be aligned to | |
4b6694ef | 777 | the same level of indentation as the code. Such comments usually |
7015aca4 RS |
778 | describe the purpose of the following lines or the state of the program |
779 | at that point. For example: | |
780 | ||
781 | @smallexample | |
782 | @group | |
783 | (prog1 (setq auto-fill-function | |
784 | @dots{} | |
785 | @dots{} | |
4b6694ef | 786 | ;; update mode line |
7015aca4 RS |
787 | (force-mode-line-update))) |
788 | @end group | |
789 | @end smallexample | |
790 | ||
2468d0c0 | 791 | We also normally use two semicolons for comments outside functions. |
7015aca4 RS |
792 | |
793 | @smallexample | |
794 | @group | |
2468d0c0 DL |
795 | ;; This Lisp code is run in Emacs |
796 | ;; when it is to operate as a server | |
797 | ;; for other processes. | |
7015aca4 RS |
798 | @end group |
799 | @end smallexample | |
800 | ||
2468d0c0 DL |
801 | Every function that has no documentation string (presumably one that is |
802 | used only internally within the package it belongs to), should instead | |
803 | have a two-semicolon comment right before the function, explaining what | |
804 | the function does and how to call it properly. Explain precisely what | |
805 | each argument means and how the function interprets its possible values. | |
806 | ||
807 | @item ;;; | |
808 | Comments that start with three semicolons, @samp{;;;}, should start at | |
809 | the left margin. These are used, occasionally, for comments within | |
810 | functions that should start at the margin. We also use them sometimes | |
811 | for comments that are between functions---whether to use two or three | |
dd4a1d28 LT |
812 | semicolons depends on whether the comment should be considered a |
813 | ``heading'' by Outline minor mode. By default, comments starting with | |
814 | at least three semicolons (followed by a single space and a | |
815 | non-whitespace character) are considered headings, comments starting | |
816 | with two or less are not. | |
2468d0c0 | 817 | |
574efc83 | 818 | Another use for triple-semicolon comments is for commenting out lines |
2468d0c0 | 819 | within a function. We use three semicolons for this precisely so that |
dd4a1d28 LT |
820 | they remain at the left margin. By default, Outline minor mode does |
821 | not consider a comment to be a heading (even if it starts with at | |
822 | least three semicolons) if the semicolons are followed by at least two | |
823 | spaces. Thus, if you add an introductory comment to the commented out | |
824 | code, make sure to indent it by at least two spaces after the three | |
825 | semicolons. | |
4b6694ef RS |
826 | |
827 | @smallexample | |
828 | (defun foo (a) | |
dd4a1d28 | 829 | ;;; This is no longer necessary. |
4b6694ef RS |
830 | ;;; (force-mode-line-update) |
831 | (message "Finished with %s" a)) | |
832 | @end smallexample | |
833 | ||
dd4a1d28 LT |
834 | When commenting out entire functions, use two semicolons. |
835 | ||
7015aca4 RS |
836 | @item ;;;; |
837 | Comments that start with four semicolons, @samp{;;;;}, should be aligned | |
838 | to the left margin and are used for headings of major sections of a | |
839 | program. For example: | |
840 | ||
841 | @smallexample | |
842 | ;;;; The kill ring | |
843 | @end smallexample | |
844 | @end table | |
845 | ||
846 | @noindent | |
847 | The indentation commands of the Lisp modes in Emacs, such as @kbd{M-;} | |
969fe9b5 | 848 | (@code{indent-for-comment}) and @key{TAB} (@code{lisp-indent-line}), |
7015aca4 | 849 | automatically indent comments according to these conventions, |
574efc83 | 850 | depending on the number of semicolons. @xref{Comments,, |
7015aca4 RS |
851 | Manipulating Comments, emacs, The GNU Emacs Manual}. |
852 | ||
7015aca4 RS |
853 | @node Library Headers |
854 | @section Conventional Headers for Emacs Libraries | |
855 | @cindex header comments | |
856 | @cindex library header comments | |
857 | ||
f9f59935 | 858 | Emacs has conventions for using special comments in Lisp libraries |
7015aca4 | 859 | to divide them into sections and give information such as who wrote |
8241495d RS |
860 | them. This section explains these conventions. |
861 | ||
862 | We'll start with an example, a package that is included in the Emacs | |
863 | distribution. | |
864 | ||
865 | Parts of this example reflect its status as part of Emacs; for | |
866 | example, the copyright notice lists the Free Software Foundation as the | |
867 | copyright holder, and the copying permission says the file is part of | |
868 | Emacs. When you write a package and post it, the copyright holder would | |
869 | be you (unless your employer claims to own it instead), and you should | |
870 | get the suggested copying permission from the end of the GNU General | |
871 | Public License itself. Don't say your file is part of Emacs | |
872 | if we haven't installed it in Emacs yet! | |
873 | ||
874 | With that warning out of the way, on to the example: | |
7015aca4 RS |
875 | |
876 | @smallexample | |
877 | @group | |
878 | ;;; lisp-mnt.el --- minor mode for Emacs Lisp maintainers | |
879 | ||
880 | ;; Copyright (C) 1992 Free Software Foundation, Inc. | |
881 | @end group | |
882 | ||
883 | ;; Author: Eric S. Raymond <esr@@snark.thyrsus.com> | |
884 | ;; Maintainer: Eric S. Raymond <esr@@snark.thyrsus.com> | |
885 | ;; Created: 14 Jul 1992 | |
886 | ;; Version: 1.2 | |
887 | @group | |
888 | ;; Keywords: docs | |
889 | ||
890 | ;; This file is part of GNU Emacs. | |
969fe9b5 RS |
891 | @dots{} |
892 | ;; Free Software Foundation, Inc., 59 Temple Place - Suite 330, | |
893 | ;; Boston, MA 02111-1307, USA. | |
7015aca4 RS |
894 | @end group |
895 | @end smallexample | |
896 | ||
897 | The very first line should have this format: | |
898 | ||
899 | @example | |
900 | ;;; @var{filename} --- @var{description} | |
901 | @end example | |
902 | ||
903 | @noindent | |
e4317c8c RS |
904 | The description should be complete in one line. If the file |
905 | needs a @samp{-*-} specification, put it after @var{description}. | |
7015aca4 RS |
906 | |
907 | After the copyright notice come several @dfn{header comment} lines, | |
4b6694ef | 908 | each beginning with @samp{;; @var{header-name}:}. Here is a table of |
7015aca4 RS |
909 | the conventional possibilities for @var{header-name}: |
910 | ||
911 | @table @samp | |
912 | @item Author | |
913 | This line states the name and net address of at least the principal | |
914 | author of the library. | |
915 | ||
916 | If there are multiple authors, you can list them on continuation lines | |
4b6694ef | 917 | led by @code{;;} and a tab character, like this: |
7015aca4 RS |
918 | |
919 | @smallexample | |
920 | @group | |
921 | ;; Author: Ashwin Ram <Ram-Ashwin@@cs.yale.edu> | |
4b6694ef RS |
922 | ;; Dave Sill <de5@@ornl.gov> |
923 | ;; Dave Brennan <brennan@@hal.com> | |
924 | ;; Eric Raymond <esr@@snark.thyrsus.com> | |
7015aca4 RS |
925 | @end group |
926 | @end smallexample | |
927 | ||
928 | @item Maintainer | |
929 | This line should contain a single name/address as in the Author line, or | |
4b6694ef RS |
930 | an address only, or the string @samp{FSF}. If there is no maintainer |
931 | line, the person(s) in the Author field are presumed to be the | |
932 | maintainers. The example above is mildly bogus because the maintainer | |
933 | line is redundant. | |
7015aca4 RS |
934 | |
935 | The idea behind the @samp{Author} and @samp{Maintainer} lines is to make | |
936 | possible a Lisp function to ``send mail to the maintainer'' without | |
937 | having to mine the name out by hand. | |
938 | ||
939 | Be sure to surround the network address with @samp{<@dots{}>} if | |
940 | you include the person's full name as well as the network address. | |
941 | ||
942 | @item Created | |
943 | This optional line gives the original creation date of the | |
944 | file. For historical interest only. | |
945 | ||
946 | @item Version | |
947 | If you wish to record version numbers for the individual Lisp program, put | |
948 | them in this line. | |
949 | ||
950 | @item Adapted-By | |
951 | In this header line, place the name of the person who adapted the | |
952 | library for installation (to make it fit the style conventions, for | |
953 | example). | |
954 | ||
955 | @item Keywords | |
956 | This line lists keywords for the @code{finder-by-keyword} help command. | |
a9f0a989 RS |
957 | Please use that command to see a list of the meaningful keywords. |
958 | ||
7015aca4 | 959 | This field is important; it's how people will find your package when |
2c62739d RS |
960 | they're looking for things by topic area. To separate the keywords, you |
961 | can use spaces, commas, or both. | |
7015aca4 RS |
962 | @end table |
963 | ||
964 | Just about every Lisp library ought to have the @samp{Author} and | |
965 | @samp{Keywords} header comment lines. Use the others if they are | |
966 | appropriate. You can also put in header lines with other header | |
967 | names---they have no standard meanings, so they can't do any harm. | |
968 | ||
969 | We use additional stylized comments to subdivide the contents of the | |
2468d0c0 DL |
970 | library file. These should be separated by blank lines from anything |
971 | else. Here is a table of them: | |
7015aca4 RS |
972 | |
973 | @table @samp | |
974 | @item ;;; Commentary: | |
975 | This begins introductory comments that explain how the library works. | |
a9f0a989 RS |
976 | It should come right after the copying permissions, terminated by a |
977 | @samp{Change Log}, @samp{History} or @samp{Code} comment line. This | |
978 | text is used by the Finder package, so it should make sense in that | |
979 | context. | |
980 | ||
fd423b79 | 981 | @item ;;; Documentation: |
3ff91798 RS |
982 | This was used in some files in place of @samp{;;; Commentary:}, |
983 | but it is deprecated. | |
7015aca4 | 984 | |
a9f0a989 | 985 | @item ;;; Change Log: |
7015aca4 | 986 | This begins change log information stored in the library file (if you |
2468d0c0 DL |
987 | store the change history there). For Lisp files distributed with Emacs, |
988 | the change history is kept in the file @file{ChangeLog} and not in the | |
989 | source file at all; these files generally do not have a @samp{;;; Change | |
990 | Log:} line. @samp{History} is an alternative to @samp{Change Log}. | |
7015aca4 RS |
991 | |
992 | @item ;;; Code: | |
993 | This begins the actual code of the program. | |
994 | ||
995 | @item ;;; @var{filename} ends here | |
996 | This is the @dfn{footer line}; it appears at the very end of the file. | |
997 | Its purpose is to enable people to detect truncated versions of the file | |
998 | from the lack of a footer line. | |
999 | @end table | |
ab5796a9 MB |
1000 | |
1001 | @ignore | |
1002 | arch-tag: 9ea911c2-6b1d-47dd-88b7-0a94e8b27c2e | |
1003 | @end ignore |