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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 | |
8241495d RS |
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 | |
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49 | primitives that are not primitives in Emacs Lisp---even to |
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 | |
104 | time, for the sake of macros. You do that like this: | |
105 | ||
106 | @example | |
107 | (eval-when-compile (require 'cl)) | |
108 | @end example | |
109 | ||
7015aca4 | 110 | @item |
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111 | When defining a major mode, please follow the major mode |
112 | conventions. @xref{Major Mode Conventions}. | |
113 | ||
114 | @item | |
115 | When defining a minor mode, please follow the minor mode | |
116 | conventions. @xref{Minor Mode Conventions}. | |
7015aca4 | 117 | |
6cbf476c RS |
118 | @item |
119 | If the purpose of a function is to tell you whether a certain condition | |
120 | is true or false, give the function a name that ends in @samp{p}. If | |
121 | the name is one word, add just @samp{p}; if the name is multiple words, | |
122 | add @samp{-p}. Examples are @code{framep} and @code{frame-live-p}. | |
123 | ||
124 | @item | |
125 | If a user option variable records a true-or-false condition, give it a | |
126 | name that ends in @samp{-flag}. | |
127 | ||
49247521 LK |
128 | @item |
129 | If the purpose of a variable is to store a single function, give it a | |
130 | name that ends in @samp{-function}. If the purpose of a variable is | |
131 | to store a list of functions (i.e., the variable is a hook), please | |
132 | follow the naming conventions for hooks. @xref{Hooks}. | |
133 | ||
7015aca4 | 134 | @item |
a9f0a989 RS |
135 | @cindex reserved keys |
136 | @cindex keys, reserved | |
7015aca4 | 137 | Please do not define @kbd{C-c @var{letter}} as a key in your major |
17d4b8ce RS |
138 | modes. Sequences consisting of @kbd{C-c} and a letter (either upper |
139 | or lower case) are reserved for users; they are the @strong{only} | |
140 | sequences reserved for users, so do not block them. | |
7015aca4 | 141 | |
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142 | Changing all the Emacs major modes to respect this convention was a |
143 | lot of work; abandoning this convention would make that work go to | |
144 | waste, and inconvenience users. Please comply with it. | |
7015aca4 | 145 | |
17d4b8ce RS |
146 | @item |
147 | Sequences consisting of @kbd{C-c} followed by a control character or a | |
148 | digit are reserved for major modes. | |
00d96ada RS |
149 | |
150 | @item | |
151 | Sequences consisting of @kbd{C-c} followed by @kbd{@{}, @kbd{@}}, | |
152 | @kbd{<}, @kbd{>}, @kbd{:} or @kbd{;} are also reserved for major modes. | |
153 | ||
154 | @item | |
155 | Sequences consisting of @kbd{C-c} followed by any other punctuation | |
156 | character are allocated for minor modes. Using them in a major mode is | |
157 | not absolutely prohibited, but if you do that, the major mode binding | |
158 | may be shadowed from time to time by minor modes. | |
7015aca4 | 159 | |
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160 | @item |
161 | Function keys @key{F5} through @key{F9} without modifier keys are | |
162 | reserved for users to define. | |
163 | ||
7015aca4 | 164 | @item |
f9f59935 | 165 | Do not bind @kbd{C-h} following any prefix character (including |
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166 | @kbd{C-c}). If you don't bind @kbd{C-h}, it is automatically available |
167 | as a help character for listing the subcommands of the prefix character. | |
168 | ||
169 | @item | |
f9f59935 | 170 | Do not bind a key sequence ending in @key{ESC} except following |
969fe9b5 | 171 | another @key{ESC}. (That is, it is OK to bind a sequence ending in |
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172 | @kbd{@key{ESC} @key{ESC}}.) |
173 | ||
174 | The reason for this rule is that a non-prefix binding for @key{ESC} in | |
175 | any context prevents recognition of escape sequences as function keys in | |
176 | that context. | |
177 | ||
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178 | @item |
179 | Anything which acts like a temporary mode or state which the user can | |
b6ae404e | 180 | enter and leave should define @kbd{@key{ESC} @key{ESC}} or |
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181 | @kbd{@key{ESC} @key{ESC} @key{ESC}} as a way to escape. |
182 | ||
183 | For a state which accepts ordinary Emacs commands, or more generally any | |
184 | kind of state in which @key{ESC} followed by a function key or arrow key | |
185 | is potentially meaningful, then you must not define @kbd{@key{ESC} | |
186 | @key{ESC}}, since that would preclude recognizing an escape sequence | |
187 | after @key{ESC}. In these states, you should define @kbd{@key{ESC} | |
188 | @key{ESC} @key{ESC}} as the way to escape. Otherwise, define | |
189 | @kbd{@key{ESC} @key{ESC}} instead. | |
190 | ||
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191 | @item |
192 | Applications should not bind mouse events based on button 1 with the | |
193 | shift key held down. These events include @kbd{S-mouse-1}, | |
194 | @kbd{M-S-mouse-1}, @kbd{C-S-mouse-1}, and so on. They are reserved for | |
195 | users. | |
196 | ||
197 | @item | |
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198 | @cindex mouse-2 |
199 | @cindex references, following | |
f9f59935 RS |
200 | Special major modes used for read-only text should usually redefine |
201 | @kbd{mouse-2} and @key{RET} to trace some sort of reference in the text. | |
202 | Modes such as Dired, Info, Compilation, and Occur redefine it in this | |
203 | way. | |
4b6694ef | 204 | |
7015aca4 | 205 | @item |
8414f615 | 206 | When a package provides a modification of ordinary Emacs behavior, it is |
b8fbee64 | 207 | good to include a command to enable and disable the feature, provide a |
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208 | command named @code{@var{whatever}-mode} which turns the feature on or |
209 | off, and make it autoload (@pxref{Autoload}). Design the package so | |
210 | that simply loading it has no visible effect---that should not enable | |
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211 | the feature.@footnote{Consider that the package may be loaded |
212 | arbitrarily by Custom for instance.} Users will request the feature by | |
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213 | invoking the command. It is a good idea to define this command |
214 | as a minor mode. | |
b68c6256 DL |
215 | |
216 | @cindex unloading packages | |
3ff91798 RS |
217 | If loading the file adds functions to hooks, define a function |
218 | @code{@var{feature}-unload-hook}, where @var{feature} is the name of | |
219 | the feature the package provides, and make it undo any such changes. | |
220 | Using @code{unload-feature} to unload the file will run this function. | |
221 | @xref{Unloading}. | |
8414f615 RS |
222 | |
223 | @item | |
224 | It is a bad idea to define aliases for the Emacs primitives. Use the | |
225 | standard names instead. | |
7015aca4 | 226 | |
c7a401dd DL |
227 | @item |
228 | If a package needs to define an alias or a new function for | |
bbac5699 | 229 | compatibility with some other version of Emacs, name it with the package |
c7a401dd DL |
230 | prefix, not with the raw name with which it occurs in the other version. |
231 | Here is an example from Gnus, which provides many examples of such | |
232 | compatibility issues. | |
233 | ||
234 | @example | |
235 | (defalias 'gnus-point-at-bol | |
236 | (if (fboundp 'point-at-bol) | |
237 | 'point-at-bol | |
238 | 'line-beginning-position)) | |
239 | @end example | |
240 | ||
7015aca4 | 241 | @item |
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242 | Redefining (or advising) an Emacs primitive is discouraged. It may do |
243 | the right thing for a particular program, but there is no telling what | |
244 | other programs might break as a result. | |
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245 | |
246 | @item | |
247 | If a file does replace any of the functions or library programs of | |
248 | standard Emacs, prominent comments at the beginning of the file should | |
249 | say which functions are replaced, and how the behavior of the | |
250 | replacements differs from that of the originals. | |
251 | ||
3ff91798 RS |
252 | @item |
253 | Avoid using macros that define functions and variables with names that | |
ee49c7d9 | 254 | are constructed. It is best for maintenance when the name of the |
3ff91798 RS |
255 | function or variable being defined is given explicitly in the source |
256 | code, as the second element of the list---as it is when you use | |
e8d6f886 | 257 | @code{defun}, @code{defalias}, @code{defvar} and @code{defcustom}. |
3ff91798 | 258 | |
7015aca4 RS |
259 | @item |
260 | Please keep the names of your Emacs Lisp source files to 13 characters | |
261 | or less. This way, if the files are compiled, the compiled files' names | |
262 | will be 14 characters or less, which is short enough to fit on all kinds | |
263 | of Unix systems. | |
264 | ||
265 | @item | |
266 | Don't use @code{next-line} or @code{previous-line} in programs; nearly | |
267 | always, @code{forward-line} is more convenient as well as more | |
268 | predictable and robust. @xref{Text Lines}. | |
269 | ||
270 | @item | |
574efc83 RS |
271 | Don't call functions that set the mark, unless setting the mark is one |
272 | of the intended features of your program. The mark is a user-level | |
273 | feature, so it is incorrect to change the mark except to supply a value | |
274 | for the user's benefit. @xref{The Mark}. | |
7015aca4 | 275 | |
f9f59935 | 276 | In particular, don't use any of these functions: |
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277 | |
278 | @itemize @bullet | |
279 | @item | |
280 | @code{beginning-of-buffer}, @code{end-of-buffer} | |
281 | @item | |
282 | @code{replace-string}, @code{replace-regexp} | |
283 | @end itemize | |
284 | ||
285 | If you just want to move point, or replace a certain string, without any | |
286 | of the other features intended for interactive users, you can replace | |
287 | these functions with one or two lines of simple Lisp code. | |
288 | ||
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289 | @item |
290 | Use lists rather than vectors, except when there is a particular reason | |
291 | to use a vector. Lisp has more facilities for manipulating lists than | |
292 | for vectors, and working with lists is usually more convenient. | |
293 | ||
294 | Vectors are advantageous for tables that are substantial in size and are | |
295 | accessed in random order (not searched front to back), provided there is | |
296 | no need to insert or delete elements (only lists allow that). | |
297 | ||
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298 | @item |
299 | The recommended way to print a message in the echo area is with | |
300 | the @code{message} function, not @code{princ}. @xref{The Echo Area}. | |
301 | ||
302 | @item | |
303 | When you encounter an error condition, call the function @code{error} | |
304 | (or @code{signal}). The function @code{error} does not return. | |
305 | @xref{Signaling Errors}. | |
306 | ||
307 | Do not use @code{message}, @code{throw}, @code{sleep-for}, | |
308 | or @code{beep} to report errors. | |
309 | ||
bfe721d1 KH |
310 | @item |
311 | An error message should start with a capital letter but should not end | |
312 | with a period. | |
313 | ||
01e3636e RS |
314 | @item |
315 | In @code{interactive}, if you use a Lisp expression to produce a list | |
316 | of arguments, don't try to provide the ``correct'' default values for | |
317 | region or position arguments. Instead, provide @code{nil} for those | |
318 | arguments if they were not specified, and have the function body | |
319 | compute the default value when the argument is @code{nil}. For | |
320 | instance, write this: | |
321 | ||
322 | @example | |
323 | (defun foo (pos) | |
324 | (interactive | |
325 | (list (if @var{specified} @var{specified-pos}))) | |
326 | (unless pos (setq pos @var{default-pos})) | |
327 | ...) | |
328 | @end example | |
329 | ||
330 | @noindent | |
331 | rather than this: | |
332 | ||
333 | @example | |
334 | (defun foo (pos) | |
335 | (interactive | |
336 | (list (if @var{specified} @var{specified-pos} | |
337 | @var{default-pos}))) | |
338 | ...) | |
339 | @end example | |
340 | ||
341 | @noindent | |
342 | This is so that repetition of the command will recompute | |
343 | these defaults based on the current circumstances. | |
344 | ||
345 | You do not need to take such precautions when you use interactive | |
346 | specs @samp{d}, @samp{m} and @samp{r}, because they make special | |
347 | arrangements to recompute the argument values on repetition of the | |
348 | command. | |
349 | ||
2089b41a RS |
350 | @item |
351 | Many commands that take a long time to execute display a message that | |
01e3636e | 352 | says something like @samp{Operating...} when they start, and change it to |
2089b41a RS |
353 | @samp{Operating...done} when they finish. Please keep the style of |
354 | these messages uniform: @emph{no} space around the ellipsis, and | |
01e3636e | 355 | @emph{no} period after @samp{done}. |
2089b41a | 356 | |
7015aca4 | 357 | @item |
4b6694ef RS |
358 | Try to avoid using recursive edits. Instead, do what the Rmail @kbd{e} |
359 | command does: use a new local keymap that contains one command defined | |
360 | to switch back to the old local keymap. Or do what the | |
361 | @code{edit-options} command does: switch to another buffer and let the | |
362 | user switch back at will. @xref{Recursive Editing}. | |
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363 | |
364 | @item | |
365 | In some other systems there is a convention of choosing variable names | |
366 | that begin and end with @samp{*}. We don't use that convention in Emacs | |
4b6694ef | 367 | Lisp, so please don't use it in your programs. (Emacs uses such names |
969fe9b5 | 368 | only for special-purpose buffers.) The users will find Emacs more |
4b6694ef | 369 | coherent if all libraries use the same conventions. |
7015aca4 | 370 | |
6a994023 RS |
371 | @item |
372 | Try to avoid compiler warnings about undefined free variables, by adding | |
378f6042 | 373 | @code{defvar} definitions for these variables. |
6a994023 | 374 | |
8241495d RS |
375 | Sometimes adding a @code{require} for another package is useful to avoid |
376 | compilation warnings for variables and functions defined in that | |
513331d3 | 377 | package. If you do this, often it is better if the @code{require} acts |
8241495d RS |
378 | only at compile time. Here's how to do that: |
379 | ||
380 | @example | |
381 | (eval-when-compile | |
382 | (require 'foo) | |
383 | (defvar bar-baz)) | |
384 | @end example | |
385 | ||
6a994023 RS |
386 | If you bind a variable in one function, and use it or set it in another |
387 | function, the compiler warns about the latter function unless the | |
388 | variable has a definition. But often these variables have short names, | |
a9f0a989 | 389 | and it is not clean for Lisp packages to define such variable names. |
6a994023 RS |
390 | Therefore, you should rename the variable to start with the name prefix |
391 | used for the other functions and variables in your package. | |
392 | ||
7015aca4 RS |
393 | @item |
394 | Indent each function with @kbd{C-M-q} (@code{indent-sexp}) using the | |
395 | default indentation parameters. | |
396 | ||
397 | @item | |
398 | Don't make a habit of putting close-parentheses on lines by themselves; | |
399 | Lisp programmers find this disconcerting. Once in a while, when there | |
400 | is a sequence of many consecutive close-parentheses, it may make sense | |
969fe9b5 | 401 | to split the sequence in one or two significant places. |
7015aca4 RS |
402 | |
403 | @item | |
404 | Please put a copyright notice on the file if you give copies to anyone. | |
f9f59935 RS |
405 | Use a message like this one: |
406 | ||
407 | @smallexample | |
408 | ;; Copyright (C) @var{year} @var{name} | |
409 | ||
410 | ;; This program is free software; you can redistribute it and/or | |
411 | ;; modify it under the terms of the GNU General Public License as | |
412 | ;; published by the Free Software Foundation; either version 2 of | |
413 | ;; the License, or (at your option) any later version. | |
414 | ||
415 | ;; This program is distributed in the hope that it will be | |
416 | ;; useful, but WITHOUT ANY WARRANTY; without even the implied | |
417 | ;; warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR | |
418 | ;; PURPOSE. See the GNU General Public License for more details. | |
419 | ||
420 | ;; You should have received a copy of the GNU General Public | |
421 | ;; License along with this program; if not, write to the Free | |
422 | ;; Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, | |
423 | ;; MA 02111-1307 USA | |
424 | @end smallexample | |
425 | ||
426 | If you have signed papers to assign the copyright to the Foundation, | |
427 | then use @samp{Free Software Foundation, Inc.} as @var{name}. | |
428 | Otherwise, use your name. | |
7015aca4 RS |
429 | @end itemize |
430 | ||
431 | @node Compilation Tips | |
432 | @section Tips for Making Compiled Code Fast | |
433 | @cindex execution speed | |
434 | @cindex speedups | |
435 | ||
436 | Here are ways of improving the execution speed of byte-compiled | |
4b6694ef | 437 | Lisp programs. |
7015aca4 RS |
438 | |
439 | @itemize @bullet | |
440 | @item | |
441 | @cindex profiling | |
442 | @cindex timing programs | |
a9f0a989 | 443 | @cindex @file{elp.el} |
5f7eb05d EZ |
444 | Profile your program with the @file{elp} library. See the file |
445 | @file{elp.el} for instructions. | |
7015aca4 RS |
446 | |
447 | @item | |
448 | Use iteration rather than recursion whenever possible. | |
449 | Function calls are slow in Emacs Lisp even when a compiled function | |
450 | is calling another compiled function. | |
451 | ||
452 | @item | |
bfe721d1 KH |
453 | Using the primitive list-searching functions @code{memq}, @code{member}, |
454 | @code{assq}, or @code{assoc} is even faster than explicit iteration. It | |
f9f59935 | 455 | can be worth rearranging a data structure so that one of these primitive |
bfe721d1 | 456 | search functions can be used. |
7015aca4 RS |
457 | |
458 | @item | |
177c0ea7 | 459 | Certain built-in functions are handled specially in byte-compiled code, |
7015aca4 RS |
460 | avoiding the need for an ordinary function call. It is a good idea to |
461 | use these functions rather than alternatives. To see whether a function | |
462 | is handled specially by the compiler, examine its @code{byte-compile} | |
463 | property. If the property is non-@code{nil}, then the function is | |
464 | handled specially. | |
465 | ||
466 | For example, the following input will show you that @code{aref} is | |
a9f0a989 | 467 | compiled specially (@pxref{Array Functions}): |
7015aca4 | 468 | |
4b6694ef | 469 | @example |
7015aca4 RS |
470 | @group |
471 | (get 'aref 'byte-compile) | |
472 | @result{} byte-compile-two-args | |
473 | @end group | |
4b6694ef | 474 | @end example |
7015aca4 RS |
475 | |
476 | @item | |
1911e6e5 | 477 | If calling a small function accounts for a substantial part of your |
7015aca4 RS |
478 | program's running time, make the function inline. This eliminates |
479 | the function call overhead. Since making a function inline reduces | |
480 | the flexibility of changing the program, don't do it unless it gives | |
4b6694ef | 481 | a noticeable speedup in something slow enough that users care about |
7015aca4 RS |
482 | the speed. @xref{Inline Functions}. |
483 | @end itemize | |
484 | ||
485 | @node Documentation Tips | |
486 | @section Tips for Documentation Strings | |
487 | ||
969fe9b5 RS |
488 | @findex checkdoc-minor-mode |
489 | Here are some tips and conventions for the writing of documentation | |
490 | strings. You can check many of these conventions by running the command | |
491 | @kbd{M-x checkdoc-minor-mode}. | |
7015aca4 RS |
492 | |
493 | @itemize @bullet | |
494 | @item | |
574efc83 | 495 | Every command, function, or variable intended for users to know about |
7015aca4 RS |
496 | should have a documentation string. |
497 | ||
498 | @item | |
e0d32668 RS |
499 | An internal variable or subroutine of a Lisp program might as well have |
500 | a documentation string. In earlier Emacs versions, you could save space | |
501 | by using a comment instead of a documentation string, but that is no | |
2468d0c0 DL |
502 | longer the case---documentation strings now take up very little space in |
503 | a running Emacs. | |
7015aca4 | 504 | |
b090d792 RS |
505 | @item |
506 | Format the documentation string so that it fits in an Emacs window on an | |
507 | 80-column screen. It is a good idea for most lines to be no wider than | |
508 | 60 characters. The first line should not be wider than 67 characters | |
509 | or it will look bad in the output of @code{apropos}. | |
510 | ||
511 | You can fill the text if that looks good. However, rather than blindly | |
512 | filling the entire documentation string, you can often make it much more | |
513 | readable by choosing certain line breaks with care. Use blank lines | |
514 | between topics if the documentation string is long. | |
515 | ||
7015aca4 RS |
516 | @item |
517 | The first line of the documentation string should consist of one or two | |
574efc83 | 518 | complete sentences that stand on their own as a summary. @kbd{M-x |
2468d0c0 DL |
519 | apropos} displays just the first line, and if that line's contents don't |
520 | stand on their own, the result looks bad. In particular, start the | |
521 | first line with a capital letter and end with a period. | |
7015aca4 | 522 | |
aa5dbf7b RS |
523 | For a function, the first line should briefly answer the question, |
524 | ``What does this function do?'' For a variable, the first line should | |
525 | briefly answer the question, ``What does this value mean?'' | |
526 | ||
527 | Don't limit the documentation string to one line; use as many lines as | |
528 | you need to explain the details of how to use the function or | |
529 | variable. Please use complete sentences for the rest of the text too. | |
7015aca4 | 530 | |
7878d6b6 RS |
531 | @item |
532 | The first line should mention all the important arguments of the | |
533 | function, and should mention them in the order that they are written | |
534 | in a function call. If the function has many arguments, then it is | |
535 | not feasible to mention them all in the first line; in that case, the | |
536 | first line should mention the first few arguments, including the most | |
537 | important arguments. | |
538 | ||
4b6694ef | 539 | @item |
8241495d | 540 | For consistency, phrase the verb in the first sentence of a function's |
321a3725 | 541 | documentation string as an imperative---for instance, use ``Return the |
8241495d RS |
542 | cons of A and B.'' in preference to ``Returns the cons of A and B@.'' |
543 | Usually it looks good to do likewise for the rest of the first | |
544 | paragraph. Subsequent paragraphs usually look better if each sentence | |
b090d792 | 545 | is indicative and has a proper subject. |
4b6694ef | 546 | |
7015aca4 RS |
547 | @item |
548 | Write documentation strings in the active voice, not the passive, and in | |
549 | the present tense, not the future. For instance, use ``Return a list | |
550 | containing A and B.'' instead of ``A list containing A and B will be | |
551 | returned.'' | |
552 | ||
553 | @item | |
554 | Avoid using the word ``cause'' (or its equivalents) unnecessarily. | |
555 | Instead of, ``Cause Emacs to display text in boldface,'' write just | |
556 | ``Display text in boldface.'' | |
557 | ||
2468d0c0 DL |
558 | @item |
559 | When a command is meaningful only in a certain mode or situation, | |
560 | do mention that in the documentation string. For example, | |
561 | the documentation of @code{dired-find-file} is: | |
562 | ||
563 | @example | |
564 | In Dired, visit the file or directory named on this line. | |
565 | @end example | |
566 | ||
7015aca4 RS |
567 | @item |
568 | Do not start or end a documentation string with whitespace. | |
177c0ea7 | 569 | |
7015aca4 RS |
570 | @item |
571 | @strong{Do not} indent subsequent lines of a documentation string so | |
572 | that the text is lined up in the source code with the text of the first | |
573 | line. This looks nice in the source code, but looks bizarre when users | |
574 | view the documentation. Remember that the indentation before the | |
575 | starting double-quote is not part of the string! | |
576 | ||
75d97f47 RS |
577 | @item |
578 | When the user tries to use a disabled command, Emacs displays just the | |
579 | first paragraph of its documentation string---everything through the | |
580 | first blank line. If you wish, you can choose which information to | |
581 | include before the first blank line so as to make this display useful. | |
582 | ||
7015aca4 RS |
583 | @item |
584 | A variable's documentation string should start with @samp{*} if the | |
4b6694ef | 585 | variable is one that users would often want to set interactively. If |
574efc83 RS |
586 | the value is a long list, or a function, or if the variable would be set |
587 | only in init files, then don't start the documentation string with | |
7015aca4 RS |
588 | @samp{*}. @xref{Defining Variables}. |
589 | ||
590 | @item | |
591 | The documentation string for a variable that is a yes-or-no flag should | |
4b6694ef RS |
592 | start with words such as ``Non-nil means@dots{}'', to make it clear that |
593 | all non-@code{nil} values are equivalent and indicate explicitly what | |
594 | @code{nil} and non-@code{nil} mean. | |
7015aca4 | 595 | |
5c5b7d3e RS |
596 | @item |
597 | The documentation string for a function that is a yes-or-no predicate | |
598 | should start with words such as ``Return t if @dots{}'', to indicate | |
599 | explicitly what constitutes ``truth''. The word ``return'' avoids | |
600 | starting the sentence with lower-case ``t'', which is somewhat | |
601 | distracting. | |
602 | ||
7015aca4 RS |
603 | @item |
604 | When a function's documentation string mentions the value of an argument | |
605 | of the function, use the argument name in capital letters as if it were | |
606 | a name for that value. Thus, the documentation string of the function | |
2468d0c0 DL |
607 | @code{eval} refers to its second argument as @samp{FORM}, because the |
608 | actual argument name is @code{form}: | |
609 | ||
610 | @example | |
611 | Evaluate FORM and return its value. | |
612 | @end example | |
7015aca4 | 613 | |
2468d0c0 DL |
614 | Also write metasyntactic variables in capital letters, such as when you |
615 | show the decomposition of a list or vector into subunits, some of which | |
616 | may vary. @samp{KEY} and @samp{VALUE} in the following example | |
8241495d RS |
617 | illustrate this practice: |
618 | ||
619 | @example | |
620 | The argument TABLE should be an alist whose elements | |
621 | have the form (KEY . VALUE). Here, KEY is ... | |
622 | @end example | |
7015aca4 | 623 | |
5c5b7d3e RS |
624 | @item |
625 | Never change the case of a Lisp symbol when you mention it in a doc | |
626 | string. If the symbol's name is @code{foo}, write ``foo'', not | |
627 | ``Foo'' (which is a different symbol). | |
628 | ||
629 | This might appear to contradict the policy of writing function | |
630 | argument values, but there is no real contradiction; the argument | |
631 | @emph{value} is not the same thing as the @emph{symbol} which the | |
632 | function uses to hold the value. | |
633 | ||
634 | If this puts a lower-case letter at the beginning of a sentence | |
635 | and that annoys you, rewrite the sentence so that the symbol | |
636 | is not at the start of it. | |
637 | ||
2468d0c0 DL |
638 | @item |
639 | If a line in a documentation string begins with an open-parenthesis, | |
640 | write a backslash before the open-parenthesis, like this: | |
641 | ||
642 | @example | |
643 | The argument FOO can be either a number | |
644 | \(a buffer position) or a string (a file name). | |
645 | @end example | |
646 | ||
647 | This prevents the open-parenthesis from being treated as the start of a | |
648 | defun (@pxref{Defuns,, Defuns, emacs, The GNU Emacs Manual}). | |
649 | ||
70057f39 | 650 | @anchor{Docstring hyperlinks} |
7015aca4 RS |
651 | @item |
652 | @iftex | |
653 | When a documentation string refers to a Lisp symbol, write it as it | |
654 | would be printed (which usually means in lower case), with single-quotes | |
655 | around it. For example: @samp{`lambda'}. There are two exceptions: | |
656 | write @code{t} and @code{nil} without single-quotes. | |
657 | @end iftex | |
37680279 | 658 | @ifnottex |
7015aca4 RS |
659 | When a documentation string refers to a Lisp symbol, write it as it |
660 | would be printed (which usually means in lower case), with single-quotes | |
661 | around it. For example: @samp{lambda}. There are two exceptions: write | |
969fe9b5 RS |
662 | t and nil without single-quotes. (In this manual, we use a different |
663 | convention, with single-quotes for all symbols.) | |
37680279 | 664 | @end ifnottex |
7015aca4 | 665 | |
1911e6e5 RS |
666 | Help mode automatically creates a hyperlink when a documentation string |
667 | uses a symbol name inside single quotes, if the symbol has either a | |
a9f0a989 RS |
668 | function or a variable definition. You do not need to do anything |
669 | special to make use of this feature. However, when a symbol has both a | |
670 | function definition and a variable definition, and you want to refer to | |
671 | just one of them, you can specify which one by writing one of the words | |
672 | @samp{variable}, @samp{option}, @samp{function}, or @samp{command}, | |
673 | immediately before the symbol name. (Case makes no difference in | |
674 | recognizing these indicator words.) For example, if you write | |
675 | ||
676 | @example | |
677 | This function sets the variable `buffer-file-name'. | |
678 | @end example | |
679 | ||
680 | @noindent | |
681 | then the hyperlink will refer only to the variable documentation of | |
682 | @code{buffer-file-name}, and not to its function documentation. | |
683 | ||
684 | If a symbol has a function definition and/or a variable definition, but | |
685 | those are irrelevant to the use of the symbol that you are documenting, | |
686 | you can write the word @samp{symbol} before the symbol name to prevent | |
687 | making any hyperlink. For example, | |
969fe9b5 RS |
688 | |
689 | @example | |
a9f0a989 RS |
690 | If the argument KIND-OF-RESULT is the symbol `list', |
691 | this function returns a list of all the objects | |
692 | that satisfy the criterion. | |
969fe9b5 RS |
693 | @end example |
694 | ||
a9f0a989 RS |
695 | @noindent |
696 | does not make a hyperlink to the documentation, irrelevant here, of the | |
697 | function @code{list}. | |
698 | ||
e72850d5 LT |
699 | Normally, no hyperlink is made for a variable without variable |
700 | documentation. You can force a hyperlink for such variables by | |
701 | preceding them with one of the words @samp{variable} or | |
702 | @samp{option}. | |
703 | ||
704 | Hyperlinks for faces are only made if the face name is preceded or | |
705 | followed by the word @samp{face}. In that case, only the face | |
706 | documentation will be shown, even if the symbol is also defined as a | |
707 | variable or as a function. | |
708 | ||
8241495d | 709 | To make a hyperlink to Info documentation, write the name of the Info |
727c9159 LT |
710 | node (or anchor) in single quotes, preceded by @samp{info node}, |
711 | @samp{Info node}, @samp{info anchor} or @samp{Info anchor}. The Info | |
712 | file name defaults to @samp{emacs}. For example, | |
8241495d RS |
713 | |
714 | @smallexample | |
715 | See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'. | |
716 | @end smallexample | |
717 | ||
7015aca4 RS |
718 | @item |
719 | Don't write key sequences directly in documentation strings. Instead, | |
720 | use the @samp{\\[@dots{}]} construct to stand for them. For example, | |
9e2b495b RS |
721 | instead of writing @samp{C-f}, write the construct |
722 | @samp{\\[forward-char]}. When Emacs displays the documentation string, | |
723 | it substitutes whatever key is currently bound to @code{forward-char}. | |
724 | (This is normally @samp{C-f}, but it may be some other character if the | |
725 | user has moved key bindings.) @xref{Keys in Documentation}. | |
7015aca4 RS |
726 | |
727 | @item | |
728 | In documentation strings for a major mode, you will want to refer to the | |
729 | key bindings of that mode's local map, rather than global ones. | |
730 | Therefore, use the construct @samp{\\<@dots{}>} once in the | |
731 | documentation string to specify which key map to use. Do this before | |
732 | the first use of @samp{\\[@dots{}]}. The text inside the | |
733 | @samp{\\<@dots{}>} should be the name of the variable containing the | |
734 | local keymap for the major mode. | |
735 | ||
736 | It is not practical to use @samp{\\[@dots{}]} very many times, because | |
737 | display of the documentation string will become slow. So use this to | |
738 | describe the most important commands in your major mode, and then use | |
739 | @samp{\\@{@dots{}@}} to display the rest of the mode's keymap. | |
7015aca4 RS |
740 | @end itemize |
741 | ||
742 | @node Comment Tips | |
743 | @section Tips on Writing Comments | |
744 | ||
745 | We recommend these conventions for where to put comments and how to | |
746 | indent them: | |
747 | ||
748 | @table @samp | |
749 | @item ; | |
750 | Comments that start with a single semicolon, @samp{;}, should all be | |
751 | aligned to the same column on the right of the source code. Such | |
752 | comments usually explain how the code on the same line does its job. In | |
753 | Lisp mode and related modes, the @kbd{M-;} (@code{indent-for-comment}) | |
754 | command automatically inserts such a @samp{;} in the right place, or | |
4b6694ef | 755 | aligns such a comment if it is already present. |
7015aca4 | 756 | |
574efc83 | 757 | This and following examples are taken from the Emacs sources. |
7015aca4 RS |
758 | |
759 | @smallexample | |
760 | @group | |
761 | (setq base-version-list ; there was a base | |
762 | (assoc (substring fn 0 start-vn) ; version to which | |
763 | file-version-assoc-list)) ; this looks like | |
764 | ; a subversion | |
765 | @end group | |
766 | @end smallexample | |
767 | ||
768 | @item ;; | |
769 | Comments that start with two semicolons, @samp{;;}, should be aligned to | |
4b6694ef | 770 | the same level of indentation as the code. Such comments usually |
7015aca4 RS |
771 | describe the purpose of the following lines or the state of the program |
772 | at that point. For example: | |
773 | ||
774 | @smallexample | |
775 | @group | |
776 | (prog1 (setq auto-fill-function | |
777 | @dots{} | |
778 | @dots{} | |
4b6694ef | 779 | ;; update mode line |
7015aca4 RS |
780 | (force-mode-line-update))) |
781 | @end group | |
782 | @end smallexample | |
783 | ||
2468d0c0 | 784 | We also normally use two semicolons for comments outside functions. |
7015aca4 RS |
785 | |
786 | @smallexample | |
787 | @group | |
2468d0c0 DL |
788 | ;; This Lisp code is run in Emacs |
789 | ;; when it is to operate as a server | |
790 | ;; for other processes. | |
7015aca4 RS |
791 | @end group |
792 | @end smallexample | |
793 | ||
2468d0c0 DL |
794 | Every function that has no documentation string (presumably one that is |
795 | used only internally within the package it belongs to), should instead | |
796 | have a two-semicolon comment right before the function, explaining what | |
797 | the function does and how to call it properly. Explain precisely what | |
798 | each argument means and how the function interprets its possible values. | |
799 | ||
800 | @item ;;; | |
801 | Comments that start with three semicolons, @samp{;;;}, should start at | |
802 | the left margin. These are used, occasionally, for comments within | |
803 | functions that should start at the margin. We also use them sometimes | |
804 | for comments that are between functions---whether to use two or three | |
dd4a1d28 LT |
805 | semicolons depends on whether the comment should be considered a |
806 | ``heading'' by Outline minor mode. By default, comments starting with | |
807 | at least three semicolons (followed by a single space and a | |
808 | non-whitespace character) are considered headings, comments starting | |
809 | with two or less are not. | |
2468d0c0 | 810 | |
574efc83 | 811 | Another use for triple-semicolon comments is for commenting out lines |
2468d0c0 | 812 | within a function. We use three semicolons for this precisely so that |
dd4a1d28 LT |
813 | they remain at the left margin. By default, Outline minor mode does |
814 | not consider a comment to be a heading (even if it starts with at | |
815 | least three semicolons) if the semicolons are followed by at least two | |
816 | spaces. Thus, if you add an introductory comment to the commented out | |
817 | code, make sure to indent it by at least two spaces after the three | |
818 | semicolons. | |
4b6694ef RS |
819 | |
820 | @smallexample | |
821 | (defun foo (a) | |
dd4a1d28 | 822 | ;;; This is no longer necessary. |
4b6694ef RS |
823 | ;;; (force-mode-line-update) |
824 | (message "Finished with %s" a)) | |
825 | @end smallexample | |
826 | ||
dd4a1d28 LT |
827 | When commenting out entire functions, use two semicolons. |
828 | ||
7015aca4 RS |
829 | @item ;;;; |
830 | Comments that start with four semicolons, @samp{;;;;}, should be aligned | |
831 | to the left margin and are used for headings of major sections of a | |
832 | program. For example: | |
833 | ||
834 | @smallexample | |
835 | ;;;; The kill ring | |
836 | @end smallexample | |
837 | @end table | |
838 | ||
839 | @noindent | |
840 | The indentation commands of the Lisp modes in Emacs, such as @kbd{M-;} | |
969fe9b5 | 841 | (@code{indent-for-comment}) and @key{TAB} (@code{lisp-indent-line}), |
7015aca4 | 842 | automatically indent comments according to these conventions, |
574efc83 | 843 | depending on the number of semicolons. @xref{Comments,, |
7015aca4 RS |
844 | Manipulating Comments, emacs, The GNU Emacs Manual}. |
845 | ||
7015aca4 RS |
846 | @node Library Headers |
847 | @section Conventional Headers for Emacs Libraries | |
848 | @cindex header comments | |
849 | @cindex library header comments | |
850 | ||
f9f59935 | 851 | Emacs has conventions for using special comments in Lisp libraries |
7015aca4 | 852 | to divide them into sections and give information such as who wrote |
8241495d RS |
853 | them. This section explains these conventions. |
854 | ||
855 | We'll start with an example, a package that is included in the Emacs | |
856 | distribution. | |
857 | ||
858 | Parts of this example reflect its status as part of Emacs; for | |
859 | example, the copyright notice lists the Free Software Foundation as the | |
860 | copyright holder, and the copying permission says the file is part of | |
861 | Emacs. When you write a package and post it, the copyright holder would | |
862 | be you (unless your employer claims to own it instead), and you should | |
863 | get the suggested copying permission from the end of the GNU General | |
864 | Public License itself. Don't say your file is part of Emacs | |
865 | if we haven't installed it in Emacs yet! | |
866 | ||
867 | With that warning out of the way, on to the example: | |
7015aca4 RS |
868 | |
869 | @smallexample | |
870 | @group | |
871 | ;;; lisp-mnt.el --- minor mode for Emacs Lisp maintainers | |
872 | ||
873 | ;; Copyright (C) 1992 Free Software Foundation, Inc. | |
874 | @end group | |
875 | ||
876 | ;; Author: Eric S. Raymond <esr@@snark.thyrsus.com> | |
877 | ;; Maintainer: Eric S. Raymond <esr@@snark.thyrsus.com> | |
878 | ;; Created: 14 Jul 1992 | |
879 | ;; Version: 1.2 | |
880 | @group | |
881 | ;; Keywords: docs | |
882 | ||
883 | ;; This file is part of GNU Emacs. | |
969fe9b5 RS |
884 | @dots{} |
885 | ;; Free Software Foundation, Inc., 59 Temple Place - Suite 330, | |
886 | ;; Boston, MA 02111-1307, USA. | |
7015aca4 RS |
887 | @end group |
888 | @end smallexample | |
889 | ||
890 | The very first line should have this format: | |
891 | ||
892 | @example | |
893 | ;;; @var{filename} --- @var{description} | |
894 | @end example | |
895 | ||
896 | @noindent | |
e4317c8c RS |
897 | The description should be complete in one line. If the file |
898 | needs a @samp{-*-} specification, put it after @var{description}. | |
7015aca4 RS |
899 | |
900 | After the copyright notice come several @dfn{header comment} lines, | |
4b6694ef | 901 | each beginning with @samp{;; @var{header-name}:}. Here is a table of |
7015aca4 RS |
902 | the conventional possibilities for @var{header-name}: |
903 | ||
904 | @table @samp | |
905 | @item Author | |
906 | This line states the name and net address of at least the principal | |
907 | author of the library. | |
908 | ||
909 | If there are multiple authors, you can list them on continuation lines | |
4b6694ef | 910 | led by @code{;;} and a tab character, like this: |
7015aca4 RS |
911 | |
912 | @smallexample | |
913 | @group | |
914 | ;; Author: Ashwin Ram <Ram-Ashwin@@cs.yale.edu> | |
4b6694ef RS |
915 | ;; Dave Sill <de5@@ornl.gov> |
916 | ;; Dave Brennan <brennan@@hal.com> | |
917 | ;; Eric Raymond <esr@@snark.thyrsus.com> | |
7015aca4 RS |
918 | @end group |
919 | @end smallexample | |
920 | ||
921 | @item Maintainer | |
922 | This line should contain a single name/address as in the Author line, or | |
4b6694ef RS |
923 | an address only, or the string @samp{FSF}. If there is no maintainer |
924 | line, the person(s) in the Author field are presumed to be the | |
925 | maintainers. The example above is mildly bogus because the maintainer | |
926 | line is redundant. | |
7015aca4 RS |
927 | |
928 | The idea behind the @samp{Author} and @samp{Maintainer} lines is to make | |
929 | possible a Lisp function to ``send mail to the maintainer'' without | |
930 | having to mine the name out by hand. | |
931 | ||
932 | Be sure to surround the network address with @samp{<@dots{}>} if | |
933 | you include the person's full name as well as the network address. | |
934 | ||
935 | @item Created | |
936 | This optional line gives the original creation date of the | |
937 | file. For historical interest only. | |
938 | ||
939 | @item Version | |
940 | If you wish to record version numbers for the individual Lisp program, put | |
941 | them in this line. | |
942 | ||
943 | @item Adapted-By | |
944 | In this header line, place the name of the person who adapted the | |
945 | library for installation (to make it fit the style conventions, for | |
946 | example). | |
947 | ||
948 | @item Keywords | |
949 | This line lists keywords for the @code{finder-by-keyword} help command. | |
a9f0a989 RS |
950 | Please use that command to see a list of the meaningful keywords. |
951 | ||
7015aca4 | 952 | This field is important; it's how people will find your package when |
2c62739d RS |
953 | they're looking for things by topic area. To separate the keywords, you |
954 | can use spaces, commas, or both. | |
7015aca4 RS |
955 | @end table |
956 | ||
957 | Just about every Lisp library ought to have the @samp{Author} and | |
958 | @samp{Keywords} header comment lines. Use the others if they are | |
959 | appropriate. You can also put in header lines with other header | |
960 | names---they have no standard meanings, so they can't do any harm. | |
961 | ||
962 | We use additional stylized comments to subdivide the contents of the | |
2468d0c0 DL |
963 | library file. These should be separated by blank lines from anything |
964 | else. Here is a table of them: | |
7015aca4 RS |
965 | |
966 | @table @samp | |
967 | @item ;;; Commentary: | |
968 | This begins introductory comments that explain how the library works. | |
a9f0a989 RS |
969 | It should come right after the copying permissions, terminated by a |
970 | @samp{Change Log}, @samp{History} or @samp{Code} comment line. This | |
971 | text is used by the Finder package, so it should make sense in that | |
972 | context. | |
973 | ||
fd423b79 | 974 | @item ;;; Documentation: |
3ff91798 RS |
975 | This was used in some files in place of @samp{;;; Commentary:}, |
976 | but it is deprecated. | |
7015aca4 | 977 | |
a9f0a989 | 978 | @item ;;; Change Log: |
7015aca4 | 979 | This begins change log information stored in the library file (if you |
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980 | store the change history there). For Lisp files distributed with Emacs, |
981 | the change history is kept in the file @file{ChangeLog} and not in the | |
982 | source file at all; these files generally do not have a @samp{;;; Change | |
983 | Log:} line. @samp{History} is an alternative to @samp{Change Log}. | |
7015aca4 RS |
984 | |
985 | @item ;;; Code: | |
986 | This begins the actual code of the program. | |
987 | ||
988 | @item ;;; @var{filename} ends here | |
989 | This is the @dfn{footer line}; it appears at the very end of the file. | |
990 | Its purpose is to enable people to detect truncated versions of the file | |
991 | from the lack of a footer line. | |
992 | @end table | |
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993 | |
994 | @ignore | |
995 | arch-tag: 9ea911c2-6b1d-47dd-88b7-0a94e8b27c2e | |
996 | @end ignore |