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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
fd897522 GM |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1995, 1998, 1999 |
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 | ||
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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 | |
40 | Since all global variables share the same name space, and all functions | |
41 | share another name space, you should choose a short word to distinguish | |
42 | your program from other Lisp programs. Then take care to begin the | |
43 | names of all global variables, constants, and functions with the chosen | |
44 | prefix. This helps avoid name conflicts. | |
45 | ||
46 | This recommendation applies even to names for traditional Lisp | |
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47 | primitives that are not primitives in Emacs Lisp---even to |
48 | @code{copy-list}. Believe it or not, there is more than one plausible | |
49 | way to define @code{copy-list}. Play it safe; append your name prefix | |
50 | to produce a name like @code{foo-copy-list} or @code{mylib-copy-list} | |
51 | instead. | |
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52 | |
53 | If you write a function that you think ought to be added to Emacs under | |
54 | a certain name, such as @code{twiddle-files}, don't call it by that name | |
55 | in your program. Call it @code{mylib-twiddle-files} in your program, | |
a9f0a989 | 56 | and send mail to @samp{bug-gnu-emacs@@gnu.org} suggesting we add |
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57 | it to Emacs. If and when we do, we can change the name easily enough. |
58 | ||
59 | If one prefix is insufficient, your package may use two or three | |
60 | alternative common prefixes, so long as they make sense. | |
61 | ||
62 | Separate the prefix from the rest of the symbol name with a hyphen, | |
63 | @samp{-}. This will be consistent with Emacs itself and with most Emacs | |
64 | Lisp programs. | |
65 | ||
66 | @item | |
67 | It is often useful to put a call to @code{provide} in each separate | |
68 | library program, at least if there is more than one entry point to the | |
69 | program. | |
70 | ||
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71 | @item |
72 | If a file requires certain other library programs to be loaded | |
73 | beforehand, then the comments at the beginning of the file should say | |
74 | so. Also, use @code{require} to make sure they are loaded. | |
75 | ||
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76 | @item |
77 | If one file @var{foo} uses a macro defined in another file @var{bar}, | |
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78 | @var{foo} should contain this expression before the first use of the |
79 | macro: | |
80 | ||
81 | @example | |
82 | (eval-when-compile (require '@var{bar})) | |
83 | @end example | |
84 | ||
85 | @noindent | |
969fe9b5 RS |
86 | (And the library @var{bar} should contain @code{(provide '@var{bar})}, |
87 | to make the @code{require} work.) This will cause @var{bar} to be | |
88 | loaded when you byte-compile @var{foo}. Otherwise, you risk compiling | |
89 | @var{foo} without the necessary macro loaded, and that would produce | |
90 | compiled code that won't work right. @xref{Compiling Macros}. | |
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91 | |
92 | Using @code{eval-when-compile} avoids loading @var{bar} when | |
93 | the compiled version of @var{foo} is @emph{used}. | |
7015aca4 | 94 | |
becd5943 KH |
95 | @item |
96 | Please don't require the @code{cl} package of Common Lisp extensions at | |
97 | run time. Use of this package is optional, and it is not part of the | |
98 | standard Emacs namespace. If your package loads @code{cl} at run time, | |
99 | that could cause name clashes for users who don't use that package. | |
100 | ||
101 | However, there is no problem with using the @code{cl} package at compile | |
102 | time, for the sake of macros. You do that like this: | |
103 | ||
104 | @example | |
105 | (eval-when-compile (require 'cl)) | |
106 | @end example | |
107 | ||
7015aca4 | 108 | @item |
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109 | When defining a major mode, please follow the major mode |
110 | conventions. @xref{Major Mode Conventions}. | |
111 | ||
112 | @item | |
113 | When defining a minor mode, please follow the minor mode | |
114 | conventions. @xref{Minor Mode Conventions}. | |
7015aca4 | 115 | |
6cbf476c RS |
116 | @item |
117 | If the purpose of a function is to tell you whether a certain condition | |
118 | is true or false, give the function a name that ends in @samp{p}. If | |
119 | the name is one word, add just @samp{p}; if the name is multiple words, | |
120 | add @samp{-p}. Examples are @code{framep} and @code{frame-live-p}. | |
121 | ||
122 | @item | |
123 | If a user option variable records a true-or-false condition, give it a | |
124 | name that ends in @samp{-flag}. | |
125 | ||
7015aca4 | 126 | @item |
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127 | @cindex reserved keys |
128 | @cindex keys, reserved | |
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129 | Please do not define @kbd{C-c @var{letter}} as a key in your major |
130 | modes. These sequences are reserved for users; they are the | |
f9f59935 | 131 | @strong{only} sequences reserved for users, so do not block them. |
7015aca4 | 132 | |
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133 | Instead, define sequences consisting of @kbd{C-c} followed by a control |
134 | character, a digit, or certain punctuation characters. These sequences | |
135 | are reserved for major modes. | |
7015aca4 | 136 | |
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137 | Changing all the Emacs major modes to follow this convention was a lot |
138 | of work. Abandoning this convention would make that work go to waste, | |
139 | and inconvenience users. | |
00d96ada RS |
140 | |
141 | @item | |
142 | Sequences consisting of @kbd{C-c} followed by @kbd{@{}, @kbd{@}}, | |
143 | @kbd{<}, @kbd{>}, @kbd{:} or @kbd{;} are also reserved for major modes. | |
144 | ||
145 | @item | |
146 | Sequences consisting of @kbd{C-c} followed by any other punctuation | |
147 | character are allocated for minor modes. Using them in a major mode is | |
148 | not absolutely prohibited, but if you do that, the major mode binding | |
149 | may be shadowed from time to time by minor modes. | |
7015aca4 | 150 | |
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151 | @item |
152 | Function keys @key{F5} through @key{F9} without modifier keys are | |
153 | reserved for users to define. | |
154 | ||
7015aca4 | 155 | @item |
f9f59935 | 156 | Do not bind @kbd{C-h} following any prefix character (including |
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157 | @kbd{C-c}). If you don't bind @kbd{C-h}, it is automatically available |
158 | as a help character for listing the subcommands of the prefix character. | |
159 | ||
160 | @item | |
f9f59935 | 161 | Do not bind a key sequence ending in @key{ESC} except following |
969fe9b5 | 162 | another @key{ESC}. (That is, it is OK to bind a sequence ending in |
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163 | @kbd{@key{ESC} @key{ESC}}.) |
164 | ||
165 | The reason for this rule is that a non-prefix binding for @key{ESC} in | |
166 | any context prevents recognition of escape sequences as function keys in | |
167 | that context. | |
168 | ||
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169 | @item |
170 | Anything which acts like a temporary mode or state which the user can | |
b6ae404e | 171 | enter and leave should define @kbd{@key{ESC} @key{ESC}} or |
52c90d84 RS |
172 | @kbd{@key{ESC} @key{ESC} @key{ESC}} as a way to escape. |
173 | ||
174 | For a state which accepts ordinary Emacs commands, or more generally any | |
175 | kind of state in which @key{ESC} followed by a function key or arrow key | |
176 | is potentially meaningful, then you must not define @kbd{@key{ESC} | |
177 | @key{ESC}}, since that would preclude recognizing an escape sequence | |
178 | after @key{ESC}. In these states, you should define @kbd{@key{ESC} | |
179 | @key{ESC} @key{ESC}} as the way to escape. Otherwise, define | |
180 | @kbd{@key{ESC} @key{ESC}} instead. | |
181 | ||
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182 | @item |
183 | Applications should not bind mouse events based on button 1 with the | |
184 | shift key held down. These events include @kbd{S-mouse-1}, | |
185 | @kbd{M-S-mouse-1}, @kbd{C-S-mouse-1}, and so on. They are reserved for | |
186 | users. | |
187 | ||
188 | @item | |
f9f59935 RS |
189 | Special major modes used for read-only text should usually redefine |
190 | @kbd{mouse-2} and @key{RET} to trace some sort of reference in the text. | |
191 | Modes such as Dired, Info, Compilation, and Occur redefine it in this | |
192 | way. | |
4b6694ef | 193 | |
7015aca4 | 194 | @item |
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195 | When a package provides a modification of ordinary Emacs behavior, it is |
196 | good to include a command to enable and disable the feature, Provide a | |
197 | command named @code{@var{whatever}-mode} which turns the feature on or | |
198 | off, and make it autoload (@pxref{Autoload}). Design the package so | |
199 | that simply loading it has no visible effect---that should not enable | |
200 | the feature. Users will request the feature by invoking the command. | |
201 | ||
202 | @item | |
203 | It is a bad idea to define aliases for the Emacs primitives. Use the | |
204 | standard names instead. | |
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205 | |
206 | @item | |
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207 | Redefining (or advising) an Emacs primitive is discouraged. It may do |
208 | the right thing for a particular program, but there is no telling what | |
209 | other programs might break as a result. | |
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210 | |
211 | @item | |
212 | If a file does replace any of the functions or library programs of | |
213 | standard Emacs, prominent comments at the beginning of the file should | |
214 | say which functions are replaced, and how the behavior of the | |
215 | replacements differs from that of the originals. | |
216 | ||
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217 | @item |
218 | Please keep the names of your Emacs Lisp source files to 13 characters | |
219 | or less. This way, if the files are compiled, the compiled files' names | |
220 | will be 14 characters or less, which is short enough to fit on all kinds | |
221 | of Unix systems. | |
222 | ||
223 | @item | |
224 | Don't use @code{next-line} or @code{previous-line} in programs; nearly | |
225 | always, @code{forward-line} is more convenient as well as more | |
226 | predictable and robust. @xref{Text Lines}. | |
227 | ||
228 | @item | |
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229 | Don't call functions that set the mark, unless setting the mark is one |
230 | of the intended features of your program. The mark is a user-level | |
231 | feature, so it is incorrect to change the mark except to supply a value | |
232 | for the user's benefit. @xref{The Mark}. | |
7015aca4 | 233 | |
f9f59935 | 234 | In particular, don't use any of these functions: |
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235 | |
236 | @itemize @bullet | |
237 | @item | |
238 | @code{beginning-of-buffer}, @code{end-of-buffer} | |
239 | @item | |
240 | @code{replace-string}, @code{replace-regexp} | |
241 | @end itemize | |
242 | ||
243 | If you just want to move point, or replace a certain string, without any | |
244 | of the other features intended for interactive users, you can replace | |
245 | these functions with one or two lines of simple Lisp code. | |
246 | ||
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247 | @item |
248 | Use lists rather than vectors, except when there is a particular reason | |
249 | to use a vector. Lisp has more facilities for manipulating lists than | |
250 | for vectors, and working with lists is usually more convenient. | |
251 | ||
252 | Vectors are advantageous for tables that are substantial in size and are | |
253 | accessed in random order (not searched front to back), provided there is | |
254 | no need to insert or delete elements (only lists allow that). | |
255 | ||
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256 | @item |
257 | The recommended way to print a message in the echo area is with | |
258 | the @code{message} function, not @code{princ}. @xref{The Echo Area}. | |
259 | ||
260 | @item | |
261 | When you encounter an error condition, call the function @code{error} | |
262 | (or @code{signal}). The function @code{error} does not return. | |
263 | @xref{Signaling Errors}. | |
264 | ||
265 | Do not use @code{message}, @code{throw}, @code{sleep-for}, | |
266 | or @code{beep} to report errors. | |
267 | ||
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268 | @item |
269 | An error message should start with a capital letter but should not end | |
270 | with a period. | |
271 | ||
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272 | @item |
273 | Many commands that take a long time to execute display a message that | |
274 | says @samp{Operating...} when they start, and change it to | |
275 | @samp{Operating...done} when they finish. Please keep the style of | |
276 | these messages uniform: @emph{no} space around the ellipsis, and | |
277 | @emph{no} period at the end. | |
278 | ||
7015aca4 | 279 | @item |
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280 | Try to avoid using recursive edits. Instead, do what the Rmail @kbd{e} |
281 | command does: use a new local keymap that contains one command defined | |
282 | to switch back to the old local keymap. Or do what the | |
283 | @code{edit-options} command does: switch to another buffer and let the | |
284 | user switch back at will. @xref{Recursive Editing}. | |
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285 | |
286 | @item | |
287 | In some other systems there is a convention of choosing variable names | |
288 | that begin and end with @samp{*}. We don't use that convention in Emacs | |
4b6694ef | 289 | Lisp, so please don't use it in your programs. (Emacs uses such names |
969fe9b5 | 290 | only for special-purpose buffers.) The users will find Emacs more |
4b6694ef | 291 | coherent if all libraries use the same conventions. |
7015aca4 | 292 | |
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293 | @item |
294 | Try to avoid compiler warnings about undefined free variables, by adding | |
378f6042 | 295 | @code{defvar} definitions for these variables. |
6a994023 | 296 | |
8241495d RS |
297 | Sometimes adding a @code{require} for another package is useful to avoid |
298 | compilation warnings for variables and functions defined in that | |
513331d3 | 299 | package. If you do this, often it is better if the @code{require} acts |
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300 | only at compile time. Here's how to do that: |
301 | ||
302 | @example | |
303 | (eval-when-compile | |
304 | (require 'foo) | |
305 | (defvar bar-baz)) | |
306 | @end example | |
307 | ||
6a994023 RS |
308 | If you bind a variable in one function, and use it or set it in another |
309 | function, the compiler warns about the latter function unless the | |
310 | variable has a definition. But often these variables have short names, | |
a9f0a989 | 311 | and it is not clean for Lisp packages to define such variable names. |
6a994023 RS |
312 | Therefore, you should rename the variable to start with the name prefix |
313 | used for the other functions and variables in your package. | |
314 | ||
7015aca4 RS |
315 | @item |
316 | Indent each function with @kbd{C-M-q} (@code{indent-sexp}) using the | |
317 | default indentation parameters. | |
318 | ||
319 | @item | |
320 | Don't make a habit of putting close-parentheses on lines by themselves; | |
321 | Lisp programmers find this disconcerting. Once in a while, when there | |
322 | is a sequence of many consecutive close-parentheses, it may make sense | |
969fe9b5 | 323 | to split the sequence in one or two significant places. |
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324 | |
325 | @item | |
326 | Please put a copyright notice on the file if you give copies to anyone. | |
f9f59935 RS |
327 | Use a message like this one: |
328 | ||
329 | @smallexample | |
330 | ;; Copyright (C) @var{year} @var{name} | |
331 | ||
332 | ;; This program is free software; you can redistribute it and/or | |
333 | ;; modify it under the terms of the GNU General Public License as | |
334 | ;; published by the Free Software Foundation; either version 2 of | |
335 | ;; the License, or (at your option) any later version. | |
336 | ||
337 | ;; This program is distributed in the hope that it will be | |
338 | ;; useful, but WITHOUT ANY WARRANTY; without even the implied | |
339 | ;; warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR | |
340 | ;; PURPOSE. See the GNU General Public License for more details. | |
341 | ||
342 | ;; You should have received a copy of the GNU General Public | |
343 | ;; License along with this program; if not, write to the Free | |
344 | ;; Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, | |
345 | ;; MA 02111-1307 USA | |
346 | @end smallexample | |
347 | ||
348 | If you have signed papers to assign the copyright to the Foundation, | |
349 | then use @samp{Free Software Foundation, Inc.} as @var{name}. | |
350 | Otherwise, use your name. | |
7015aca4 RS |
351 | @end itemize |
352 | ||
353 | @node Compilation Tips | |
354 | @section Tips for Making Compiled Code Fast | |
355 | @cindex execution speed | |
356 | @cindex speedups | |
357 | ||
358 | Here are ways of improving the execution speed of byte-compiled | |
4b6694ef | 359 | Lisp programs. |
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360 | |
361 | @itemize @bullet | |
362 | @item | |
363 | @cindex profiling | |
364 | @cindex timing programs | |
365 | @cindex @file{profile.el} | |
a9f0a989 RS |
366 | @cindex @file{elp.el} |
367 | Profile your program with the @file{profile} library or the @file{elp} | |
368 | library. See the files @file{profile.el} and @file{elp.el} for | |
369 | instructions. | |
7015aca4 RS |
370 | |
371 | @item | |
372 | Use iteration rather than recursion whenever possible. | |
373 | Function calls are slow in Emacs Lisp even when a compiled function | |
374 | is calling another compiled function. | |
375 | ||
376 | @item | |
bfe721d1 KH |
377 | Using the primitive list-searching functions @code{memq}, @code{member}, |
378 | @code{assq}, or @code{assoc} is even faster than explicit iteration. It | |
f9f59935 | 379 | can be worth rearranging a data structure so that one of these primitive |
bfe721d1 | 380 | search functions can be used. |
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381 | |
382 | @item | |
4b6694ef | 383 | Certain built-in functions are handled specially in byte-compiled code, |
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384 | avoiding the need for an ordinary function call. It is a good idea to |
385 | use these functions rather than alternatives. To see whether a function | |
386 | is handled specially by the compiler, examine its @code{byte-compile} | |
387 | property. If the property is non-@code{nil}, then the function is | |
388 | handled specially. | |
389 | ||
390 | For example, the following input will show you that @code{aref} is | |
a9f0a989 | 391 | compiled specially (@pxref{Array Functions}): |
7015aca4 | 392 | |
4b6694ef | 393 | @example |
7015aca4 RS |
394 | @group |
395 | (get 'aref 'byte-compile) | |
396 | @result{} byte-compile-two-args | |
397 | @end group | |
4b6694ef | 398 | @end example |
7015aca4 RS |
399 | |
400 | @item | |
1911e6e5 | 401 | If calling a small function accounts for a substantial part of your |
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402 | program's running time, make the function inline. This eliminates |
403 | the function call overhead. Since making a function inline reduces | |
404 | the flexibility of changing the program, don't do it unless it gives | |
4b6694ef | 405 | a noticeable speedup in something slow enough that users care about |
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406 | the speed. @xref{Inline Functions}. |
407 | @end itemize | |
408 | ||
409 | @node Documentation Tips | |
410 | @section Tips for Documentation Strings | |
411 | ||
969fe9b5 RS |
412 | @findex checkdoc-minor-mode |
413 | Here are some tips and conventions for the writing of documentation | |
414 | strings. You can check many of these conventions by running the command | |
415 | @kbd{M-x checkdoc-minor-mode}. | |
7015aca4 RS |
416 | |
417 | @itemize @bullet | |
418 | @item | |
574efc83 | 419 | Every command, function, or variable intended for users to know about |
7015aca4 RS |
420 | should have a documentation string. |
421 | ||
422 | @item | |
e0d32668 RS |
423 | An internal variable or subroutine of a Lisp program might as well have |
424 | a documentation string. In earlier Emacs versions, you could save space | |
425 | by using a comment instead of a documentation string, but that is no | |
2468d0c0 DL |
426 | longer the case---documentation strings now take up very little space in |
427 | a running Emacs. | |
7015aca4 RS |
428 | |
429 | @item | |
430 | The first line of the documentation string should consist of one or two | |
574efc83 | 431 | complete sentences that stand on their own as a summary. @kbd{M-x |
2468d0c0 DL |
432 | apropos} displays just the first line, and if that line's contents don't |
433 | stand on their own, the result looks bad. In particular, start the | |
434 | first line with a capital letter and end with a period. | |
7015aca4 | 435 | |
2468d0c0 DL |
436 | The documentation string is not limited to one line; use as many lines |
437 | as you need to explain the details of how to use the function or | |
438 | variable. Please use complete sentences in the additional lines. | |
7015aca4 | 439 | |
4b6694ef | 440 | @item |
8241495d RS |
441 | For consistency, phrase the verb in the first sentence of a function's |
442 | documentation string as an imperative--for instance, use ``Return the | |
443 | cons of A and B.'' in preference to ``Returns the cons of A and B@.'' | |
444 | Usually it looks good to do likewise for the rest of the first | |
445 | paragraph. Subsequent paragraphs usually look better if each sentence | |
446 | has a proper subject. | |
4b6694ef | 447 | |
7015aca4 RS |
448 | @item |
449 | Write documentation strings in the active voice, not the passive, and in | |
450 | the present tense, not the future. For instance, use ``Return a list | |
451 | containing A and B.'' instead of ``A list containing A and B will be | |
452 | returned.'' | |
453 | ||
454 | @item | |
455 | Avoid using the word ``cause'' (or its equivalents) unnecessarily. | |
456 | Instead of, ``Cause Emacs to display text in boldface,'' write just | |
457 | ``Display text in boldface.'' | |
458 | ||
2468d0c0 DL |
459 | @item |
460 | When a command is meaningful only in a certain mode or situation, | |
461 | do mention that in the documentation string. For example, | |
462 | the documentation of @code{dired-find-file} is: | |
463 | ||
464 | @example | |
465 | In Dired, visit the file or directory named on this line. | |
466 | @end example | |
467 | ||
7015aca4 RS |
468 | @item |
469 | Do not start or end a documentation string with whitespace. | |
470 | ||
471 | @item | |
472 | Format the documentation string so that it fits in an Emacs window on an | |
574efc83 | 473 | 80-column screen. It is a good idea for most lines to be no wider than |
2468d0c0 DL |
474 | 60 characters. The first line should not be wider than 67 characters |
475 | or it will look bad in the output of @code{apropos}. | |
7015aca4 | 476 | |
2468d0c0 DL |
477 | You can fill the text if that looks good. However, rather than blindly |
478 | filling the entire documentation string, you can often make it much more | |
479 | readable by choosing certain line breaks with care. Use blank lines | |
480 | between topics if the documentation string is long. | |
7015aca4 RS |
481 | |
482 | @item | |
483 | @strong{Do not} indent subsequent lines of a documentation string so | |
484 | that the text is lined up in the source code with the text of the first | |
485 | line. This looks nice in the source code, but looks bizarre when users | |
486 | view the documentation. Remember that the indentation before the | |
487 | starting double-quote is not part of the string! | |
488 | ||
75d97f47 RS |
489 | @item |
490 | When the user tries to use a disabled command, Emacs displays just the | |
491 | first paragraph of its documentation string---everything through the | |
492 | first blank line. If you wish, you can choose which information to | |
493 | include before the first blank line so as to make this display useful. | |
494 | ||
7015aca4 RS |
495 | @item |
496 | A variable's documentation string should start with @samp{*} if the | |
4b6694ef | 497 | variable is one that users would often want to set interactively. If |
574efc83 RS |
498 | the value is a long list, or a function, or if the variable would be set |
499 | only in init files, then don't start the documentation string with | |
7015aca4 RS |
500 | @samp{*}. @xref{Defining Variables}. |
501 | ||
502 | @item | |
503 | The documentation string for a variable that is a yes-or-no flag should | |
4b6694ef RS |
504 | start with words such as ``Non-nil means@dots{}'', to make it clear that |
505 | all non-@code{nil} values are equivalent and indicate explicitly what | |
506 | @code{nil} and non-@code{nil} mean. | |
7015aca4 RS |
507 | |
508 | @item | |
509 | When a function's documentation string mentions the value of an argument | |
510 | of the function, use the argument name in capital letters as if it were | |
511 | a name for that value. Thus, the documentation string of the function | |
2468d0c0 DL |
512 | @code{eval} refers to its second argument as @samp{FORM}, because the |
513 | actual argument name is @code{form}: | |
514 | ||
515 | @example | |
516 | Evaluate FORM and return its value. | |
517 | @end example | |
7015aca4 | 518 | |
2468d0c0 DL |
519 | Also write metasyntactic variables in capital letters, such as when you |
520 | show the decomposition of a list or vector into subunits, some of which | |
521 | may vary. @samp{KEY} and @samp{VALUE} in the following example | |
8241495d RS |
522 | illustrate this practice: |
523 | ||
524 | @example | |
525 | The argument TABLE should be an alist whose elements | |
526 | have the form (KEY . VALUE). Here, KEY is ... | |
527 | @end example | |
7015aca4 | 528 | |
2468d0c0 DL |
529 | @item |
530 | If a line in a documentation string begins with an open-parenthesis, | |
531 | write a backslash before the open-parenthesis, like this: | |
532 | ||
533 | @example | |
534 | The argument FOO can be either a number | |
535 | \(a buffer position) or a string (a file name). | |
536 | @end example | |
537 | ||
538 | This prevents the open-parenthesis from being treated as the start of a | |
539 | defun (@pxref{Defuns,, Defuns, emacs, The GNU Emacs Manual}). | |
540 | ||
7015aca4 RS |
541 | @item |
542 | @iftex | |
543 | When a documentation string refers to a Lisp symbol, write it as it | |
544 | would be printed (which usually means in lower case), with single-quotes | |
545 | around it. For example: @samp{`lambda'}. There are two exceptions: | |
546 | write @code{t} and @code{nil} without single-quotes. | |
547 | @end iftex | |
37680279 | 548 | @ifnottex |
7015aca4 RS |
549 | When a documentation string refers to a Lisp symbol, write it as it |
550 | would be printed (which usually means in lower case), with single-quotes | |
551 | around it. For example: @samp{lambda}. There are two exceptions: write | |
969fe9b5 RS |
552 | t and nil without single-quotes. (In this manual, we use a different |
553 | convention, with single-quotes for all symbols.) | |
37680279 | 554 | @end ifnottex |
7015aca4 | 555 | |
1911e6e5 RS |
556 | Help mode automatically creates a hyperlink when a documentation string |
557 | uses a symbol name inside single quotes, if the symbol has either a | |
a9f0a989 RS |
558 | function or a variable definition. You do not need to do anything |
559 | special to make use of this feature. However, when a symbol has both a | |
560 | function definition and a variable definition, and you want to refer to | |
561 | just one of them, you can specify which one by writing one of the words | |
562 | @samp{variable}, @samp{option}, @samp{function}, or @samp{command}, | |
563 | immediately before the symbol name. (Case makes no difference in | |
564 | recognizing these indicator words.) For example, if you write | |
565 | ||
566 | @example | |
567 | This function sets the variable `buffer-file-name'. | |
568 | @end example | |
569 | ||
570 | @noindent | |
571 | then the hyperlink will refer only to the variable documentation of | |
572 | @code{buffer-file-name}, and not to its function documentation. | |
573 | ||
574 | If a symbol has a function definition and/or a variable definition, but | |
575 | those are irrelevant to the use of the symbol that you are documenting, | |
576 | you can write the word @samp{symbol} before the symbol name to prevent | |
577 | making any hyperlink. For example, | |
969fe9b5 RS |
578 | |
579 | @example | |
a9f0a989 RS |
580 | If the argument KIND-OF-RESULT is the symbol `list', |
581 | this function returns a list of all the objects | |
582 | that satisfy the criterion. | |
969fe9b5 RS |
583 | @end example |
584 | ||
a9f0a989 RS |
585 | @noindent |
586 | does not make a hyperlink to the documentation, irrelevant here, of the | |
587 | function @code{list}. | |
588 | ||
8241495d RS |
589 | To make a hyperlink to Info documentation, write the name of the Info |
590 | node in single quotes, preceded by @samp{info node} or @samp{Info | |
591 | node}. The Info file name defaults to @samp{emacs}. For example, | |
592 | ||
593 | @smallexample | |
594 | See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'. | |
595 | @end smallexample | |
596 | ||
7015aca4 RS |
597 | @item |
598 | Don't write key sequences directly in documentation strings. Instead, | |
599 | use the @samp{\\[@dots{}]} construct to stand for them. For example, | |
9e2b495b RS |
600 | instead of writing @samp{C-f}, write the construct |
601 | @samp{\\[forward-char]}. When Emacs displays the documentation string, | |
602 | it substitutes whatever key is currently bound to @code{forward-char}. | |
603 | (This is normally @samp{C-f}, but it may be some other character if the | |
604 | user has moved key bindings.) @xref{Keys in Documentation}. | |
7015aca4 RS |
605 | |
606 | @item | |
607 | In documentation strings for a major mode, you will want to refer to the | |
608 | key bindings of that mode's local map, rather than global ones. | |
609 | Therefore, use the construct @samp{\\<@dots{}>} once in the | |
610 | documentation string to specify which key map to use. Do this before | |
611 | the first use of @samp{\\[@dots{}]}. The text inside the | |
612 | @samp{\\<@dots{}>} should be the name of the variable containing the | |
613 | local keymap for the major mode. | |
614 | ||
615 | It is not practical to use @samp{\\[@dots{}]} very many times, because | |
616 | display of the documentation string will become slow. So use this to | |
617 | describe the most important commands in your major mode, and then use | |
618 | @samp{\\@{@dots{}@}} to display the rest of the mode's keymap. | |
7015aca4 RS |
619 | @end itemize |
620 | ||
621 | @node Comment Tips | |
622 | @section Tips on Writing Comments | |
623 | ||
624 | We recommend these conventions for where to put comments and how to | |
625 | indent them: | |
626 | ||
627 | @table @samp | |
628 | @item ; | |
629 | Comments that start with a single semicolon, @samp{;}, should all be | |
630 | aligned to the same column on the right of the source code. Such | |
631 | comments usually explain how the code on the same line does its job. In | |
632 | Lisp mode and related modes, the @kbd{M-;} (@code{indent-for-comment}) | |
633 | command automatically inserts such a @samp{;} in the right place, or | |
4b6694ef | 634 | aligns such a comment if it is already present. |
7015aca4 | 635 | |
574efc83 | 636 | This and following examples are taken from the Emacs sources. |
7015aca4 RS |
637 | |
638 | @smallexample | |
639 | @group | |
640 | (setq base-version-list ; there was a base | |
641 | (assoc (substring fn 0 start-vn) ; version to which | |
642 | file-version-assoc-list)) ; this looks like | |
643 | ; a subversion | |
644 | @end group | |
645 | @end smallexample | |
646 | ||
647 | @item ;; | |
648 | Comments that start with two semicolons, @samp{;;}, should be aligned to | |
4b6694ef | 649 | the same level of indentation as the code. Such comments usually |
7015aca4 RS |
650 | describe the purpose of the following lines or the state of the program |
651 | at that point. For example: | |
652 | ||
653 | @smallexample | |
654 | @group | |
655 | (prog1 (setq auto-fill-function | |
656 | @dots{} | |
657 | @dots{} | |
4b6694ef | 658 | ;; update mode line |
7015aca4 RS |
659 | (force-mode-line-update))) |
660 | @end group | |
661 | @end smallexample | |
662 | ||
2468d0c0 | 663 | We also normally use two semicolons for comments outside functions. |
7015aca4 RS |
664 | |
665 | @smallexample | |
666 | @group | |
2468d0c0 DL |
667 | ;; This Lisp code is run in Emacs |
668 | ;; when it is to operate as a server | |
669 | ;; for other processes. | |
7015aca4 RS |
670 | @end group |
671 | @end smallexample | |
672 | ||
2468d0c0 DL |
673 | Every function that has no documentation string (presumably one that is |
674 | used only internally within the package it belongs to), should instead | |
675 | have a two-semicolon comment right before the function, explaining what | |
676 | the function does and how to call it properly. Explain precisely what | |
677 | each argument means and how the function interprets its possible values. | |
678 | ||
679 | @item ;;; | |
680 | Comments that start with three semicolons, @samp{;;;}, should start at | |
681 | the left margin. These are used, occasionally, for comments within | |
682 | functions that should start at the margin. We also use them sometimes | |
683 | for comments that are between functions---whether to use two or three | |
684 | semicolons there is a matter of style. | |
685 | ||
574efc83 | 686 | Another use for triple-semicolon comments is for commenting out lines |
2468d0c0 | 687 | within a function. We use three semicolons for this precisely so that |
4b6694ef RS |
688 | they remain at the left margin. |
689 | ||
690 | @smallexample | |
691 | (defun foo (a) | |
692 | ;;; This is no longer necessary. | |
693 | ;;; (force-mode-line-update) | |
694 | (message "Finished with %s" a)) | |
695 | @end smallexample | |
696 | ||
7015aca4 RS |
697 | @item ;;;; |
698 | Comments that start with four semicolons, @samp{;;;;}, should be aligned | |
699 | to the left margin and are used for headings of major sections of a | |
700 | program. For example: | |
701 | ||
702 | @smallexample | |
703 | ;;;; The kill ring | |
704 | @end smallexample | |
705 | @end table | |
706 | ||
707 | @noindent | |
708 | The indentation commands of the Lisp modes in Emacs, such as @kbd{M-;} | |
969fe9b5 | 709 | (@code{indent-for-comment}) and @key{TAB} (@code{lisp-indent-line}), |
7015aca4 | 710 | automatically indent comments according to these conventions, |
574efc83 | 711 | depending on the number of semicolons. @xref{Comments,, |
7015aca4 RS |
712 | Manipulating Comments, emacs, The GNU Emacs Manual}. |
713 | ||
7015aca4 RS |
714 | @node Library Headers |
715 | @section Conventional Headers for Emacs Libraries | |
716 | @cindex header comments | |
717 | @cindex library header comments | |
718 | ||
f9f59935 | 719 | Emacs has conventions for using special comments in Lisp libraries |
7015aca4 | 720 | to divide them into sections and give information such as who wrote |
8241495d RS |
721 | them. This section explains these conventions. |
722 | ||
723 | We'll start with an example, a package that is included in the Emacs | |
724 | distribution. | |
725 | ||
726 | Parts of this example reflect its status as part of Emacs; for | |
727 | example, the copyright notice lists the Free Software Foundation as the | |
728 | copyright holder, and the copying permission says the file is part of | |
729 | Emacs. When you write a package and post it, the copyright holder would | |
730 | be you (unless your employer claims to own it instead), and you should | |
731 | get the suggested copying permission from the end of the GNU General | |
732 | Public License itself. Don't say your file is part of Emacs | |
733 | if we haven't installed it in Emacs yet! | |
734 | ||
735 | With that warning out of the way, on to the example: | |
7015aca4 RS |
736 | |
737 | @smallexample | |
738 | @group | |
739 | ;;; lisp-mnt.el --- minor mode for Emacs Lisp maintainers | |
740 | ||
741 | ;; Copyright (C) 1992 Free Software Foundation, Inc. | |
742 | @end group | |
743 | ||
744 | ;; Author: Eric S. Raymond <esr@@snark.thyrsus.com> | |
745 | ;; Maintainer: Eric S. Raymond <esr@@snark.thyrsus.com> | |
746 | ;; Created: 14 Jul 1992 | |
747 | ;; Version: 1.2 | |
748 | @group | |
749 | ;; Keywords: docs | |
750 | ||
751 | ;; This file is part of GNU Emacs. | |
969fe9b5 RS |
752 | @dots{} |
753 | ;; Free Software Foundation, Inc., 59 Temple Place - Suite 330, | |
754 | ;; Boston, MA 02111-1307, USA. | |
7015aca4 RS |
755 | @end group |
756 | @end smallexample | |
757 | ||
758 | The very first line should have this format: | |
759 | ||
760 | @example | |
761 | ;;; @var{filename} --- @var{description} | |
762 | @end example | |
763 | ||
764 | @noindent | |
765 | The description should be complete in one line. | |
766 | ||
767 | After the copyright notice come several @dfn{header comment} lines, | |
4b6694ef | 768 | each beginning with @samp{;; @var{header-name}:}. Here is a table of |
7015aca4 RS |
769 | the conventional possibilities for @var{header-name}: |
770 | ||
771 | @table @samp | |
772 | @item Author | |
773 | This line states the name and net address of at least the principal | |
774 | author of the library. | |
775 | ||
776 | If there are multiple authors, you can list them on continuation lines | |
4b6694ef | 777 | led by @code{;;} and a tab character, like this: |
7015aca4 RS |
778 | |
779 | @smallexample | |
780 | @group | |
781 | ;; Author: Ashwin Ram <Ram-Ashwin@@cs.yale.edu> | |
4b6694ef RS |
782 | ;; Dave Sill <de5@@ornl.gov> |
783 | ;; Dave Brennan <brennan@@hal.com> | |
784 | ;; Eric Raymond <esr@@snark.thyrsus.com> | |
7015aca4 RS |
785 | @end group |
786 | @end smallexample | |
787 | ||
788 | @item Maintainer | |
789 | This line should contain a single name/address as in the Author line, or | |
4b6694ef RS |
790 | an address only, or the string @samp{FSF}. If there is no maintainer |
791 | line, the person(s) in the Author field are presumed to be the | |
792 | maintainers. The example above is mildly bogus because the maintainer | |
793 | line is redundant. | |
7015aca4 RS |
794 | |
795 | The idea behind the @samp{Author} and @samp{Maintainer} lines is to make | |
796 | possible a Lisp function to ``send mail to the maintainer'' without | |
797 | having to mine the name out by hand. | |
798 | ||
799 | Be sure to surround the network address with @samp{<@dots{}>} if | |
800 | you include the person's full name as well as the network address. | |
801 | ||
802 | @item Created | |
803 | This optional line gives the original creation date of the | |
804 | file. For historical interest only. | |
805 | ||
806 | @item Version | |
807 | If you wish to record version numbers for the individual Lisp program, put | |
808 | them in this line. | |
809 | ||
810 | @item Adapted-By | |
811 | In this header line, place the name of the person who adapted the | |
812 | library for installation (to make it fit the style conventions, for | |
813 | example). | |
814 | ||
815 | @item Keywords | |
816 | This line lists keywords for the @code{finder-by-keyword} help command. | |
a9f0a989 RS |
817 | Please use that command to see a list of the meaningful keywords. |
818 | ||
7015aca4 | 819 | This field is important; it's how people will find your package when |
2c62739d RS |
820 | they're looking for things by topic area. To separate the keywords, you |
821 | can use spaces, commas, or both. | |
7015aca4 RS |
822 | @end table |
823 | ||
824 | Just about every Lisp library ought to have the @samp{Author} and | |
825 | @samp{Keywords} header comment lines. Use the others if they are | |
826 | appropriate. You can also put in header lines with other header | |
827 | names---they have no standard meanings, so they can't do any harm. | |
828 | ||
829 | We use additional stylized comments to subdivide the contents of the | |
2468d0c0 DL |
830 | library file. These should be separated by blank lines from anything |
831 | else. Here is a table of them: | |
7015aca4 RS |
832 | |
833 | @table @samp | |
834 | @item ;;; Commentary: | |
835 | This begins introductory comments that explain how the library works. | |
a9f0a989 RS |
836 | It should come right after the copying permissions, terminated by a |
837 | @samp{Change Log}, @samp{History} or @samp{Code} comment line. This | |
838 | text is used by the Finder package, so it should make sense in that | |
839 | context. | |
840 | ||
841 | @item ;;; Documentation | |
842 | This has been used in some files in place of @samp{;;; Commentary:}, | |
843 | but @samp{;;; Commentary:} is preferred. | |
7015aca4 | 844 | |
a9f0a989 | 845 | @item ;;; Change Log: |
7015aca4 | 846 | This begins change log information stored in the library file (if you |
2468d0c0 DL |
847 | store the change history there). For Lisp files distributed with Emacs, |
848 | the change history is kept in the file @file{ChangeLog} and not in the | |
849 | source file at all; these files generally do not have a @samp{;;; Change | |
850 | Log:} line. @samp{History} is an alternative to @samp{Change Log}. | |
7015aca4 RS |
851 | |
852 | @item ;;; Code: | |
853 | This begins the actual code of the program. | |
854 | ||
855 | @item ;;; @var{filename} ends here | |
856 | This is the @dfn{footer line}; it appears at the very end of the file. | |
857 | Its purpose is to enable people to detect truncated versions of the file | |
858 | from the lack of a footer line. | |
859 | @end table |