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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/tips | |
6 | @node Tips, GNU Emacs Internals, Calendar, Top | |
2323275b | 7 | @appendix Tips and Conventions |
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8 | @cindex tips |
9 | @cindex standards of coding style | |
10 | @cindex coding standards | |
11 | ||
2323275b RS |
12 | This chapter describes no additional features of Emacs Lisp. Instead |
13 | it gives advice on making effective use of the features described in the | |
14 | previous chapters, and describes conventions Emacs Lisp programmers | |
15 | should follow. | |
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16 | |
17 | @menu | |
2323275b | 18 | * Coding Conventions:: Conventions for clean and robust programs. |
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19 | * Compilation Tips:: Making compiled code run fast. |
20 | * Documentation Tips:: Writing readable documentation strings. | |
21 | * Comment Tips:: Conventions for writing comments. | |
22 | * Library Headers:: Standard headers for library packages. | |
23 | @end menu | |
24 | ||
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25 | @node Coding Conventions |
26 | @section Emacs Lisp Coding Conventions | |
7015aca4 | 27 | |
2323275b RS |
28 | Here are conventions that you should follow when writing Emacs Lisp |
29 | code intended for widespread use: | |
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30 | |
31 | @itemize @bullet | |
32 | @item | |
33 | Since all global variables share the same name space, and all functions | |
34 | share another name space, you should choose a short word to distinguish | |
35 | your program from other Lisp programs. Then take care to begin the | |
36 | names of all global variables, constants, and functions with the chosen | |
37 | prefix. This helps avoid name conflicts. | |
38 | ||
39 | This recommendation applies even to names for traditional Lisp | |
40 | primitives that are not primitives in Emacs Lisp---even to @code{cadr}. | |
41 | Believe it or not, there is more than one plausible way to define | |
42 | @code{cadr}. Play it safe; append your name prefix to produce a name | |
43 | like @code{foo-cadr} or @code{mylib-cadr} instead. | |
44 | ||
45 | If you write a function that you think ought to be added to Emacs under | |
46 | a certain name, such as @code{twiddle-files}, don't call it by that name | |
47 | in your program. Call it @code{mylib-twiddle-files} in your program, | |
48 | and send mail to @samp{bug-gnu-emacs@@prep.ai.mit.edu} suggesting we add | |
49 | it to Emacs. If and when we do, we can change the name easily enough. | |
50 | ||
51 | If one prefix is insufficient, your package may use two or three | |
52 | alternative common prefixes, so long as they make sense. | |
53 | ||
54 | Separate the prefix from the rest of the symbol name with a hyphen, | |
55 | @samp{-}. This will be consistent with Emacs itself and with most Emacs | |
56 | Lisp programs. | |
57 | ||
58 | @item | |
59 | It is often useful to put a call to @code{provide} in each separate | |
60 | library program, at least if there is more than one entry point to the | |
61 | program. | |
62 | ||
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63 | @item |
64 | If a file requires certain other library programs to be loaded | |
65 | beforehand, then the comments at the beginning of the file should say | |
66 | so. Also, use @code{require} to make sure they are loaded. | |
67 | ||
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68 | @item |
69 | If one file @var{foo} uses a macro defined in another file @var{bar}, | |
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70 | @var{foo} should contain this expression before the first use of the |
71 | macro: | |
72 | ||
73 | @example | |
74 | (eval-when-compile (require '@var{bar})) | |
75 | @end example | |
76 | ||
77 | @noindent | |
78 | (And @var{bar} should contain @code{(provide '@var{bar})}, to make the | |
79 | @code{require} work.) This will cause @var{bar} to be loaded when you | |
80 | byte-compile @var{foo}. Otherwise, you risk compiling @var{foo} without | |
81 | the necessary macro loaded, and that would produce compiled code that | |
82 | won't work right. @xref{Compiling Macros}. | |
83 | ||
84 | Using @code{eval-when-compile} avoids loading @var{bar} when | |
85 | the compiled version of @var{foo} is @emph{used}. | |
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86 | |
87 | @item | |
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88 | When defining a major mode, please follow the major mode |
89 | conventions. @xref{Major Mode Conventions}. | |
90 | ||
91 | @item | |
92 | When defining a minor mode, please follow the minor mode | |
93 | conventions. @xref{Minor Mode Conventions}. | |
7015aca4 | 94 | |
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95 | @item |
96 | If the purpose of a function is to tell you whether a certain condition | |
97 | is true or false, give the function a name that ends in @samp{p}. If | |
98 | the name is one word, add just @samp{p}; if the name is multiple words, | |
99 | add @samp{-p}. Examples are @code{framep} and @code{frame-live-p}. | |
100 | ||
101 | @item | |
102 | If a user option variable records a true-or-false condition, give it a | |
103 | name that ends in @samp{-flag}. | |
104 | ||
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105 | @item |
106 | Please do not define @kbd{C-c @var{letter}} as a key in your major | |
107 | modes. These sequences are reserved for users; they are the | |
108 | @strong{only} sequences reserved for users, so we cannot do without | |
109 | them. | |
110 | ||
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111 | Instead, define sequences consisting of @kbd{C-c} followed by a control |
112 | character, a digit, or certain punctuation characters. These sequences | |
113 | are reserved for major modes. | |
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114 | |
115 | Changing all the major modes in Emacs 18 so they would follow this | |
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116 | convention was a lot of work. Abandoning this convention would make |
117 | that work go to waste, and inconvenience users. | |
118 | ||
119 | @item | |
120 | Sequences consisting of @kbd{C-c} followed by @kbd{@{}, @kbd{@}}, | |
121 | @kbd{<}, @kbd{>}, @kbd{:} or @kbd{;} are also reserved for major modes. | |
122 | ||
123 | @item | |
124 | Sequences consisting of @kbd{C-c} followed by any other punctuation | |
125 | character are allocated for minor modes. Using them in a major mode is | |
126 | not absolutely prohibited, but if you do that, the major mode binding | |
127 | may be shadowed from time to time by minor modes. | |
7015aca4 | 128 | |
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129 | @item |
130 | Function keys @key{F5} through @key{F9} without modifier keys are | |
131 | reserved for users to define. | |
132 | ||
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133 | @item |
134 | You should not bind @kbd{C-h} following any prefix character (including | |
135 | @kbd{C-c}). If you don't bind @kbd{C-h}, it is automatically available | |
136 | as a help character for listing the subcommands of the prefix character. | |
137 | ||
138 | @item | |
139 | You should not bind a key sequence ending in @key{ESC} except following | |
140 | another @key{ESC}. (That is, it is ok to bind a sequence ending in | |
141 | @kbd{@key{ESC} @key{ESC}}.) | |
142 | ||
143 | The reason for this rule is that a non-prefix binding for @key{ESC} in | |
144 | any context prevents recognition of escape sequences as function keys in | |
145 | that context. | |
146 | ||
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147 | @item |
148 | Applications should not bind mouse events based on button 1 with the | |
149 | shift key held down. These events include @kbd{S-mouse-1}, | |
150 | @kbd{M-S-mouse-1}, @kbd{C-S-mouse-1}, and so on. They are reserved for | |
151 | users. | |
152 | ||
153 | @item | |
154 | Modes should redefine @kbd{mouse-2} as a command to follow some sort of | |
155 | reference in the text of a buffer, if users usually would not want to | |
156 | alter the text in that buffer by hand. Modes such as Dired, Info, | |
157 | Compilation, and Occur redefine it in this way. | |
158 | ||
7015aca4 | 159 | @item |
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160 | When a package provides a modification of ordinary Emacs behavior, it is |
161 | good to include a command to enable and disable the feature, Provide a | |
162 | command named @code{@var{whatever}-mode} which turns the feature on or | |
163 | off, and make it autoload (@pxref{Autoload}). Design the package so | |
164 | that simply loading it has no visible effect---that should not enable | |
165 | the feature. Users will request the feature by invoking the command. | |
166 | ||
167 | @item | |
168 | It is a bad idea to define aliases for the Emacs primitives. Use the | |
169 | standard names instead. | |
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170 | |
171 | @item | |
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172 | Redefining (or advising) an Emacs primitive is discouraged. It may do |
173 | the right thing for a particular program, but there is no telling what | |
174 | other programs might break as a result. | |
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175 | |
176 | @item | |
177 | If a file does replace any of the functions or library programs of | |
178 | standard Emacs, prominent comments at the beginning of the file should | |
179 | say which functions are replaced, and how the behavior of the | |
180 | replacements differs from that of the originals. | |
181 | ||
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182 | @item |
183 | Please keep the names of your Emacs Lisp source files to 13 characters | |
184 | or less. This way, if the files are compiled, the compiled files' names | |
185 | will be 14 characters or less, which is short enough to fit on all kinds | |
186 | of Unix systems. | |
187 | ||
188 | @item | |
189 | Don't use @code{next-line} or @code{previous-line} in programs; nearly | |
190 | always, @code{forward-line} is more convenient as well as more | |
191 | predictable and robust. @xref{Text Lines}. | |
192 | ||
193 | @item | |
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194 | Don't call functions that set the mark, unless setting the mark is one |
195 | of the intended features of your program. The mark is a user-level | |
196 | feature, so it is incorrect to change the mark except to supply a value | |
197 | for the user's benefit. @xref{The Mark}. | |
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198 | |
199 | In particular, don't use these functions: | |
200 | ||
201 | @itemize @bullet | |
202 | @item | |
203 | @code{beginning-of-buffer}, @code{end-of-buffer} | |
204 | @item | |
205 | @code{replace-string}, @code{replace-regexp} | |
206 | @end itemize | |
207 | ||
208 | If you just want to move point, or replace a certain string, without any | |
209 | of the other features intended for interactive users, you can replace | |
210 | these functions with one or two lines of simple Lisp code. | |
211 | ||
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212 | @item |
213 | Use lists rather than vectors, except when there is a particular reason | |
214 | to use a vector. Lisp has more facilities for manipulating lists than | |
215 | for vectors, and working with lists is usually more convenient. | |
216 | ||
217 | Vectors are advantageous for tables that are substantial in size and are | |
218 | accessed in random order (not searched front to back), provided there is | |
219 | no need to insert or delete elements (only lists allow that). | |
220 | ||
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221 | @item |
222 | The recommended way to print a message in the echo area is with | |
223 | the @code{message} function, not @code{princ}. @xref{The Echo Area}. | |
224 | ||
225 | @item | |
226 | When you encounter an error condition, call the function @code{error} | |
227 | (or @code{signal}). The function @code{error} does not return. | |
228 | @xref{Signaling Errors}. | |
229 | ||
230 | Do not use @code{message}, @code{throw}, @code{sleep-for}, | |
231 | or @code{beep} to report errors. | |
232 | ||
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233 | @item |
234 | An error message should start with a capital letter but should not end | |
235 | with a period. | |
236 | ||
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237 | @item |
238 | Many commands that take a long time to execute display a message that | |
239 | says @samp{Operating...} when they start, and change it to | |
240 | @samp{Operating...done} when they finish. Please keep the style of | |
241 | these messages uniform: @emph{no} space around the ellipsis, and | |
242 | @emph{no} period at the end. | |
243 | ||
7015aca4 | 244 | @item |
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245 | Try to avoid using recursive edits. Instead, do what the Rmail @kbd{e} |
246 | command does: use a new local keymap that contains one command defined | |
247 | to switch back to the old local keymap. Or do what the | |
248 | @code{edit-options} command does: switch to another buffer and let the | |
249 | user switch back at will. @xref{Recursive Editing}. | |
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250 | |
251 | @item | |
252 | In some other systems there is a convention of choosing variable names | |
253 | that begin and end with @samp{*}. We don't use that convention in Emacs | |
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254 | Lisp, so please don't use it in your programs. (Emacs uses such names |
255 | only for program-generated buffers.) The users will find Emacs more | |
256 | coherent if all libraries use the same conventions. | |
7015aca4 | 257 | |
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258 | @item |
259 | Try to avoid compiler warnings about undefined free variables, by adding | |
378f6042 | 260 | @code{defvar} definitions for these variables. |
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261 | |
262 | If you bind a variable in one function, and use it or set it in another | |
263 | function, the compiler warns about the latter function unless the | |
264 | variable has a definition. But often these variables have short names, | |
265 | and it is not clean for Lisp packages to define such variables names. | |
266 | Therefore, you should rename the variable to start with the name prefix | |
267 | used for the other functions and variables in your package. | |
268 | ||
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269 | @item |
270 | Indent each function with @kbd{C-M-q} (@code{indent-sexp}) using the | |
271 | default indentation parameters. | |
272 | ||
273 | @item | |
274 | Don't make a habit of putting close-parentheses on lines by themselves; | |
275 | Lisp programmers find this disconcerting. Once in a while, when there | |
276 | is a sequence of many consecutive close-parentheses, it may make sense | |
277 | to split them in one or two significant places. | |
278 | ||
279 | @item | |
280 | Please put a copyright notice on the file if you give copies to anyone. | |
281 | Use the same lines that appear at the top of the Lisp files in Emacs | |
282 | itself. If you have not signed papers to assign the copyright to the | |
283 | Foundation, then place your name in the copyright notice in place of the | |
284 | Foundation's name. | |
285 | @end itemize | |
286 | ||
287 | @node Compilation Tips | |
288 | @section Tips for Making Compiled Code Fast | |
289 | @cindex execution speed | |
290 | @cindex speedups | |
291 | ||
292 | Here are ways of improving the execution speed of byte-compiled | |
4b6694ef | 293 | Lisp programs. |
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294 | |
295 | @itemize @bullet | |
296 | @item | |
297 | @cindex profiling | |
298 | @cindex timing programs | |
299 | @cindex @file{profile.el} | |
300 | Use the @file{profile} library to profile your program. See the file | |
301 | @file{profile.el} for instructions. | |
302 | ||
303 | @item | |
304 | Use iteration rather than recursion whenever possible. | |
305 | Function calls are slow in Emacs Lisp even when a compiled function | |
306 | is calling another compiled function. | |
307 | ||
308 | @item | |
bfe721d1 KH |
309 | Using the primitive list-searching functions @code{memq}, @code{member}, |
310 | @code{assq}, or @code{assoc} is even faster than explicit iteration. It | |
311 | may be worth rearranging a data structure so that one of these primitive | |
312 | search functions can be used. | |
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313 | |
314 | @item | |
4b6694ef | 315 | Certain built-in functions are handled specially in byte-compiled code, |
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316 | avoiding the need for an ordinary function call. It is a good idea to |
317 | use these functions rather than alternatives. To see whether a function | |
318 | is handled specially by the compiler, examine its @code{byte-compile} | |
319 | property. If the property is non-@code{nil}, then the function is | |
320 | handled specially. | |
321 | ||
322 | For example, the following input will show you that @code{aref} is | |
323 | compiled specially (@pxref{Array Functions}) while @code{elt} is not | |
324 | (@pxref{Sequence Functions}): | |
325 | ||
4b6694ef | 326 | @example |
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327 | @group |
328 | (get 'aref 'byte-compile) | |
329 | @result{} byte-compile-two-args | |
330 | @end group | |
331 | ||
332 | @group | |
333 | (get 'elt 'byte-compile) | |
334 | @result{} nil | |
335 | @end group | |
4b6694ef | 336 | @end example |
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337 | |
338 | @item | |
339 | If calling a small function accounts for a substantial part of your | |
340 | program's running time, make the function inline. This eliminates | |
341 | the function call overhead. Since making a function inline reduces | |
342 | the flexibility of changing the program, don't do it unless it gives | |
4b6694ef | 343 | a noticeable speedup in something slow enough that users care about |
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344 | the speed. @xref{Inline Functions}. |
345 | @end itemize | |
346 | ||
347 | @node Documentation Tips | |
348 | @section Tips for Documentation Strings | |
349 | ||
350 | Here are some tips for the writing of documentation strings. | |
351 | ||
352 | @itemize @bullet | |
353 | @item | |
574efc83 | 354 | Every command, function, or variable intended for users to know about |
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355 | should have a documentation string. |
356 | ||
357 | @item | |
e0d32668 RS |
358 | An internal variable or subroutine of a Lisp program might as well have |
359 | a documentation string. In earlier Emacs versions, you could save space | |
360 | by using a comment instead of a documentation string, but that is no | |
361 | longer the case. | |
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362 | |
363 | @item | |
364 | The first line of the documentation string should consist of one or two | |
574efc83 | 365 | complete sentences that stand on their own as a summary. @kbd{M-x |
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366 | apropos} displays just the first line, and if it doesn't stand on its |
367 | own, the result looks bad. In particular, start the first line with a | |
368 | capital letter and end with a period. | |
7015aca4 | 369 | |
574efc83 | 370 | The documentation string can have additional lines that expand on the |
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371 | details of how to use the function or variable. The additional lines |
372 | should be made up of complete sentences also, but they may be filled if | |
373 | that looks good. | |
374 | ||
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375 | @item |
376 | For consistency, phrase the verb in the first sentence of a | |
377 | documentation string as an infinitive with ``to'' omitted. For | |
378 | instance, use ``Return the cons of A and B.'' in preference to ``Returns | |
379 | the cons of A and B@.'' Usually it looks good to do likewise for the | |
380 | rest of the first paragraph. Subsequent paragraphs usually look better | |
381 | if they have proper subjects. | |
382 | ||
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383 | @item |
384 | Write documentation strings in the active voice, not the passive, and in | |
385 | the present tense, not the future. For instance, use ``Return a list | |
386 | containing A and B.'' instead of ``A list containing A and B will be | |
387 | returned.'' | |
388 | ||
389 | @item | |
390 | Avoid using the word ``cause'' (or its equivalents) unnecessarily. | |
391 | Instead of, ``Cause Emacs to display text in boldface,'' write just | |
392 | ``Display text in boldface.'' | |
393 | ||
394 | @item | |
395 | Do not start or end a documentation string with whitespace. | |
396 | ||
397 | @item | |
398 | Format the documentation string so that it fits in an Emacs window on an | |
574efc83 | 399 | 80-column screen. It is a good idea for most lines to be no wider than |
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400 | 60 characters. The first line can be wider if necessary to fit the |
401 | information that ought to be there. | |
402 | ||
403 | However, rather than simply filling the entire documentation string, you | |
404 | can make it much more readable by choosing line breaks with care. | |
405 | Use blank lines between topics if the documentation string is long. | |
406 | ||
407 | @item | |
408 | @strong{Do not} indent subsequent lines of a documentation string so | |
409 | that the text is lined up in the source code with the text of the first | |
410 | line. This looks nice in the source code, but looks bizarre when users | |
411 | view the documentation. Remember that the indentation before the | |
412 | starting double-quote is not part of the string! | |
413 | ||
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414 | @item |
415 | When the user tries to use a disabled command, Emacs displays just the | |
416 | first paragraph of its documentation string---everything through the | |
417 | first blank line. If you wish, you can choose which information to | |
418 | include before the first blank line so as to make this display useful. | |
419 | ||
7015aca4 RS |
420 | @item |
421 | A variable's documentation string should start with @samp{*} if the | |
4b6694ef | 422 | variable is one that users would often want to set interactively. If |
574efc83 RS |
423 | the value is a long list, or a function, or if the variable would be set |
424 | only in init files, then don't start the documentation string with | |
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425 | @samp{*}. @xref{Defining Variables}. |
426 | ||
427 | @item | |
428 | The documentation string for a variable that is a yes-or-no flag should | |
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429 | start with words such as ``Non-nil means@dots{}'', to make it clear that |
430 | all non-@code{nil} values are equivalent and indicate explicitly what | |
431 | @code{nil} and non-@code{nil} mean. | |
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432 | |
433 | @item | |
434 | When a function's documentation string mentions the value of an argument | |
435 | of the function, use the argument name in capital letters as if it were | |
436 | a name for that value. Thus, the documentation string of the function | |
4b6694ef RS |
437 | @code{/} refers to its second argument as @samp{DIVISOR}, because the |
438 | actual argument name is @code{divisor}. | |
7015aca4 RS |
439 | |
440 | Also use all caps for meta-syntactic variables, such as when you show | |
441 | the decomposition of a list or vector into subunits, some of which may | |
442 | vary. | |
443 | ||
444 | @item | |
445 | @iftex | |
446 | When a documentation string refers to a Lisp symbol, write it as it | |
447 | would be printed (which usually means in lower case), with single-quotes | |
448 | around it. For example: @samp{`lambda'}. There are two exceptions: | |
449 | write @code{t} and @code{nil} without single-quotes. | |
450 | @end iftex | |
451 | @ifinfo | |
452 | When a documentation string refers to a Lisp symbol, write it as it | |
453 | would be printed (which usually means in lower case), with single-quotes | |
454 | around it. For example: @samp{lambda}. There are two exceptions: write | |
455 | t and nil without single-quotes. (In this manual, we normally do use | |
456 | single-quotes for those symbols.) | |
457 | @end ifinfo | |
458 | ||
459 | @item | |
460 | Don't write key sequences directly in documentation strings. Instead, | |
461 | use the @samp{\\[@dots{}]} construct to stand for them. For example, | |
9e2b495b RS |
462 | instead of writing @samp{C-f}, write the construct |
463 | @samp{\\[forward-char]}. When Emacs displays the documentation string, | |
464 | it substitutes whatever key is currently bound to @code{forward-char}. | |
465 | (This is normally @samp{C-f}, but it may be some other character if the | |
466 | user has moved key bindings.) @xref{Keys in Documentation}. | |
7015aca4 RS |
467 | |
468 | @item | |
469 | In documentation strings for a major mode, you will want to refer to the | |
470 | key bindings of that mode's local map, rather than global ones. | |
471 | Therefore, use the construct @samp{\\<@dots{}>} once in the | |
472 | documentation string to specify which key map to use. Do this before | |
473 | the first use of @samp{\\[@dots{}]}. The text inside the | |
474 | @samp{\\<@dots{}>} should be the name of the variable containing the | |
475 | local keymap for the major mode. | |
476 | ||
477 | It is not practical to use @samp{\\[@dots{}]} very many times, because | |
478 | display of the documentation string will become slow. So use this to | |
479 | describe the most important commands in your major mode, and then use | |
480 | @samp{\\@{@dots{}@}} to display the rest of the mode's keymap. | |
7015aca4 RS |
481 | @end itemize |
482 | ||
483 | @node Comment Tips | |
484 | @section Tips on Writing Comments | |
485 | ||
486 | We recommend these conventions for where to put comments and how to | |
487 | indent them: | |
488 | ||
489 | @table @samp | |
490 | @item ; | |
491 | Comments that start with a single semicolon, @samp{;}, should all be | |
492 | aligned to the same column on the right of the source code. Such | |
493 | comments usually explain how the code on the same line does its job. In | |
494 | Lisp mode and related modes, the @kbd{M-;} (@code{indent-for-comment}) | |
495 | command automatically inserts such a @samp{;} in the right place, or | |
4b6694ef | 496 | aligns such a comment if it is already present. |
7015aca4 | 497 | |
574efc83 | 498 | This and following examples are taken from the Emacs sources. |
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499 | |
500 | @smallexample | |
501 | @group | |
502 | (setq base-version-list ; there was a base | |
503 | (assoc (substring fn 0 start-vn) ; version to which | |
504 | file-version-assoc-list)) ; this looks like | |
505 | ; a subversion | |
506 | @end group | |
507 | @end smallexample | |
508 | ||
509 | @item ;; | |
510 | Comments that start with two semicolons, @samp{;;}, should be aligned to | |
4b6694ef | 511 | the same level of indentation as the code. Such comments usually |
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512 | describe the purpose of the following lines or the state of the program |
513 | at that point. For example: | |
514 | ||
515 | @smallexample | |
516 | @group | |
517 | (prog1 (setq auto-fill-function | |
518 | @dots{} | |
519 | @dots{} | |
4b6694ef | 520 | ;; update mode line |
7015aca4 RS |
521 | (force-mode-line-update))) |
522 | @end group | |
523 | @end smallexample | |
524 | ||
4b6694ef RS |
525 | Every function that has no documentation string (because it is use only |
526 | internally within the package it belongs to), should have instead a | |
527 | two-semicolon comment right before the function, explaining what the | |
528 | function does and how to call it properly. Explain precisely what each | |
574efc83 | 529 | argument means and how the function interprets its possible values. |
7015aca4 RS |
530 | |
531 | @item ;;; | |
532 | Comments that start with three semicolons, @samp{;;;}, should start at | |
4b6694ef RS |
533 | the left margin. Such comments are used outside function definitions to |
534 | make general statements explaining the design principles of the program. | |
535 | For example: | |
7015aca4 RS |
536 | |
537 | @smallexample | |
538 | @group | |
539 | ;;; This Lisp code is run in Emacs | |
540 | ;;; when it is to operate as a server | |
541 | ;;; for other processes. | |
542 | @end group | |
543 | @end smallexample | |
544 | ||
574efc83 | 545 | Another use for triple-semicolon comments is for commenting out lines |
4b6694ef RS |
546 | within a function. We use triple-semicolons for this precisely so that |
547 | they remain at the left margin. | |
548 | ||
549 | @smallexample | |
550 | (defun foo (a) | |
551 | ;;; This is no longer necessary. | |
552 | ;;; (force-mode-line-update) | |
553 | (message "Finished with %s" a)) | |
554 | @end smallexample | |
555 | ||
7015aca4 RS |
556 | @item ;;;; |
557 | Comments that start with four semicolons, @samp{;;;;}, should be aligned | |
558 | to the left margin and are used for headings of major sections of a | |
559 | program. For example: | |
560 | ||
561 | @smallexample | |
562 | ;;;; The kill ring | |
563 | @end smallexample | |
564 | @end table | |
565 | ||
566 | @noindent | |
567 | The indentation commands of the Lisp modes in Emacs, such as @kbd{M-;} | |
568 | (@code{indent-for-comment}) and @key{TAB} (@code{lisp-indent-line}) | |
569 | automatically indent comments according to these conventions, | |
574efc83 | 570 | depending on the number of semicolons. @xref{Comments,, |
7015aca4 RS |
571 | Manipulating Comments, emacs, The GNU Emacs Manual}. |
572 | ||
7015aca4 RS |
573 | @node Library Headers |
574 | @section Conventional Headers for Emacs Libraries | |
575 | @cindex header comments | |
576 | @cindex library header comments | |
577 | ||
578 | Emacs 19 has conventions for using special comments in Lisp libraries | |
579 | to divide them into sections and give information such as who wrote | |
580 | them. This section explains these conventions. First, an example: | |
581 | ||
582 | @smallexample | |
583 | @group | |
584 | ;;; lisp-mnt.el --- minor mode for Emacs Lisp maintainers | |
585 | ||
586 | ;; Copyright (C) 1992 Free Software Foundation, Inc. | |
587 | @end group | |
588 | ||
589 | ;; Author: Eric S. Raymond <esr@@snark.thyrsus.com> | |
590 | ;; Maintainer: Eric S. Raymond <esr@@snark.thyrsus.com> | |
591 | ;; Created: 14 Jul 1992 | |
592 | ;; Version: 1.2 | |
593 | @group | |
594 | ;; Keywords: docs | |
595 | ||
596 | ;; This file is part of GNU Emacs. | |
574efc83 | 597 | @var{copying permissions}@dots{} |
7015aca4 RS |
598 | @end group |
599 | @end smallexample | |
600 | ||
601 | The very first line should have this format: | |
602 | ||
603 | @example | |
604 | ;;; @var{filename} --- @var{description} | |
605 | @end example | |
606 | ||
607 | @noindent | |
608 | The description should be complete in one line. | |
609 | ||
610 | After the copyright notice come several @dfn{header comment} lines, | |
4b6694ef | 611 | each beginning with @samp{;; @var{header-name}:}. Here is a table of |
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612 | the conventional possibilities for @var{header-name}: |
613 | ||
614 | @table @samp | |
615 | @item Author | |
616 | This line states the name and net address of at least the principal | |
617 | author of the library. | |
618 | ||
619 | If there are multiple authors, you can list them on continuation lines | |
4b6694ef | 620 | led by @code{;;} and a tab character, like this: |
7015aca4 RS |
621 | |
622 | @smallexample | |
623 | @group | |
624 | ;; Author: Ashwin Ram <Ram-Ashwin@@cs.yale.edu> | |
4b6694ef RS |
625 | ;; Dave Sill <de5@@ornl.gov> |
626 | ;; Dave Brennan <brennan@@hal.com> | |
627 | ;; Eric Raymond <esr@@snark.thyrsus.com> | |
7015aca4 RS |
628 | @end group |
629 | @end smallexample | |
630 | ||
631 | @item Maintainer | |
632 | This line should contain a single name/address as in the Author line, or | |
4b6694ef RS |
633 | an address only, or the string @samp{FSF}. If there is no maintainer |
634 | line, the person(s) in the Author field are presumed to be the | |
635 | maintainers. The example above is mildly bogus because the maintainer | |
636 | line is redundant. | |
7015aca4 RS |
637 | |
638 | The idea behind the @samp{Author} and @samp{Maintainer} lines is to make | |
639 | possible a Lisp function to ``send mail to the maintainer'' without | |
640 | having to mine the name out by hand. | |
641 | ||
642 | Be sure to surround the network address with @samp{<@dots{}>} if | |
643 | you include the person's full name as well as the network address. | |
644 | ||
645 | @item Created | |
646 | This optional line gives the original creation date of the | |
647 | file. For historical interest only. | |
648 | ||
649 | @item Version | |
650 | If you wish to record version numbers for the individual Lisp program, put | |
651 | them in this line. | |
652 | ||
653 | @item Adapted-By | |
654 | In this header line, place the name of the person who adapted the | |
655 | library for installation (to make it fit the style conventions, for | |
656 | example). | |
657 | ||
658 | @item Keywords | |
659 | This line lists keywords for the @code{finder-by-keyword} help command. | |
660 | This field is important; it's how people will find your package when | |
2c62739d RS |
661 | they're looking for things by topic area. To separate the keywords, you |
662 | can use spaces, commas, or both. | |
7015aca4 RS |
663 | @end table |
664 | ||
665 | Just about every Lisp library ought to have the @samp{Author} and | |
666 | @samp{Keywords} header comment lines. Use the others if they are | |
667 | appropriate. You can also put in header lines with other header | |
668 | names---they have no standard meanings, so they can't do any harm. | |
669 | ||
670 | We use additional stylized comments to subdivide the contents of the | |
671 | library file. Here is a table of them: | |
672 | ||
673 | @table @samp | |
674 | @item ;;; Commentary: | |
675 | This begins introductory comments that explain how the library works. | |
676 | It should come right after the copying permissions. | |
677 | ||
678 | @item ;;; Change log: | |
679 | This begins change log information stored in the library file (if you | |
680 | store the change history there). For most of the Lisp | |
681 | files distributed with Emacs, the change history is kept in the file | |
682 | @file{ChangeLog} and not in the source file at all; these files do | |
683 | not have a @samp{;;; Change log:} line. | |
684 | ||
685 | @item ;;; Code: | |
686 | This begins the actual code of the program. | |
687 | ||
688 | @item ;;; @var{filename} ends here | |
689 | This is the @dfn{footer line}; it appears at the very end of the file. | |
690 | Its purpose is to enable people to detect truncated versions of the file | |
691 | from the lack of a footer line. | |
692 | @end table |