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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, 1994 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/loading | |
6 | @node Loading, Byte Compilation, Macros, Top | |
7 | @chapter Loading | |
8 | @cindex loading | |
9 | @cindex library | |
10 | @cindex Lisp library | |
11 | ||
12 | Loading a file of Lisp code means bringing its contents into the Lisp | |
13 | environment in the form of Lisp objects. Emacs finds and opens the | |
14 | file, reads the text, evaluates each form, and then closes the file. | |
15 | ||
16 | The load functions evaluate all the expressions in a file just | |
17 | as the @code{eval-current-buffer} function evaluates all the | |
18 | expressions in a buffer. The difference is that the load functions | |
19 | read and evaluate the text in the file as found on disk, not the text | |
20 | in an Emacs buffer. | |
21 | ||
22 | @cindex top-level form | |
23 | The loaded file must contain Lisp expressions, either as source code | |
78c71a98 RS |
24 | or as byte-compiled code. Each form in the file is called a |
25 | @dfn{top-level form}. There is no special format for the forms in a | |
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26 | loadable file; any form in a file may equally well be typed directly |
27 | into a buffer and evaluated there. (Indeed, most code is tested this | |
28 | way.) Most often, the forms are function definitions and variable | |
29 | definitions. | |
30 | ||
31 | A file containing Lisp code is often called a @dfn{library}. Thus, | |
32 | the ``Rmail library'' is a file containing code for Rmail mode. | |
33 | Similarly, a ``Lisp library directory'' is a directory of files | |
34 | containing Lisp code. | |
35 | ||
36 | @menu | |
37 | * How Programs Do Loading:: The @code{load} function and others. | |
38 | * Autoload:: Setting up a function to autoload. | |
39 | * Repeated Loading:: Precautions about loading a file twice. | |
bfe721d1 | 40 | * Named Features:: Loading a library if it isn't already loaded. |
83ac6b45 RS |
41 | * Unloading:: How to ``unload'' a library that was loaded. |
42 | * Hooks for Loading:: Providing code to be run when | |
43 | particular libraries are loaded. | |
44 | @end menu | |
45 | ||
46 | @node How Programs Do Loading | |
47 | @section How Programs Do Loading | |
48 | ||
49 | Emacs Lisp has several interfaces for loading. For example, | |
50 | @code{autoload} creates a placeholder object for a function in a file; | |
51 | trying to call the autoloading function loads the file to get the | |
52 | function's real definition (@pxref{Autoload}). @code{require} loads a | |
bfe721d1 | 53 | file if it isn't already loaded (@pxref{Named Features}). Ultimately, all |
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54 | these facilities call the @code{load} function to do the work. |
55 | ||
56 | @defun load filename &optional missing-ok nomessage nosuffix | |
57 | This function finds and opens a file of Lisp code, evaluates all the | |
58 | forms in it, and closes the file. | |
59 | ||
60 | To find the file, @code{load} first looks for a file named | |
61 | @file{@var{filename}.elc}, that is, for a file whose name is | |
62 | @var{filename} with @samp{.elc} appended. If such a file exists, it is | |
63 | loaded. If there is no file by that name, then @code{load} looks for a | |
78c71a98 | 64 | file named @file{@var{filename}.el}. If that file exists, it is loaded. |
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65 | Finally, if neither of those names is found, @code{load} looks for a |
66 | file named @var{filename} with nothing appended, and loads it if it | |
67 | exists. (The @code{load} function is not clever about looking at | |
68 | @var{filename}. In the perverse case of a file named @file{foo.el.el}, | |
69 | evaluation of @code{(load "foo.el")} will indeed find it.) | |
70 | ||
71 | If the optional argument @var{nosuffix} is non-@code{nil}, then the | |
72 | suffixes @samp{.elc} and @samp{.el} are not tried. In this case, you | |
73 | must specify the precise file name you want. | |
74 | ||
75 | If @var{filename} is a relative file name, such as @file{foo} or | |
76 | @file{baz/foo.bar}, @code{load} searches for the file using the variable | |
77 | @code{load-path}. It appends @var{filename} to each of the directories | |
78 | listed in @code{load-path}, and loads the first file it finds whose name | |
79 | matches. The current default directory is tried only if it is specified | |
80 | in @code{load-path}, where @code{nil} stands for the default directory. | |
81 | @code{load} tries all three possible suffixes in the first directory in | |
82 | @code{load-path}, then all three suffixes in the second directory, and | |
83 | so on. | |
84 | ||
85 | If you get a warning that @file{foo.elc} is older than @file{foo.el}, it | |
86 | means you should consider recompiling @file{foo.el}. @xref{Byte | |
87 | Compilation}. | |
88 | ||
89 | Messages like @samp{Loading foo...} and @samp{Loading foo...done} appear | |
90 | in the echo area during loading unless @var{nomessage} is | |
91 | non-@code{nil}. | |
92 | ||
93 | @cindex load errors | |
94 | Any unhandled errors while loading a file terminate loading. If the | |
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95 | load was done for the sake of @code{autoload}, any function definitions |
96 | made during the loading are undone. | |
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97 | |
98 | @kindex file-error | |
99 | If @code{load} can't find the file to load, then normally it signals the | |
100 | error @code{file-error} (with @samp{Cannot open load file | |
101 | @var{filename}}). But if @var{missing-ok} is non-@code{nil}, then | |
102 | @code{load} just returns @code{nil}. | |
103 | ||
22697dac KH |
104 | You can use the variable @code{load-read-function} to specify a function |
105 | for @code{load} to use instead of @code{read} for reading expressions. | |
106 | See below. | |
107 | ||
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108 | @code{load} returns @code{t} if the file loads successfully. |
109 | @end defun | |
110 | ||
111 | @ignore | |
112 | @deffn Command load-file filename | |
113 | This function loads the file @var{filename}. If @var{filename} is an | |
114 | absolute file name, then it is loaded. If it is relative, then the | |
115 | current default directory is assumed. @code{load-path} is not used, and | |
116 | suffixes are not appended. Use this function if you wish to specify | |
117 | the file to be loaded exactly. | |
118 | @end deffn | |
119 | ||
120 | @deffn Command load-library library | |
121 | This function loads the library named @var{library}. A library is | |
122 | nothing more than a file that may be loaded as described earlier. This | |
123 | function is identical to @code{load}, save that it reads a file name | |
124 | interactively with completion. | |
125 | @end deffn | |
126 | @end ignore | |
127 | ||
128 | @defopt load-path | |
129 | @cindex @code{EMACSLOADPATH} environment variable | |
130 | The value of this variable is a list of directories to search when | |
131 | loading files with @code{load}. Each element is a string (which must be | |
132 | a directory name) or @code{nil} (which stands for the current working | |
133 | directory). The value of @code{load-path} is initialized from the | |
134 | environment variable @code{EMACSLOADPATH}, if that exists; otherwise its | |
135 | default value is specified in @file{emacs/src/paths.h} when Emacs is | |
136 | built. | |
137 | ||
138 | The syntax of @code{EMACSLOADPATH} is the same as used for @code{PATH}; | |
bfe721d1 KH |
139 | @samp{:} (or @samp{;}, according to the operating system) separates |
140 | directory names, and @samp{.} is used for the current default directory. | |
141 | Here is an example of how to set your @code{EMACSLOADPATH} variable from | |
142 | a @code{csh} @file{.login} file: | |
83ac6b45 RS |
143 | |
144 | @c This overfull hbox is OK. --rjc 16mar92 | |
145 | @smallexample | |
146 | setenv EMACSLOADPATH .:/user/bil/emacs:/usr/lib/emacs/lisp | |
147 | @end smallexample | |
148 | ||
149 | Here is how to set it using @code{sh}: | |
150 | ||
151 | @smallexample | |
152 | export EMACSLOADPATH | |
153 | EMACSLOADPATH=.:/user/bil/emacs:/usr/local/lib/emacs/lisp | |
154 | @end smallexample | |
155 | ||
156 | Here is an example of code you can place in a @file{.emacs} file to add | |
157 | several directories to the front of your default @code{load-path}: | |
158 | ||
159 | @smallexample | |
160 | (setq load-path | |
161 | (append (list nil "/user/bil/emacs" | |
162 | "/usr/local/lisplib" | |
163 | (expand-file-name "~/emacs")) | |
164 | load-path)) | |
165 | @end smallexample | |
166 | ||
167 | @c Wordy to rid us of an overfull hbox. --rjc 15mar92 | |
168 | @noindent | |
169 | In this example, the path searches the current working directory first, | |
170 | followed then by the @file{/user/bil/emacs} directory and then by | |
171 | the @file{/usr/local/lisplib} directory, | |
172 | which are then followed by the standard directories for Lisp code. | |
173 | ||
78c71a98 RS |
174 | The command line options @samp{-l} or @samp{-load} specify a Lisp |
175 | library to load as part of Emacs startup. Since this file might be in | |
176 | the current directory, Emacs 18 temporarily adds the current directory | |
177 | to the front of @code{load-path} so the file can be found there. Newer | |
178 | Emacs versions also find such files in the current directory, but | |
179 | without altering @code{load-path}. | |
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180 | |
181 | Dumping Emacs uses a special value of @code{load-path}. If the value of | |
182 | @code{load-path} at the end of dumping is unchanged (that is, still the | |
183 | same special value), the dumped Emacs switches to the ordinary | |
184 | @code{load-path} value when it starts up, as decribed above. But if | |
185 | @code{load-path} has any other value at the end of dumping, that value | |
186 | is used for execution of the dumped Emacs also. | |
187 | ||
188 | Therefore, if you want to change @code{load-path} temporarily for | |
189 | loading a few libraries in @file{site-init.el} or @file{site-load.el}, | |
190 | you should bind @code{load-path} locally with @code{let} around the | |
191 | calls to @code{load}. | |
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192 | @end defopt |
193 | ||
194 | @defvar load-in-progress | |
195 | This variable is non-@code{nil} if Emacs is in the process of loading a | |
bfe721d1 | 196 | file, and it is @code{nil} otherwise. |
22697dac KH |
197 | @end defvar |
198 | ||
199 | @defvar load-read-function | |
200 | This variable specifies an alternate expression-reading function for | |
201 | @code{load} and @code{eval-region} to use instead of @code{read}. | |
202 | The function should accept one argument, just as @code{read} does. | |
203 | ||
204 | Normally, the variable's value is @code{nil}, which means those | |
205 | functions should use @code{read}. | |
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206 | @end defvar |
207 | ||
208 | To learn how @code{load} is used to build Emacs, see @ref{Building Emacs}. | |
209 | ||
210 | @node Autoload | |
211 | @section Autoload | |
212 | @cindex autoload | |
213 | ||
214 | The @dfn{autoload} facility allows you to make a function or macro | |
bfe721d1 KH |
215 | known in Lisp, but put off loading the file that defines it. The first |
216 | call to the function automatically reads the proper file to install the | |
217 | real definition and other associated code, then runs the real definition | |
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218 | as if it had been loaded all along. |
219 | ||
220 | There are two ways to set up an autoloaded function: by calling | |
221 | @code{autoload}, and by writing a special ``magic'' comment in the | |
222 | source before the real definition. @code{autoload} is the low-level | |
223 | primitive for autoloading; any Lisp program can call @code{autoload} at | |
224 | any time. Magic comments do nothing on their own; they serve as a guide | |
225 | for the command @code{update-file-autoloads}, which constructs calls to | |
226 | @code{autoload} and arranges to execute them when Emacs is built. Magic | |
227 | comments are the most convenient way to make a function autoload, but | |
228 | only for packages installed along with Emacs. | |
229 | ||
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230 | @defun autoload function filename &optional docstring interactive type |
231 | This function defines the function (or macro) named @var{function} so as | |
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232 | to load automatically from @var{filename}. The string @var{filename} |
233 | specifies the file to load to get the real definition of @var{function}. | |
234 | ||
235 | The argument @var{docstring} is the documentation string for the | |
236 | function. Normally, this is the identical to the documentation string | |
237 | in the function definition itself. Specifying the documentation string | |
238 | in the call to @code{autoload} makes it possible to look at the | |
239 | documentation without loading the function's real definition. | |
240 | ||
241 | If @var{interactive} is non-@code{nil}, then the function can be called | |
242 | interactively. This lets completion in @kbd{M-x} work without loading | |
243 | the function's real definition. The complete interactive specification | |
244 | need not be given here; it's not needed unless the user actually calls | |
245 | @var{function}, and when that happens, it's time to load the real | |
246 | definition. | |
247 | ||
248 | You can autoload macros and keymaps as well as ordinary functions. | |
249 | Specify @var{type} as @code{macro} if @var{function} is really a macro. | |
250 | Specify @var{type} as @code{keymap} if @var{function} is really a | |
251 | keymap. Various parts of Emacs need to know this information without | |
252 | loading the real definition. | |
253 | ||
254 | @cindex function cell in autoload | |
78c71a98 | 255 | If @var{function} already has a non-void function definition that is not |
83ac6b45 | 256 | an autoload object, @code{autoload} does nothing and returns @code{nil}. |
78c71a98 | 257 | If the function cell of @var{function} is void, or is already an autoload |
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258 | object, then it is defined as an autoload object like this: |
259 | ||
260 | @example | |
261 | (autoload @var{filename} @var{docstring} @var{interactive} @var{type}) | |
262 | @end example | |
263 | ||
264 | For example, | |
265 | ||
266 | @example | |
267 | (symbol-function 'run-prolog) | |
268 | @result{} (autoload "prolog" 169681 t nil) | |
269 | @end example | |
270 | ||
271 | @noindent | |
272 | In this case, @code{"prolog"} is the name of the file to load, 169681 | |
273 | refers to the documentation string in the @file{emacs/etc/DOC} file | |
274 | (@pxref{Documentation Basics}), @code{t} means the function is | |
275 | interactive, and @code{nil} that it is not a macro or a keymap. | |
276 | @end defun | |
277 | ||
278 | @cindex autoload errors | |
279 | The autoloaded file usually contains other definitions and may require | |
280 | or provide one or more features. If the file is not completely loaded | |
281 | (due to an error in the evaluation of its contents), any function | |
282 | definitions or @code{provide} calls that occurred during the load are | |
283 | undone. This is to ensure that the next attempt to call any function | |
284 | autoloading from this file will try again to load the file. If not for | |
285 | this, then some of the functions in the file might appear defined, but | |
286 | they might fail to work properly for the lack of certain subroutines | |
287 | defined later in the file and not loaded successfully. | |
288 | ||
289 | If the autoloaded file fails to define the desired Lisp function or | |
290 | macro, then an error is signaled with data @code{"Autoloading failed to | |
291 | define function @var{function-name}"}. | |
292 | ||
293 | @findex update-file-autoloads | |
294 | @findex update-directory-autoloads | |
295 | A magic autoload comment looks like @samp{;;;###autoload}, on a line | |
296 | by itself, just before the real definition of the function in its | |
297 | autoloadable source file. The command @kbd{M-x update-file-autoloads} | |
298 | writes a corresponding @code{autoload} call into @file{loaddefs.el}. | |
299 | Building Emacs loads @file{loaddefs.el} and thus calls @code{autoload}. | |
300 | @kbd{M-x update-directory-autoloads} is even more powerful; it updates | |
301 | autoloads for all files in the current directory. | |
302 | ||
303 | The same magic comment can copy any kind of form into | |
304 | @file{loaddefs.el}. If the form following the magic comment is not a | |
305 | function definition, it is copied verbatim. You can also use a magic | |
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306 | comment to execute a form at build time @emph{without} executing it when |
307 | the file itself is loaded. To do this, write the form @dfn{on the same | |
308 | line} as the magic comment. Since it is in a comment, it does nothing | |
309 | when you load the source file; but @code{update-file-autoloads} copies | |
310 | it to @file{loaddefs.el}, where it is executed while building Emacs. | |
83ac6b45 RS |
311 | |
312 | The following example shows how @code{doctor} is prepared for | |
313 | autoloading with a magic comment: | |
314 | ||
315 | @smallexample | |
316 | ;;;###autoload | |
317 | (defun doctor () | |
318 | "Switch to *doctor* buffer and start giving psychotherapy." | |
319 | (interactive) | |
320 | (switch-to-buffer "*doctor*") | |
321 | (doctor-mode)) | |
322 | @end smallexample | |
323 | ||
324 | @noindent | |
325 | Here's what that produces in @file{loaddefs.el}: | |
326 | ||
327 | @smallexample | |
328 | (autoload 'doctor "doctor" | |
329 | "\ | |
330 | Switch to *doctor* buffer and start giving psychotherapy." | |
331 | t) | |
332 | @end smallexample | |
333 | ||
334 | @noindent | |
335 | The backslash and newline immediately following the double-quote are a | |
336 | convention used only in the preloaded Lisp files such as | |
337 | @file{loaddefs.el}; they tell @code{make-docfile} to put the | |
338 | documentation string in the @file{etc/DOC} file. @xref{Building Emacs}. | |
339 | ||
340 | @node Repeated Loading | |
83ac6b45 RS |
341 | @section Repeated Loading |
342 | @cindex repeated loading | |
343 | ||
344 | You may load one file more than once in an Emacs session. For | |
345 | example, after you have rewritten and reinstalled a function definition | |
346 | by editing it in a buffer, you may wish to return to the original | |
347 | version; you can do this by reloading the file it came from. | |
348 | ||
349 | When you load or reload files, bear in mind that the @code{load} and | |
350 | @code{load-library} functions automatically load a byte-compiled file | |
351 | rather than a non-compiled file of similar name. If you rewrite a file | |
352 | that you intend to save and reinstall, remember to byte-compile it if | |
353 | necessary; otherwise you may find yourself inadvertently reloading the | |
354 | older, byte-compiled file instead of your newer, non-compiled file! | |
355 | ||
356 | When writing the forms in a Lisp library file, keep in mind that the | |
357 | file might be loaded more than once. For example, the choice of | |
358 | @code{defvar} vs.@: @code{defconst} for defining a variable depends on | |
359 | whether it is desirable to reinitialize the variable if the library is | |
360 | reloaded: @code{defconst} does so, and @code{defvar} does not. | |
361 | (@xref{Defining Variables}.) | |
362 | ||
363 | The simplest way to add an element to an alist is like this: | |
364 | ||
365 | @example | |
366 | (setq minor-mode-alist | |
367 | (cons '(leif-mode " Leif") minor-mode-alist)) | |
368 | @end example | |
369 | ||
370 | @noindent | |
371 | But this would add multiple elements if the library is reloaded. | |
372 | To avoid the problem, write this: | |
373 | ||
374 | @example | |
375 | (or (assq 'leif-mode minor-mode-alist) | |
376 | (setq minor-mode-alist | |
377 | (cons '(leif-mode " Leif") minor-mode-alist))) | |
378 | @end example | |
379 | ||
bfe721d1 KH |
380 | To add an element to a list just once, use @code{add-to-list} |
381 | (@pxref{Setting Variables}). | |
382 | ||
83ac6b45 RS |
383 | Occasionally you will want to test explicitly whether a library has |
384 | already been loaded. Here's one way to test, in a library, whether it | |
385 | has been loaded before: | |
386 | ||
387 | @example | |
bfe721d1 KH |
388 | (defvar foo-was-loaded) |
389 | ||
83ac6b45 RS |
390 | (if (not (boundp 'foo-was-loaded)) |
391 | @var{execute-first-time-only}) | |
392 | ||
393 | (setq foo-was-loaded t) | |
394 | @end example | |
395 | ||
396 | @noindent | |
397 | If the library uses @code{provide} to provide a named feature, you can | |
398 | use @code{featurep} to test whether the library has been loaded. | |
78c71a98 | 399 | @ifinfo |
bfe721d1 | 400 | @xref{Named Features}. |
78c71a98 | 401 | @end ifinfo |
83ac6b45 | 402 | |
bfe721d1 | 403 | @node Named Features |
83ac6b45 RS |
404 | @section Features |
405 | @cindex features | |
406 | @cindex requiring features | |
407 | @cindex providing features | |
408 | ||
409 | @code{provide} and @code{require} are an alternative to | |
410 | @code{autoload} for loading files automatically. They work in terms of | |
411 | named @dfn{features}. Autoloading is triggered by calling a specific | |
412 | function, but a feature is loaded the first time another program asks | |
413 | for it by name. | |
414 | ||
415 | A feature name is a symbol that stands for a collection of functions, | |
416 | variables, etc. The file that defines them should @dfn{provide} the | |
417 | feature. Another program that uses them may ensure they are defined by | |
418 | @dfn{requiring} the feature. This loads the file of definitions if it | |
419 | hasn't been loaded already. | |
420 | ||
421 | To require the presence of a feature, call @code{require} with the | |
422 | feature name as argument. @code{require} looks in the global variable | |
423 | @code{features} to see whether the desired feature has been provided | |
424 | already. If not, it loads the feature from the appropriate file. This | |
78c71a98 | 425 | file should call @code{provide} at the top level to add the feature to |
83ac6b45 RS |
426 | @code{features}; if it fails to do so, @code{require} signals an error. |
427 | @cindex load error with require | |
428 | ||
429 | Features are normally named after the files that provide them, so that | |
430 | @code{require} need not be given the file name. | |
431 | ||
432 | For example, in @file{emacs/lisp/prolog.el}, | |
433 | the definition for @code{run-prolog} includes the following code: | |
434 | ||
435 | @smallexample | |
436 | (defun run-prolog () | |
437 | "Run an inferior Prolog process, input and output via buffer *prolog*." | |
438 | (interactive) | |
439 | (require 'comint) | |
440 | (switch-to-buffer (make-comint "prolog" prolog-program-name)) | |
441 | (inferior-prolog-mode)) | |
442 | @end smallexample | |
443 | ||
444 | @noindent | |
445 | The expression @code{(require 'comint)} loads the file @file{comint.el} | |
446 | if it has not yet been loaded. This ensures that @code{make-comint} is | |
447 | defined. | |
448 | ||
449 | The @file{comint.el} file contains the following top-level expression: | |
450 | ||
451 | @smallexample | |
452 | (provide 'comint) | |
453 | @end smallexample | |
454 | ||
455 | @noindent | |
456 | This adds @code{comint} to the global @code{features} list, so that | |
457 | @code{(require 'comint)} will henceforth know that nothing needs to be | |
458 | done. | |
459 | ||
460 | @cindex byte-compiling @code{require} | |
78c71a98 | 461 | When @code{require} is used at top level in a file, it takes effect |
83ac6b45 RS |
462 | when you byte-compile that file (@pxref{Byte Compilation}) as well as |
463 | when you load it. This is in case the required package contains macros | |
464 | that the byte compiler must know about. | |
465 | ||
466 | Although top-level calls to @code{require} are evaluated during | |
467 | byte compilation, @code{provide} calls are not. Therefore, you can | |
468 | ensure that a file of definitions is loaded before it is byte-compiled | |
469 | by including a @code{provide} followed by a @code{require} for the same | |
470 | feature, as in the following example. | |
471 | ||
472 | @smallexample | |
473 | @group | |
474 | (provide 'my-feature) ; @r{Ignored by byte compiler,} | |
475 | ; @r{evaluated by @code{load}.} | |
476 | (require 'my-feature) ; @r{Evaluated by byte compiler.} | |
477 | @end group | |
478 | @end smallexample | |
479 | ||
78c71a98 RS |
480 | @noindent |
481 | The compiler ignores the @code{provide}, then processes the | |
482 | @code{require} by loading the file in question. Loading the file does | |
483 | execute the @code{provide} call, so the subsequent @code{require} call | |
484 | does nothing while loading. | |
485 | ||
83ac6b45 RS |
486 | @defun provide feature |
487 | This function announces that @var{feature} is now loaded, or being | |
488 | loaded, into the current Emacs session. This means that the facilities | |
489 | associated with @var{feature} are or will be available for other Lisp | |
490 | programs. | |
491 | ||
492 | The direct effect of calling @code{provide} is to add @var{feature} to | |
493 | the front of the list @code{features} if it is not already in the list. | |
494 | The argument @var{feature} must be a symbol. @code{provide} returns | |
495 | @var{feature}. | |
496 | ||
497 | @smallexample | |
498 | features | |
499 | @result{} (bar bish) | |
500 | ||
501 | (provide 'foo) | |
502 | @result{} foo | |
503 | features | |
504 | @result{} (foo bar bish) | |
505 | @end smallexample | |
506 | ||
bfe721d1 KH |
507 | When a file is loaded to satisfy an autoload, and it stops due to an |
508 | error in the evaluating its contents, any function definitions or | |
509 | @code{provide} calls that occurred during the load are undone. | |
510 | @xref{Autoload}. | |
83ac6b45 RS |
511 | @end defun |
512 | ||
513 | @defun require feature &optional filename | |
514 | This function checks whether @var{feature} is present in the current | |
515 | Emacs session (using @code{(featurep @var{feature})}; see below). If it | |
516 | is not, then @code{require} loads @var{filename} with @code{load}. If | |
517 | @var{filename} is not supplied, then the name of the symbol | |
518 | @var{feature} is used as the file name to load. | |
519 | ||
520 | If loading the file fails to provide @var{feature}, @code{require} | |
521 | signals an error, @samp{Required feature @var{feature} was not | |
522 | provided}. | |
523 | @end defun | |
524 | ||
525 | @defun featurep feature | |
526 | This function returns @code{t} if @var{feature} has been provided in the | |
527 | current Emacs session (i.e., @var{feature} is a member of | |
528 | @code{features}.) | |
529 | @end defun | |
530 | ||
531 | @defvar features | |
532 | The value of this variable is a list of symbols that are the features | |
533 | loaded in the current Emacs session. Each symbol was put in this list | |
534 | with a call to @code{provide}. The order of the elements in the | |
535 | @code{features} list is not significant. | |
536 | @end defvar | |
537 | ||
538 | @node Unloading | |
539 | @section Unloading | |
540 | @cindex unloading | |
541 | ||
542 | @c Emacs 19 feature | |
543 | You can discard the functions and variables loaded by a library to | |
544 | reclaim memory for other Lisp objects. To do this, use the function | |
545 | @code{unload-feature}: | |
546 | ||
ee6bcc94 | 547 | @deffn Command unload-feature feature &optional force |
83ac6b45 | 548 | This command unloads the library that provided feature @var{feature}. |
78c71a98 RS |
549 | It undefines all functions, macros, and variables defined in that |
550 | library with @code{defconst}, @code{defvar}, @code{defun}, | |
551 | @code{defmacro}, @code{defsubst} and @code{defalias}. It then restores | |
bfe721d1 KH |
552 | any autoloads formerly associated with those symbols. (Loading |
553 | saves these in the @code{autoload} property of the symbol.) | |
ee6bcc94 RS |
554 | |
555 | Ordinarily, @code{unload-feature} refuses to unload a library on which | |
556 | other loaded libraries depend. (A library @var{a} depends on library | |
557 | @var{b} if @var{a} contains a @code{require} for @var{b}.) If the | |
558 | optional argument @var{force} is non-@code{nil}, dependencies are | |
559 | ignored and you can unload any library. | |
83ac6b45 RS |
560 | @end deffn |
561 | ||
562 | The @code{unload-feature} function is written in Lisp; its actions are | |
563 | based on the variable @code{load-history}. | |
564 | ||
565 | @defvar load-history | |
566 | This variable's value is an alist connecting library names with the | |
567 | names of functions and variables they define, the features they provide, | |
568 | and the features they require. | |
569 | ||
570 | Each element is a list and describes one library. The @sc{car} of the | |
571 | list is the name of the library, as a string. The rest of the list is | |
572 | composed of these kinds of objects: | |
573 | ||
574 | @itemize @bullet | |
575 | @item | |
78c71a98 | 576 | Symbols that were defined by this library. |
83ac6b45 RS |
577 | @item |
578 | Lists of the form @code{(require . @var{feature})} indicating | |
579 | features that were required. | |
580 | @item | |
581 | Lists of the form @code{(provide . @var{feature})} indicating | |
582 | features that were provided. | |
583 | @end itemize | |
584 | ||
585 | The value of @code{load-history} may have one element whose @sc{car} is | |
586 | @code{nil}. This element describes definitions made with | |
587 | @code{eval-buffer} on a buffer that is not visiting a file. | |
588 | @end defvar | |
589 | ||
590 | The command @code{eval-region} updates @code{load-history}, but does so | |
591 | by adding the symbols defined to the element for the file being visited, | |
592 | rather than replacing that element. | |
593 | ||
594 | @node Hooks for Loading | |
595 | @section Hooks for Loading | |
596 | @cindex loading hooks | |
597 | @cindex hooks for loading | |
598 | ||
599 | You can ask for code to be executed if and when a particular library is | |
600 | loaded, by calling @code{eval-after-load}. | |
601 | ||
602 | @defun eval-after-load library form | |
603 | This function arranges to evaluate @var{form} at the end of loading the | |
d2e9ee06 RS |
604 | library @var{library}, if and when @var{library} is loaded. If |
605 | @var{library} is already loaded, it evaluates @var{form} right away. | |
83ac6b45 RS |
606 | |
607 | The library name @var{library} must exactly match the argument of | |
608 | @code{load}. To get the proper results when an installed library is | |
609 | found by searching @code{load-path}, you should not include any | |
610 | directory names in @var{library}. | |
611 | ||
612 | An error in @var{form} does not undo the load, but does prevent | |
613 | execution of the rest of @var{form}. | |
614 | @end defun | |
615 | ||
d2e9ee06 RS |
616 | In general, well-designed Lisp programs should not use this feature. |
617 | The clean and modular ways to interact with a Lisp library are (1) | |
618 | examine and set the library's variables (those which are meant for | |
619 | outside use), and and (2) call the library's functions. If you wish to | |
620 | do (1), you can do it immediately---there is no need to wait for when | |
621 | the library is loaded. To do (2), you must load the library (preferably | |
622 | with @code{require}). | |
623 | ||
bfe721d1 | 624 | But it is ok to use @code{eval-after-load} in your personal customizations |
d2e9ee06 RS |
625 | if you don't feel they must meet the design standards of programs to be |
626 | released. | |
627 | ||
83ac6b45 RS |
628 | @defvar after-load-alist |
629 | An alist of expressions to evaluate if and when particular libraries are | |
630 | loaded. Each element looks like this: | |
631 | ||
632 | @example | |
633 | (@var{filename} @var{forms}@dots{}) | |
634 | @end example | |
635 | ||
636 | The function @code{load} checks @code{after-load-alist} in order to | |
637 | implement @code{eval-after-load}. | |
638 | @end defvar | |
639 | ||
640 | @c Emacs 19 feature |