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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
f9f59935 | 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 Free Software Foundation, Inc. |
83ac6b45 RS |
4 | @c See the file elisp.texi for copying conditions. |
5 | @setfilename ../info/loading | |
f9f59935 | 6 | @node Loading, Byte Compilation, Customization, Top |
83ac6b45 RS |
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 | |
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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. | |
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38 | * Library Search:: Finding a library to load. |
39 | * Loading Non-ASCII:: Non-@sc{ASCII} characters in Emacs Lisp files. | |
83ac6b45 RS |
40 | * Autoload:: Setting up a function to autoload. |
41 | * Repeated Loading:: Precautions about loading a file twice. | |
bfe721d1 | 42 | * Named Features:: Loading a library if it isn't already loaded. |
83ac6b45 RS |
43 | * Unloading:: How to ``unload'' a library that was loaded. |
44 | * Hooks for Loading:: Providing code to be run when | |
45 | particular libraries are loaded. | |
46 | @end menu | |
47 | ||
48 | @node How Programs Do Loading | |
49 | @section How Programs Do Loading | |
50 | ||
51 | Emacs Lisp has several interfaces for loading. For example, | |
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52 | @code{autoload} creates a placeholder object for a function defined in a |
53 | file; trying to call the autoloading function loads the file to get the | |
83ac6b45 | 54 | function's real definition (@pxref{Autoload}). @code{require} loads a |
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55 | file if it isn't already loaded (@pxref{Named Features}). Ultimately, |
56 | all these facilities call the @code{load} function to do the work. | |
83ac6b45 | 57 | |
a9f0a989 | 58 | @defun load filename &optional missing-ok nomessage nosuffix must-suffix |
83ac6b45 RS |
59 | This function finds and opens a file of Lisp code, evaluates all the |
60 | forms in it, and closes the file. | |
61 | ||
62 | To find the file, @code{load} first looks for a file named | |
63 | @file{@var{filename}.elc}, that is, for a file whose name is | |
64 | @var{filename} with @samp{.elc} appended. If such a file exists, it is | |
65 | loaded. If there is no file by that name, then @code{load} looks for a | |
78c71a98 | 66 | file named @file{@var{filename}.el}. If that file exists, it is loaded. |
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67 | Finally, if neither of those names is found, @code{load} looks for a |
68 | file named @var{filename} with nothing appended, and loads it if it | |
69 | exists. (The @code{load} function is not clever about looking at | |
70 | @var{filename}. In the perverse case of a file named @file{foo.el.el}, | |
71 | evaluation of @code{(load "foo.el")} will indeed find it.) | |
72 | ||
73 | If the optional argument @var{nosuffix} is non-@code{nil}, then the | |
74 | suffixes @samp{.elc} and @samp{.el} are not tried. In this case, you | |
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75 | must specify the precise file name you want. By specifying the precise |
76 | file name and using @code{t} for @var{nosuffix}, you can prevent | |
77 | perverse file names such as @file{foo.el.el} from being tried. | |
83ac6b45 | 78 | |
a9f0a989 RS |
79 | If the optional argument @var{must-suffix} is non-@code{nil}, then |
80 | @code{load} insists that the file name used must end in either | |
81 | @samp{.el} or @samp{.elc}, unless it contains an explicit directory | |
82 | name. If @var{filename} does not contain an explicit directory name, | |
83 | and does not end in a suffix, then @code{load} insists on adding one. | |
84 | ||
83ac6b45 RS |
85 | If @var{filename} is a relative file name, such as @file{foo} or |
86 | @file{baz/foo.bar}, @code{load} searches for the file using the variable | |
87 | @code{load-path}. It appends @var{filename} to each of the directories | |
88 | listed in @code{load-path}, and loads the first file it finds whose name | |
89 | matches. The current default directory is tried only if it is specified | |
90 | in @code{load-path}, where @code{nil} stands for the default directory. | |
91 | @code{load} tries all three possible suffixes in the first directory in | |
92 | @code{load-path}, then all three suffixes in the second directory, and | |
a9f0a989 | 93 | so on. @xref{Library Search}. |
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94 | |
95 | If you get a warning that @file{foo.elc} is older than @file{foo.el}, it | |
96 | means you should consider recompiling @file{foo.el}. @xref{Byte | |
97 | Compilation}. | |
98 | ||
969fe9b5 RS |
99 | When loading a source file (not compiled), @code{load} performs |
100 | character set translation just as Emacs would do when visiting the file. | |
101 | @xref{Coding Systems}. | |
102 | ||
83ac6b45 RS |
103 | Messages like @samp{Loading foo...} and @samp{Loading foo...done} appear |
104 | in the echo area during loading unless @var{nomessage} is | |
105 | non-@code{nil}. | |
106 | ||
107 | @cindex load errors | |
108 | Any unhandled errors while loading a file terminate loading. If the | |
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109 | load was done for the sake of @code{autoload}, any function definitions |
110 | made during the loading are undone. | |
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111 | |
112 | @kindex file-error | |
113 | If @code{load} can't find the file to load, then normally it signals the | |
114 | error @code{file-error} (with @samp{Cannot open load file | |
115 | @var{filename}}). But if @var{missing-ok} is non-@code{nil}, then | |
116 | @code{load} just returns @code{nil}. | |
117 | ||
22697dac KH |
118 | You can use the variable @code{load-read-function} to specify a function |
119 | for @code{load} to use instead of @code{read} for reading expressions. | |
120 | See below. | |
121 | ||
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122 | @code{load} returns @code{t} if the file loads successfully. |
123 | @end defun | |
124 | ||
83ac6b45 | 125 | @deffn Command load-file filename |
f9f59935 RS |
126 | This command loads the file @var{filename}. If @var{filename} is a |
127 | relative file name, then the current default directory is assumed. | |
128 | @code{load-path} is not used, and suffixes are not appended. Use this | |
a9f0a989 | 129 | command if you wish to specify precisely the file name to load. |
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130 | @end deffn |
131 | ||
132 | @deffn Command load-library library | |
f9f59935 RS |
133 | This command loads the library named @var{library}. It is equivalent to |
134 | @code{load}, except in how it reads its argument interactively. | |
83ac6b45 | 135 | @end deffn |
83ac6b45 | 136 | |
a9f0a989 RS |
137 | @defvar load-in-progress |
138 | This variable is non-@code{nil} if Emacs is in the process of loading a | |
139 | file, and it is @code{nil} otherwise. | |
140 | @end defvar | |
141 | ||
142 | @defvar load-read-function | |
143 | This variable specifies an alternate expression-reading function for | |
144 | @code{load} and @code{eval-region} to use instead of @code{read}. | |
145 | The function should accept one argument, just as @code{read} does. | |
146 | ||
147 | Normally, the variable's value is @code{nil}, which means those | |
148 | functions should use @code{read}. | |
55607887 RS |
149 | |
150 | @strong{Note:} Instead of using this variable, it is cleaner to use | |
151 | another, newer feature: to pass the function as the @var{read-function} | |
152 | argument to @code{eval-region}. @xref{Eval}. | |
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153 | @end defvar |
154 | ||
1911e6e5 RS |
155 | For information about how @code{load} is used in building Emacs, see |
156 | @ref{Building Emacs}. | |
a9f0a989 RS |
157 | |
158 | @node Library Search | |
159 | @section Library Search | |
160 | ||
161 | When Emacs loads a Lisp library, it searches for the library | |
162 | in a list of directories specified by the variable @code{load-path}. | |
163 | ||
83ac6b45 RS |
164 | @defopt load-path |
165 | @cindex @code{EMACSLOADPATH} environment variable | |
166 | The value of this variable is a list of directories to search when | |
167 | loading files with @code{load}. Each element is a string (which must be | |
168 | a directory name) or @code{nil} (which stands for the current working | |
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169 | directory). |
170 | @end defopt | |
171 | ||
172 | The value of @code{load-path} is initialized from the environment | |
173 | variable @code{EMACSLOADPATH}, if that exists; otherwise its default | |
174 | value is specified in @file{emacs/src/paths.h} when Emacs is built. | |
175 | Then the list is expanded by adding subdirectories of the directories | |
176 | in the list. | |
83ac6b45 | 177 | |
a9f0a989 | 178 | The syntax of @code{EMACSLOADPATH} is the same as used for @code{PATH}; |
bfe721d1 KH |
179 | @samp{:} (or @samp{;}, according to the operating system) separates |
180 | directory names, and @samp{.} is used for the current default directory. | |
181 | Here is an example of how to set your @code{EMACSLOADPATH} variable from | |
182 | a @code{csh} @file{.login} file: | |
83ac6b45 | 183 | |
83ac6b45 | 184 | @smallexample |
f1e2c45e | 185 | setenv EMACSLOADPATH .:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp |
83ac6b45 RS |
186 | @end smallexample |
187 | ||
a9f0a989 | 188 | Here is how to set it using @code{sh}: |
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189 | |
190 | @smallexample | |
191 | export EMACSLOADPATH | |
f1e2c45e | 192 | EMACSLOADPATH=.:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp |
83ac6b45 RS |
193 | @end smallexample |
194 | ||
a9f0a989 | 195 | Here is an example of code you can place in a @file{.emacs} file to add |
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196 | several directories to the front of your default @code{load-path}: |
197 | ||
198 | @smallexample | |
bda144f4 | 199 | @group |
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200 | (setq load-path |
201 | (append (list nil "/user/bil/emacs" | |
202 | "/usr/local/lisplib" | |
5e41cf03 | 203 | "~/emacs") |
83ac6b45 | 204 | load-path)) |
bda144f4 | 205 | @end group |
83ac6b45 RS |
206 | @end smallexample |
207 | ||
208 | @c Wordy to rid us of an overfull hbox. --rjc 15mar92 | |
209 | @noindent | |
210 | In this example, the path searches the current working directory first, | |
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211 | followed then by the @file{/user/bil/emacs} directory, the |
212 | @file{/usr/local/lisplib} directory, and the @file{~/emacs} directory, | |
83ac6b45 RS |
213 | which are then followed by the standard directories for Lisp code. |
214 | ||
a9f0a989 | 215 | Dumping Emacs uses a special value of @code{load-path}. If the value of |
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216 | @code{load-path} at the end of dumping is unchanged (that is, still the |
217 | same special value), the dumped Emacs switches to the ordinary | |
cc8c51f1 | 218 | @code{load-path} value when it starts up, as described above. But if |
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219 | @code{load-path} has any other value at the end of dumping, that value |
220 | is used for execution of the dumped Emacs also. | |
221 | ||
a9f0a989 | 222 | Therefore, if you want to change @code{load-path} temporarily for |
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223 | loading a few libraries in @file{site-init.el} or @file{site-load.el}, |
224 | you should bind @code{load-path} locally with @code{let} around the | |
225 | calls to @code{load}. | |
83ac6b45 | 226 | |
089e089d | 227 | The default value of @code{load-path}, when running an Emacs which has |
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228 | been installed on the system, includes two special directories (and |
229 | their subdirectories as well): | |
089e089d RS |
230 | |
231 | @smallexample | |
a9f0a989 | 232 | "/usr/local/share/emacs/@var{version}/site-lisp" |
089e089d RS |
233 | @end smallexample |
234 | ||
a9f0a989 RS |
235 | @noindent |
236 | and | |
237 | ||
238 | @smallexample | |
239 | "/usr/local/share/emacs/site-lisp" | |
240 | @end smallexample | |
241 | ||
242 | @noindent | |
243 | The first one is for locally installed packages for a particular Emacs | |
244 | version; the second is for locally installed packages meant for use with | |
245 | all installed Emacs versions. | |
089e089d RS |
246 | |
247 | There are several reasons why a Lisp package that works well in one | |
248 | Emacs version can cause trouble in another. Sometimes packages need | |
249 | updating for incompatible changes in Emacs; sometimes they depend on | |
250 | undocumented internal Emacs data that can change without notice; | |
251 | sometimes a newer Emacs version incorporates a version of the package, | |
252 | and should be used only with that version. | |
253 | ||
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254 | Emacs finds these directories' subdirectories and adds them to |
255 | @code{load-path} when it starts up. Both immediate subdirectories and | |
256 | subdirectories multiple levels down are added to @code{load-path}. | |
257 | ||
258 | Not all subdirectories are included, though. Subdirectories whose | |
259 | names do not start with a letter or digit are excluded. Subdirectories | |
260 | named @file{RCS} are excluded. Also, a subdirectory which contains a | |
261 | file named @file{.nosearch} is excluded. You can use these methods to | |
262 | prevent certain subdirectories of the @file{site-lisp} directories from | |
263 | being searched. | |
264 | ||
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265 | If you run Emacs from the directory where it was built---that is, an |
266 | executable that has not been formally installed---then @code{load-path} | |
267 | normally contains two additional directories. These are the @code{lisp} | |
268 | and @code{site-lisp} subdirectories of the main build directory. (Both | |
269 | are represented as absolute file names.) | |
270 | ||
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271 | @deffn Command locate-library library &optional nosuffix path interactive-call |
272 | This command finds the precise file name for library @var{library}. It | |
273 | searches for the library in the same way @code{load} does, and the | |
274 | argument @var{nosuffix} has the same meaning as in @code{load}: don't | |
275 | add suffixes @samp{.elc} or @samp{.el} to the specified name | |
276 | @var{library}. | |
277 | ||
278 | If the @var{path} is non-@code{nil}, that list of directories is used | |
279 | instead of @code{load-path}. | |
280 | ||
281 | When @code{locate-library} is called from a program, it returns the file | |
282 | name as a string. When the user runs @code{locate-library} | |
283 | interactively, the argument @var{interactive-call} is @code{t}, and this | |
284 | tells @code{locate-library} to display the file name in the echo area. | |
285 | @end deffn | |
286 | ||
a9f0a989 RS |
287 | @node Loading Non-ASCII |
288 | @section Loading Non-ASCII Characters | |
289 | ||
290 | When Emacs Lisp programs contain string constants with non-@sc{ASCII} | |
291 | characters, these can be represented within Emacs either as unibyte | |
292 | strings or as multibyte strings (@pxref{Text Representations}). Which | |
293 | representation is used depends on how the file is read into Emacs. If | |
294 | it is read with decoding into multibyte representation, the text of the | |
295 | Lisp program will be multibyte text, and its string constants will be | |
296 | multibyte strings. If a file containing Latin-1 characters (for | |
297 | example) is read without decoding, the text of the program will be | |
298 | unibyte text, and its string constants will be unibyte strings. | |
299 | @xref{Coding Systems}. | |
300 | ||
301 | To make the results more predictable, Emacs always performs decoding | |
302 | into the multibyte representation when loading Lisp files, even if it | |
303 | was started with the @samp{--unibyte} option. This means that string | |
304 | constants with non-@sc{ASCII} characters translate into multibyte | |
305 | strings. The only exception is when a particular file specifies no | |
306 | decoding. | |
307 | ||
308 | The reason Emacs is designed this way is so that Lisp programs give | |
309 | predictable results, regardless of how Emacs was started. In addition, | |
310 | this enables programs that depend on using multibyte text to work even | |
311 | in a unibyte Emacs. Of course, such programs should be designed to | |
312 | notice whether the user prefers unibyte or multibyte text, by checking | |
313 | @code{default-enable-multibyte-characters}, and convert representations | |
314 | appropriately. | |
315 | ||
316 | In most Emacs Lisp programs, the fact that non-@sc{ASCII} strings are | |
317 | multibyte strings should not be noticeable, since inserting them in | |
318 | unibyte buffers converts them to unibyte automatically. However, if | |
319 | this does make a difference, you can force a particular Lisp file to be | |
430f8c73 | 320 | interpreted as unibyte by writing @samp{-*-unibyte: t;-*-} in a |
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321 | comment on the file's first line. With that designator, the file will |
322 | be unconditionally be interpreted as unibyte, even in an ordinary | |
323 | multibyte Emacs session. | |
324 | ||
83ac6b45 RS |
325 | @node Autoload |
326 | @section Autoload | |
327 | @cindex autoload | |
328 | ||
329 | The @dfn{autoload} facility allows you to make a function or macro | |
bfe721d1 KH |
330 | known in Lisp, but put off loading the file that defines it. The first |
331 | call to the function automatically reads the proper file to install the | |
332 | real definition and other associated code, then runs the real definition | |
83ac6b45 RS |
333 | as if it had been loaded all along. |
334 | ||
335 | There are two ways to set up an autoloaded function: by calling | |
336 | @code{autoload}, and by writing a special ``magic'' comment in the | |
337 | source before the real definition. @code{autoload} is the low-level | |
338 | primitive for autoloading; any Lisp program can call @code{autoload} at | |
969fe9b5 | 339 | any time. Magic comments are the most convenient way to make a function |
a9f0a989 RS |
340 | autoload, for packages installed along with Emacs. These comments do |
341 | nothing on their own, but they serve as a guide for the command | |
969fe9b5 RS |
342 | @code{update-file-autoloads}, which constructs calls to @code{autoload} |
343 | and arranges to execute them when Emacs is built. | |
83ac6b45 | 344 | |
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345 | @defun autoload function filename &optional docstring interactive type |
346 | This function defines the function (or macro) named @var{function} so as | |
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347 | to load automatically from @var{filename}. The string @var{filename} |
348 | specifies the file to load to get the real definition of @var{function}. | |
349 | ||
f9f59935 RS |
350 | If @var{filename} does not contain either a directory name, or the |
351 | suffix @code{.el} or @code{.elc}, then @code{autoload} insists on adding | |
352 | one of these suffixes, and it will not load from a file whose name is | |
353 | just @var{filename} with no added suffix. | |
354 | ||
83ac6b45 | 355 | The argument @var{docstring} is the documentation string for the |
f9f59935 | 356 | function. Normally, this should be identical to the documentation string |
83ac6b45 RS |
357 | in the function definition itself. Specifying the documentation string |
358 | in the call to @code{autoload} makes it possible to look at the | |
359 | documentation without loading the function's real definition. | |
360 | ||
969fe9b5 RS |
361 | If @var{interactive} is non-@code{nil}, that says @var{function} can be |
362 | called interactively. This lets completion in @kbd{M-x} work without | |
a9f0a989 RS |
363 | loading @var{function}'s real definition. The complete interactive |
364 | specification is not given here; it's not needed unless the user | |
365 | actually calls @var{function}, and when that happens, it's time to load | |
366 | the real definition. | |
83ac6b45 RS |
367 | |
368 | You can autoload macros and keymaps as well as ordinary functions. | |
369 | Specify @var{type} as @code{macro} if @var{function} is really a macro. | |
370 | Specify @var{type} as @code{keymap} if @var{function} is really a | |
371 | keymap. Various parts of Emacs need to know this information without | |
372 | loading the real definition. | |
373 | ||
bda144f4 MW |
374 | An autoloaded keymap loads automatically during key lookup when a prefix |
375 | key's binding is the symbol @var{function}. Autoloading does not occur | |
376 | for other kinds of access to the keymap. In particular, it does not | |
377 | happen when a Lisp program gets the keymap from the value of a variable | |
378 | and calls @code{define-key}; not even if the variable name is the same | |
379 | symbol @var{function}. | |
380 | ||
83ac6b45 | 381 | @cindex function cell in autoload |
78c71a98 | 382 | If @var{function} already has a non-void function definition that is not |
83ac6b45 | 383 | an autoload object, @code{autoload} does nothing and returns @code{nil}. |
78c71a98 | 384 | If the function cell of @var{function} is void, or is already an autoload |
83ac6b45 RS |
385 | object, then it is defined as an autoload object like this: |
386 | ||
387 | @example | |
388 | (autoload @var{filename} @var{docstring} @var{interactive} @var{type}) | |
389 | @end example | |
390 | ||
391 | For example, | |
392 | ||
393 | @example | |
bda144f4 | 394 | @group |
83ac6b45 RS |
395 | (symbol-function 'run-prolog) |
396 | @result{} (autoload "prolog" 169681 t nil) | |
bda144f4 | 397 | @end group |
83ac6b45 RS |
398 | @end example |
399 | ||
400 | @noindent | |
401 | In this case, @code{"prolog"} is the name of the file to load, 169681 | |
f9f59935 RS |
402 | refers to the documentation string in the |
403 | @file{emacs/etc/DOC-@var{version}} file (@pxref{Documentation Basics}), | |
404 | @code{t} means the function is interactive, and @code{nil} that it is | |
405 | not a macro or a keymap. | |
83ac6b45 RS |
406 | @end defun |
407 | ||
408 | @cindex autoload errors | |
409 | The autoloaded file usually contains other definitions and may require | |
410 | or provide one or more features. If the file is not completely loaded | |
411 | (due to an error in the evaluation of its contents), any function | |
412 | definitions or @code{provide} calls that occurred during the load are | |
413 | undone. This is to ensure that the next attempt to call any function | |
414 | autoloading from this file will try again to load the file. If not for | |
a9f0a989 RS |
415 | this, then some of the functions in the file might be defined by the |
416 | aborted load, but fail to work properly for the lack of certain | |
417 | subroutines not loaded successfully because they come later in the file. | |
83ac6b45 RS |
418 | |
419 | If the autoloaded file fails to define the desired Lisp function or | |
420 | macro, then an error is signaled with data @code{"Autoloading failed to | |
421 | define function @var{function-name}"}. | |
422 | ||
423 | @findex update-file-autoloads | |
424 | @findex update-directory-autoloads | |
a9f0a989 | 425 | A magic autoload comment consists of @samp{;;;###autoload}, on a line |
83ac6b45 RS |
426 | by itself, just before the real definition of the function in its |
427 | autoloadable source file. The command @kbd{M-x update-file-autoloads} | |
428 | writes a corresponding @code{autoload} call into @file{loaddefs.el}. | |
429 | Building Emacs loads @file{loaddefs.el} and thus calls @code{autoload}. | |
430 | @kbd{M-x update-directory-autoloads} is even more powerful; it updates | |
431 | autoloads for all files in the current directory. | |
432 | ||
433 | The same magic comment can copy any kind of form into | |
434 | @file{loaddefs.el}. If the form following the magic comment is not a | |
435 | function definition, it is copied verbatim. You can also use a magic | |
78c71a98 | 436 | comment to execute a form at build time @emph{without} executing it when |
0f87d8d9 | 437 | the file itself is loaded. To do this, write the form @emph{on the same |
78c71a98 | 438 | line} as the magic comment. Since it is in a comment, it does nothing |
969fe9b5 RS |
439 | when you load the source file; but @kbd{M-x update-file-autoloads} |
440 | copies it to @file{loaddefs.el}, where it is executed while building | |
441 | Emacs. | |
83ac6b45 RS |
442 | |
443 | The following example shows how @code{doctor} is prepared for | |
444 | autoloading with a magic comment: | |
445 | ||
446 | @smallexample | |
447 | ;;;###autoload | |
448 | (defun doctor () | |
449 | "Switch to *doctor* buffer and start giving psychotherapy." | |
450 | (interactive) | |
451 | (switch-to-buffer "*doctor*") | |
452 | (doctor-mode)) | |
453 | @end smallexample | |
454 | ||
455 | @noindent | |
456 | Here's what that produces in @file{loaddefs.el}: | |
457 | ||
458 | @smallexample | |
459 | (autoload 'doctor "doctor" | |
460 | "\ | |
461 | Switch to *doctor* buffer and start giving psychotherapy." | |
462 | t) | |
463 | @end smallexample | |
464 | ||
465 | @noindent | |
466 | The backslash and newline immediately following the double-quote are a | |
467 | convention used only in the preloaded Lisp files such as | |
468 | @file{loaddefs.el}; they tell @code{make-docfile} to put the | |
469 | documentation string in the @file{etc/DOC} file. @xref{Building Emacs}. | |
470 | ||
471 | @node Repeated Loading | |
83ac6b45 RS |
472 | @section Repeated Loading |
473 | @cindex repeated loading | |
474 | ||
a9f0a989 | 475 | You can load a given file more than once in an Emacs session. For |
83ac6b45 RS |
476 | example, after you have rewritten and reinstalled a function definition |
477 | by editing it in a buffer, you may wish to return to the original | |
478 | version; you can do this by reloading the file it came from. | |
479 | ||
480 | When you load or reload files, bear in mind that the @code{load} and | |
481 | @code{load-library} functions automatically load a byte-compiled file | |
482 | rather than a non-compiled file of similar name. If you rewrite a file | |
f9f59935 RS |
483 | that you intend to save and reinstall, you need to byte-compile the new |
484 | version; otherwise Emacs will load the older, byte-compiled file instead | |
485 | of your newer, non-compiled file! If that happens, the message | |
a9f0a989 | 486 | displayed when loading the file includes, @samp{(compiled; note, source is |
969fe9b5 | 487 | newer)}, to remind you to recompile it. |
83ac6b45 RS |
488 | |
489 | When writing the forms in a Lisp library file, keep in mind that the | |
f9f59935 RS |
490 | file might be loaded more than once. For example, think about whether |
491 | each variable should be reinitialized when you reload the library; | |
492 | @code{defvar} does not change the value if the variable is already | |
493 | initialized. (@xref{Defining Variables}.) | |
83ac6b45 RS |
494 | |
495 | The simplest way to add an element to an alist is like this: | |
496 | ||
497 | @example | |
498 | (setq minor-mode-alist | |
499 | (cons '(leif-mode " Leif") minor-mode-alist)) | |
500 | @end example | |
501 | ||
502 | @noindent | |
503 | But this would add multiple elements if the library is reloaded. | |
504 | To avoid the problem, write this: | |
505 | ||
506 | @example | |
507 | (or (assq 'leif-mode minor-mode-alist) | |
508 | (setq minor-mode-alist | |
509 | (cons '(leif-mode " Leif") minor-mode-alist))) | |
510 | @end example | |
511 | ||
a9f0a989 | 512 | To add an element to a list just once, you can also use @code{add-to-list} |
bfe721d1 KH |
513 | (@pxref{Setting Variables}). |
514 | ||
83ac6b45 RS |
515 | Occasionally you will want to test explicitly whether a library has |
516 | already been loaded. Here's one way to test, in a library, whether it | |
517 | has been loaded before: | |
518 | ||
519 | @example | |
969fe9b5 | 520 | (defvar foo-was-loaded nil) |
bfe721d1 | 521 | |
969fe9b5 RS |
522 | (unless foo-was-loaded |
523 | @var{execute-first-time-only} | |
524 | (setq foo-was-loaded t)) | |
83ac6b45 RS |
525 | @end example |
526 | ||
527 | @noindent | |
528 | If the library uses @code{provide} to provide a named feature, you can | |
969fe9b5 RS |
529 | use @code{featurep} earlier in the file to test whether the |
530 | @code{provide} call has been executed before. | |
78c71a98 | 531 | @ifinfo |
bfe721d1 | 532 | @xref{Named Features}. |
78c71a98 | 533 | @end ifinfo |
83ac6b45 | 534 | |
bfe721d1 | 535 | @node Named Features |
83ac6b45 RS |
536 | @section Features |
537 | @cindex features | |
538 | @cindex requiring features | |
539 | @cindex providing features | |
540 | ||
541 | @code{provide} and @code{require} are an alternative to | |
542 | @code{autoload} for loading files automatically. They work in terms of | |
543 | named @dfn{features}. Autoloading is triggered by calling a specific | |
544 | function, but a feature is loaded the first time another program asks | |
545 | for it by name. | |
546 | ||
547 | A feature name is a symbol that stands for a collection of functions, | |
548 | variables, etc. The file that defines them should @dfn{provide} the | |
549 | feature. Another program that uses them may ensure they are defined by | |
550 | @dfn{requiring} the feature. This loads the file of definitions if it | |
551 | hasn't been loaded already. | |
552 | ||
553 | To require the presence of a feature, call @code{require} with the | |
554 | feature name as argument. @code{require} looks in the global variable | |
555 | @code{features} to see whether the desired feature has been provided | |
556 | already. If not, it loads the feature from the appropriate file. This | |
78c71a98 | 557 | file should call @code{provide} at the top level to add the feature to |
83ac6b45 RS |
558 | @code{features}; if it fails to do so, @code{require} signals an error. |
559 | @cindex load error with require | |
560 | ||
83ac6b45 RS |
561 | For example, in @file{emacs/lisp/prolog.el}, |
562 | the definition for @code{run-prolog} includes the following code: | |
563 | ||
564 | @smallexample | |
565 | (defun run-prolog () | |
9e2b495b | 566 | "Run an inferior Prolog process, with I/O via buffer *prolog*." |
83ac6b45 RS |
567 | (interactive) |
568 | (require 'comint) | |
569 | (switch-to-buffer (make-comint "prolog" prolog-program-name)) | |
570 | (inferior-prolog-mode)) | |
571 | @end smallexample | |
572 | ||
573 | @noindent | |
574 | The expression @code{(require 'comint)} loads the file @file{comint.el} | |
575 | if it has not yet been loaded. This ensures that @code{make-comint} is | |
969fe9b5 RS |
576 | defined. Features are normally named after the files that provide them, |
577 | so that @code{require} need not be given the file name. | |
83ac6b45 RS |
578 | |
579 | The @file{comint.el} file contains the following top-level expression: | |
580 | ||
581 | @smallexample | |
582 | (provide 'comint) | |
583 | @end smallexample | |
584 | ||
585 | @noindent | |
586 | This adds @code{comint} to the global @code{features} list, so that | |
587 | @code{(require 'comint)} will henceforth know that nothing needs to be | |
588 | done. | |
589 | ||
590 | @cindex byte-compiling @code{require} | |
78c71a98 | 591 | When @code{require} is used at top level in a file, it takes effect |
83ac6b45 RS |
592 | when you byte-compile that file (@pxref{Byte Compilation}) as well as |
593 | when you load it. This is in case the required package contains macros | |
594 | that the byte compiler must know about. | |
595 | ||
596 | Although top-level calls to @code{require} are evaluated during | |
597 | byte compilation, @code{provide} calls are not. Therefore, you can | |
598 | ensure that a file of definitions is loaded before it is byte-compiled | |
599 | by including a @code{provide} followed by a @code{require} for the same | |
600 | feature, as in the following example. | |
601 | ||
602 | @smallexample | |
603 | @group | |
604 | (provide 'my-feature) ; @r{Ignored by byte compiler,} | |
605 | ; @r{evaluated by @code{load}.} | |
606 | (require 'my-feature) ; @r{Evaluated by byte compiler.} | |
607 | @end group | |
608 | @end smallexample | |
609 | ||
78c71a98 RS |
610 | @noindent |
611 | The compiler ignores the @code{provide}, then processes the | |
612 | @code{require} by loading the file in question. Loading the file does | |
613 | execute the @code{provide} call, so the subsequent @code{require} call | |
969fe9b5 | 614 | does nothing when the file is loaded. |
78c71a98 | 615 | |
83ac6b45 RS |
616 | @defun provide feature |
617 | This function announces that @var{feature} is now loaded, or being | |
618 | loaded, into the current Emacs session. This means that the facilities | |
619 | associated with @var{feature} are or will be available for other Lisp | |
620 | programs. | |
621 | ||
622 | The direct effect of calling @code{provide} is to add @var{feature} to | |
623 | the front of the list @code{features} if it is not already in the list. | |
624 | The argument @var{feature} must be a symbol. @code{provide} returns | |
625 | @var{feature}. | |
626 | ||
627 | @smallexample | |
628 | features | |
629 | @result{} (bar bish) | |
630 | ||
631 | (provide 'foo) | |
632 | @result{} foo | |
633 | features | |
634 | @result{} (foo bar bish) | |
635 | @end smallexample | |
636 | ||
bfe721d1 KH |
637 | When a file is loaded to satisfy an autoload, and it stops due to an |
638 | error in the evaluating its contents, any function definitions or | |
639 | @code{provide} calls that occurred during the load are undone. | |
640 | @xref{Autoload}. | |
83ac6b45 RS |
641 | @end defun |
642 | ||
643 | @defun require feature &optional filename | |
644 | This function checks whether @var{feature} is present in the current | |
f9f59935 RS |
645 | Emacs session (using @code{(featurep @var{feature})}; see below). The |
646 | argument @var{feature} must be a symbol. | |
647 | ||
648 | If the feature is not present, then @code{require} loads @var{filename} | |
649 | with @code{load}. If @var{filename} is not supplied, then the name of | |
650 | the symbol @var{feature} is used as the base file name to load. | |
651 | However, in this case, @code{require} insists on finding @var{feature} | |
652 | with an added suffix; a file whose name is just @var{feature} won't be | |
653 | used. | |
83ac6b45 RS |
654 | |
655 | If loading the file fails to provide @var{feature}, @code{require} | |
656 | signals an error, @samp{Required feature @var{feature} was not | |
657 | provided}. | |
658 | @end defun | |
659 | ||
660 | @defun featurep feature | |
661 | This function returns @code{t} if @var{feature} has been provided in the | |
969fe9b5 | 662 | current Emacs session (i.e., if @var{feature} is a member of |
83ac6b45 RS |
663 | @code{features}.) |
664 | @end defun | |
665 | ||
666 | @defvar features | |
667 | The value of this variable is a list of symbols that are the features | |
668 | loaded in the current Emacs session. Each symbol was put in this list | |
669 | with a call to @code{provide}. The order of the elements in the | |
670 | @code{features} list is not significant. | |
671 | @end defvar | |
672 | ||
673 | @node Unloading | |
674 | @section Unloading | |
675 | @cindex unloading | |
676 | ||
677 | @c Emacs 19 feature | |
678 | You can discard the functions and variables loaded by a library to | |
679 | reclaim memory for other Lisp objects. To do this, use the function | |
680 | @code{unload-feature}: | |
681 | ||
ee6bcc94 | 682 | @deffn Command unload-feature feature &optional force |
83ac6b45 | 683 | This command unloads the library that provided feature @var{feature}. |
78c71a98 | 684 | It undefines all functions, macros, and variables defined in that |
969fe9b5 RS |
685 | library with @code{defun}, @code{defalias}, @code{defsubst}, |
686 | @code{defmacro}, @code{defconst}, @code{defvar}, and @code{defcustom}. | |
687 | It then restores any autoloads formerly associated with those symbols. | |
688 | (Loading saves these in the @code{autoload} property of the symbol.) | |
ee6bcc94 | 689 | |
6582d61e RS |
690 | Before restoring the previous definitions, @code{unload-feature} runs |
691 | @code{remove-hook} to remove functions in the library from certain | |
692 | hooks. These hooks include variables whose names end in @samp{hook} or | |
693 | @samp{-hooks}, plus those listed in @code{loadhist-special-hooks}. This | |
694 | is to prevent Emacs from ceasing to function because important hooks | |
695 | refer to functions that are no longer defined. | |
696 | ||
697 | @vindex @var{feature}-unload-hook | |
698 | If these measures are not sufficient to prevent malfunction, a library | |
699 | can define an explicit unload hook. If @code{@var{feature}-unload-hook} | |
700 | is defined, it is run as a normal hook before restoring the previous | |
701 | definitions, @emph{instead of} the usual hook-removing actions. The | |
702 | unload hook ought to undo all the global state changes made by the | |
703 | library that might cease to work once the library is unloaded. | |
704 | ||
ee6bcc94 RS |
705 | Ordinarily, @code{unload-feature} refuses to unload a library on which |
706 | other loaded libraries depend. (A library @var{a} depends on library | |
707 | @var{b} if @var{a} contains a @code{require} for @var{b}.) If the | |
708 | optional argument @var{force} is non-@code{nil}, dependencies are | |
709 | ignored and you can unload any library. | |
83ac6b45 RS |
710 | @end deffn |
711 | ||
712 | The @code{unload-feature} function is written in Lisp; its actions are | |
713 | based on the variable @code{load-history}. | |
714 | ||
715 | @defvar load-history | |
716 | This variable's value is an alist connecting library names with the | |
717 | names of functions and variables they define, the features they provide, | |
718 | and the features they require. | |
719 | ||
720 | Each element is a list and describes one library. The @sc{car} of the | |
721 | list is the name of the library, as a string. The rest of the list is | |
722 | composed of these kinds of objects: | |
723 | ||
724 | @itemize @bullet | |
725 | @item | |
78c71a98 | 726 | Symbols that were defined by this library. |
83ac6b45 RS |
727 | @item |
728 | Lists of the form @code{(require . @var{feature})} indicating | |
729 | features that were required. | |
730 | @item | |
731 | Lists of the form @code{(provide . @var{feature})} indicating | |
732 | features that were provided. | |
733 | @end itemize | |
734 | ||
735 | The value of @code{load-history} may have one element whose @sc{car} is | |
736 | @code{nil}. This element describes definitions made with | |
737 | @code{eval-buffer} on a buffer that is not visiting a file. | |
738 | @end defvar | |
739 | ||
740 | The command @code{eval-region} updates @code{load-history}, but does so | |
741 | by adding the symbols defined to the element for the file being visited, | |
55607887 | 742 | rather than replacing that element. @xref{Eval}. |
83ac6b45 | 743 | |
6582d61e RS |
744 | Preloaded libraries don't contribute to @code{load-history}. |
745 | ||
746 | @tindex loadhist-special-hooks | |
747 | @defvar loadhist-special-hooks | |
748 | This variable holds a list of hooks to be scanned before unloading a | |
749 | library, to remove functions defined in the library. | |
750 | @end defvar | |
751 | ||
83ac6b45 RS |
752 | @node Hooks for Loading |
753 | @section Hooks for Loading | |
754 | @cindex loading hooks | |
755 | @cindex hooks for loading | |
756 | ||
757 | You can ask for code to be executed if and when a particular library is | |
758 | loaded, by calling @code{eval-after-load}. | |
759 | ||
760 | @defun eval-after-load library form | |
761 | This function arranges to evaluate @var{form} at the end of loading the | |
d2e9ee06 RS |
762 | library @var{library}, if and when @var{library} is loaded. If |
763 | @var{library} is already loaded, it evaluates @var{form} right away. | |
83ac6b45 RS |
764 | |
765 | The library name @var{library} must exactly match the argument of | |
766 | @code{load}. To get the proper results when an installed library is | |
767 | found by searching @code{load-path}, you should not include any | |
768 | directory names in @var{library}. | |
769 | ||
770 | An error in @var{form} does not undo the load, but does prevent | |
771 | execution of the rest of @var{form}. | |
772 | @end defun | |
773 | ||
d2e9ee06 RS |
774 | In general, well-designed Lisp programs should not use this feature. |
775 | The clean and modular ways to interact with a Lisp library are (1) | |
776 | examine and set the library's variables (those which are meant for | |
cc8c51f1 | 777 | outside use), and (2) call the library's functions. If you wish to |
d2e9ee06 RS |
778 | do (1), you can do it immediately---there is no need to wait for when |
779 | the library is loaded. To do (2), you must load the library (preferably | |
780 | with @code{require}). | |
781 | ||
969fe9b5 RS |
782 | But it is OK to use @code{eval-after-load} in your personal |
783 | customizations if you don't feel they must meet the design standards for | |
784 | programs meant for wider use. | |
d2e9ee06 | 785 | |
83ac6b45 RS |
786 | @defvar after-load-alist |
787 | An alist of expressions to evaluate if and when particular libraries are | |
788 | loaded. Each element looks like this: | |
789 | ||
790 | @example | |
791 | (@var{filename} @var{forms}@dots{}) | |
792 | @end example | |
793 | ||
794 | The function @code{load} checks @code{after-load-alist} in order to | |
795 | implement @code{eval-after-load}. | |
796 | @end defvar | |
797 | ||
798 | @c Emacs 19 feature |