| 1 | @c -*-texinfo-*- |
| 2 | @c This is part of the GNU Emacs Lisp Reference Manual. |
| 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999 |
| 4 | @c Free Software Foundation, Inc. |
| 5 | @c See the file elisp.texi for copying conditions. |
| 6 | @setfilename ../info/loading |
| 7 | @node Loading, Byte Compilation, Customization, Top |
| 8 | @chapter Loading |
| 9 | @cindex loading |
| 10 | @cindex library |
| 11 | @cindex Lisp library |
| 12 | |
| 13 | Loading a file of Lisp code means bringing its contents into the Lisp |
| 14 | environment in the form of Lisp objects. Emacs finds and opens the |
| 15 | file, reads the text, evaluates each form, and then closes the file. |
| 16 | |
| 17 | The load functions evaluate all the expressions in a file just |
| 18 | as the @code{eval-current-buffer} function evaluates all the |
| 19 | expressions in a buffer. The difference is that the load functions |
| 20 | read and evaluate the text in the file as found on disk, not the text |
| 21 | in an Emacs buffer. |
| 22 | |
| 23 | @cindex top-level form |
| 24 | The loaded file must contain Lisp expressions, either as source code |
| 25 | or as byte-compiled code. Each form in the file is called a |
| 26 | @dfn{top-level form}. There is no special format for the forms in a |
| 27 | loadable file; any form in a file may equally well be typed directly |
| 28 | into a buffer and evaluated there. (Indeed, most code is tested this |
| 29 | way.) Most often, the forms are function definitions and variable |
| 30 | definitions. |
| 31 | |
| 32 | A file containing Lisp code is often called a @dfn{library}. Thus, |
| 33 | the ``Rmail library'' is a file containing code for Rmail mode. |
| 34 | Similarly, a ``Lisp library directory'' is a directory of files |
| 35 | containing Lisp code. |
| 36 | |
| 37 | @menu |
| 38 | * How Programs Do Loading:: The @code{load} function and others. |
| 39 | * Library Search:: Finding a library to load. |
| 40 | * Loading Non-ASCII:: Non-@sc{ascii} characters in Emacs Lisp files. |
| 41 | * Autoload:: Setting up a function to autoload. |
| 42 | * Repeated Loading:: Precautions about loading a file twice. |
| 43 | * Named Features:: Loading a library if it isn't already loaded. |
| 44 | * Unloading:: How to ``unload'' a library that was loaded. |
| 45 | * Hooks for Loading:: Providing code to be run when |
| 46 | particular libraries are loaded. |
| 47 | @end menu |
| 48 | |
| 49 | @node How Programs Do Loading |
| 50 | @section How Programs Do Loading |
| 51 | |
| 52 | Emacs Lisp has several interfaces for loading. For example, |
| 53 | @code{autoload} creates a placeholder object for a function defined in a |
| 54 | file; trying to call the autoloading function loads the file to get the |
| 55 | function's real definition (@pxref{Autoload}). @code{require} loads a |
| 56 | file if it isn't already loaded (@pxref{Named Features}). Ultimately, |
| 57 | all these facilities call the @code{load} function to do the work. |
| 58 | |
| 59 | @defun load filename &optional missing-ok nomessage nosuffix must-suffix |
| 60 | This function finds and opens a file of Lisp code, evaluates all the |
| 61 | forms in it, and closes the file. |
| 62 | |
| 63 | To find the file, @code{load} first looks for a file named |
| 64 | @file{@var{filename}.elc}, that is, for a file whose name is |
| 65 | @var{filename} with @samp{.elc} appended. If such a file exists, it is |
| 66 | loaded. If there is no file by that name, then @code{load} looks for a |
| 67 | file named @file{@var{filename}.el}. If that file exists, it is loaded. |
| 68 | Finally, if neither of those names is found, @code{load} looks for a |
| 69 | file named @var{filename} with nothing appended, and loads it if it |
| 70 | exists. (The @code{load} function is not clever about looking at |
| 71 | @var{filename}. In the perverse case of a file named @file{foo.el.el}, |
| 72 | evaluation of @code{(load "foo.el")} will indeed find it.) |
| 73 | |
| 74 | If the optional argument @var{nosuffix} is non-@code{nil}, then the |
| 75 | suffixes @samp{.elc} and @samp{.el} are not tried. In this case, you |
| 76 | must specify the precise file name you want. By specifying the precise |
| 77 | file name and using @code{t} for @var{nosuffix}, you can prevent |
| 78 | perverse file names such as @file{foo.el.el} from being tried. |
| 79 | |
| 80 | If the optional argument @var{must-suffix} is non-@code{nil}, then |
| 81 | @code{load} insists that the file name used must end in either |
| 82 | @samp{.el} or @samp{.elc}, unless it contains an explicit directory |
| 83 | name. If @var{filename} does not contain an explicit directory name, |
| 84 | and does not end in a suffix, then @code{load} insists on adding one. |
| 85 | |
| 86 | If @var{filename} is a relative file name, such as @file{foo} or |
| 87 | @file{baz/foo.bar}, @code{load} searches for the file using the variable |
| 88 | @code{load-path}. It appends @var{filename} to each of the directories |
| 89 | listed in @code{load-path}, and loads the first file it finds whose name |
| 90 | matches. The current default directory is tried only if it is specified |
| 91 | in @code{load-path}, where @code{nil} stands for the default directory. |
| 92 | @code{load} tries all three possible suffixes in the first directory in |
| 93 | @code{load-path}, then all three suffixes in the second directory, and |
| 94 | so on. @xref{Library Search}. |
| 95 | |
| 96 | If you get a warning that @file{foo.elc} is older than @file{foo.el}, it |
| 97 | means you should consider recompiling @file{foo.el}. @xref{Byte |
| 98 | Compilation}. |
| 99 | |
| 100 | When loading a source file (not compiled), @code{load} performs |
| 101 | character set translation just as Emacs would do when visiting the file. |
| 102 | @xref{Coding Systems}. |
| 103 | |
| 104 | Messages like @samp{Loading foo...} and @samp{Loading foo...done} appear |
| 105 | in the echo area during loading unless @var{nomessage} is |
| 106 | non-@code{nil}. |
| 107 | |
| 108 | @cindex load errors |
| 109 | Any unhandled errors while loading a file terminate loading. If the |
| 110 | load was done for the sake of @code{autoload}, any function definitions |
| 111 | made during the loading are undone. |
| 112 | |
| 113 | @kindex file-error |
| 114 | If @code{load} can't find the file to load, then normally it signals the |
| 115 | error @code{file-error} (with @samp{Cannot open load file |
| 116 | @var{filename}}). But if @var{missing-ok} is non-@code{nil}, then |
| 117 | @code{load} just returns @code{nil}. |
| 118 | |
| 119 | You can use the variable @code{load-read-function} to specify a function |
| 120 | for @code{load} to use instead of @code{read} for reading expressions. |
| 121 | See below. |
| 122 | |
| 123 | @code{load} returns @code{t} if the file loads successfully. |
| 124 | @end defun |
| 125 | |
| 126 | @deffn Command load-file filename |
| 127 | This command loads the file @var{filename}. If @var{filename} is a |
| 128 | relative file name, then the current default directory is assumed. |
| 129 | @code{load-path} is not used, and suffixes are not appended. Use this |
| 130 | command if you wish to specify precisely the file name to load. |
| 131 | @end deffn |
| 132 | |
| 133 | @deffn Command load-library library |
| 134 | This command loads the library named @var{library}. It is equivalent to |
| 135 | @code{load}, except in how it reads its argument interactively. |
| 136 | @end deffn |
| 137 | |
| 138 | @defvar load-in-progress |
| 139 | This variable is non-@code{nil} if Emacs is in the process of loading a |
| 140 | file, and it is @code{nil} otherwise. |
| 141 | @end defvar |
| 142 | |
| 143 | @defvar load-read-function |
| 144 | This variable specifies an alternate expression-reading function for |
| 145 | @code{load} and @code{eval-region} to use instead of @code{read}. |
| 146 | The function should accept one argument, just as @code{read} does. |
| 147 | |
| 148 | Normally, the variable's value is @code{nil}, which means those |
| 149 | functions should use @code{read}. |
| 150 | |
| 151 | @strong{Note:} Instead of using this variable, it is cleaner to use |
| 152 | another, newer feature: to pass the function as the @var{read-function} |
| 153 | argument to @code{eval-region}. @xref{Eval}. |
| 154 | @end defvar |
| 155 | |
| 156 | For information about how @code{load} is used in building Emacs, see |
| 157 | @ref{Building Emacs}. |
| 158 | |
| 159 | @node Library Search |
| 160 | @section Library Search |
| 161 | |
| 162 | When Emacs loads a Lisp library, it searches for the library |
| 163 | in a list of directories specified by the variable @code{load-path}. |
| 164 | |
| 165 | @defopt load-path |
| 166 | @cindex @code{EMACSLOADPATH} environment variable |
| 167 | The value of this variable is a list of directories to search when |
| 168 | loading files with @code{load}. Each element is a string (which must be |
| 169 | a directory name) or @code{nil} (which stands for the current working |
| 170 | directory). |
| 171 | @end defopt |
| 172 | |
| 173 | The value of @code{load-path} is initialized from the environment |
| 174 | variable @code{EMACSLOADPATH}, if that exists; otherwise its default |
| 175 | value is specified in @file{emacs/src/paths.h} when Emacs is built. |
| 176 | Then the list is expanded by adding subdirectories of the directories |
| 177 | in the list. |
| 178 | |
| 179 | The syntax of @code{EMACSLOADPATH} is the same as used for @code{PATH}; |
| 180 | @samp{:} (or @samp{;}, according to the operating system) separates |
| 181 | directory names, and @samp{.} is used for the current default directory. |
| 182 | Here is an example of how to set your @code{EMACSLOADPATH} variable from |
| 183 | a @code{csh} @file{.login} file: |
| 184 | |
| 185 | @smallexample |
| 186 | setenv EMACSLOADPATH .:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp |
| 187 | @end smallexample |
| 188 | |
| 189 | Here is how to set it using @code{sh}: |
| 190 | |
| 191 | @smallexample |
| 192 | export EMACSLOADPATH |
| 193 | EMACSLOADPATH=.:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp |
| 194 | @end smallexample |
| 195 | |
| 196 | Here is an example of code you can place in your init file (@pxref{Init |
| 197 | File}) to add several directories to the front of your default |
| 198 | @code{load-path}: |
| 199 | |
| 200 | @smallexample |
| 201 | @group |
| 202 | (setq load-path |
| 203 | (append (list nil "/user/bil/emacs" |
| 204 | "/usr/local/lisplib" |
| 205 | "~/emacs") |
| 206 | load-path)) |
| 207 | @end group |
| 208 | @end smallexample |
| 209 | |
| 210 | @c Wordy to rid us of an overfull hbox. --rjc 15mar92 |
| 211 | @noindent |
| 212 | In this example, the path searches the current working directory first, |
| 213 | followed then by the @file{/user/bil/emacs} directory, the |
| 214 | @file{/usr/local/lisplib} directory, and the @file{~/emacs} directory, |
| 215 | which are then followed by the standard directories for Lisp code. |
| 216 | |
| 217 | Dumping Emacs uses a special value of @code{load-path}. If the value of |
| 218 | @code{load-path} at the end of dumping is unchanged (that is, still the |
| 219 | same special value), the dumped Emacs switches to the ordinary |
| 220 | @code{load-path} value when it starts up, as described above. But if |
| 221 | @code{load-path} has any other value at the end of dumping, that value |
| 222 | is used for execution of the dumped Emacs also. |
| 223 | |
| 224 | Therefore, if you want to change @code{load-path} temporarily for |
| 225 | loading a few libraries in @file{site-init.el} or @file{site-load.el}, |
| 226 | you should bind @code{load-path} locally with @code{let} around the |
| 227 | calls to @code{load}. |
| 228 | |
| 229 | The default value of @code{load-path}, when running an Emacs which has |
| 230 | been installed on the system, includes two special directories (and |
| 231 | their subdirectories as well): |
| 232 | |
| 233 | @smallexample |
| 234 | "/usr/local/share/emacs/@var{version}/site-lisp" |
| 235 | @end smallexample |
| 236 | |
| 237 | @noindent |
| 238 | and |
| 239 | |
| 240 | @smallexample |
| 241 | "/usr/local/share/emacs/site-lisp" |
| 242 | @end smallexample |
| 243 | |
| 244 | @noindent |
| 245 | The first one is for locally installed packages for a particular Emacs |
| 246 | version; the second is for locally installed packages meant for use with |
| 247 | all installed Emacs versions. |
| 248 | |
| 249 | There are several reasons why a Lisp package that works well in one |
| 250 | Emacs version can cause trouble in another. Sometimes packages need |
| 251 | updating for incompatible changes in Emacs; sometimes they depend on |
| 252 | undocumented internal Emacs data that can change without notice; |
| 253 | sometimes a newer Emacs version incorporates a version of the package, |
| 254 | and should be used only with that version. |
| 255 | |
| 256 | Emacs finds these directories' subdirectories and adds them to |
| 257 | @code{load-path} when it starts up. Both immediate subdirectories and |
| 258 | subdirectories multiple levels down are added to @code{load-path}. |
| 259 | |
| 260 | Not all subdirectories are included, though. Subdirectories whose |
| 261 | names do not start with a letter or digit are excluded. Subdirectories |
| 262 | named @file{RCS} or @file{CVS} are excluded. Also, a subdirectory which |
| 263 | contains a file named @file{.nosearch} is excluded. You can use these |
| 264 | methods to prevent certain subdirectories of the @file{site-lisp} |
| 265 | directories from being searched. |
| 266 | |
| 267 | If you run Emacs from the directory where it was built---that is, an |
| 268 | executable that has not been formally installed---then @code{load-path} |
| 269 | normally contains two additional directories. These are the @code{lisp} |
| 270 | and @code{site-lisp} subdirectories of the main build directory. (Both |
| 271 | are represented as absolute file names.) |
| 272 | |
| 273 | @deffn Command locate-library library &optional nosuffix path interactive-call |
| 274 | This command finds the precise file name for library @var{library}. It |
| 275 | searches for the library in the same way @code{load} does, and the |
| 276 | argument @var{nosuffix} has the same meaning as in @code{load}: don't |
| 277 | add suffixes @samp{.elc} or @samp{.el} to the specified name |
| 278 | @var{library}. |
| 279 | |
| 280 | If the @var{path} is non-@code{nil}, that list of directories is used |
| 281 | instead of @code{load-path}. |
| 282 | |
| 283 | When @code{locate-library} is called from a program, it returns the file |
| 284 | name as a string. When the user runs @code{locate-library} |
| 285 | interactively, the argument @var{interactive-call} is @code{t}, and this |
| 286 | tells @code{locate-library} to display the file name in the echo area. |
| 287 | @end deffn |
| 288 | |
| 289 | @node Loading Non-ASCII |
| 290 | @section Loading Non-@sc{ascii} Characters |
| 291 | |
| 292 | When Emacs Lisp programs contain string constants with non-@sc{ascii} |
| 293 | characters, these can be represented within Emacs either as unibyte |
| 294 | strings or as multibyte strings (@pxref{Text Representations}). Which |
| 295 | representation is used depends on how the file is read into Emacs. If |
| 296 | it is read with decoding into multibyte representation, the text of the |
| 297 | Lisp program will be multibyte text, and its string constants will be |
| 298 | multibyte strings. If a file containing Latin-1 characters (for |
| 299 | example) is read without decoding, the text of the program will be |
| 300 | unibyte text, and its string constants will be unibyte strings. |
| 301 | @xref{Coding Systems}. |
| 302 | |
| 303 | To make the results more predictable, Emacs always performs decoding |
| 304 | into the multibyte representation when loading Lisp files, even if it |
| 305 | was started with the @samp{--unibyte} option. This means that string |
| 306 | constants with non-@sc{ascii} characters translate into multibyte |
| 307 | strings. The only exception is when a particular file specifies no |
| 308 | decoding. |
| 309 | |
| 310 | The reason Emacs is designed this way is so that Lisp programs give |
| 311 | predictable results, regardless of how Emacs was started. In addition, |
| 312 | this enables programs that depend on using multibyte text to work even |
| 313 | in a unibyte Emacs. Of course, such programs should be designed to |
| 314 | notice whether the user prefers unibyte or multibyte text, by checking |
| 315 | @code{default-enable-multibyte-characters}, and convert representations |
| 316 | appropriately. |
| 317 | |
| 318 | In most Emacs Lisp programs, the fact that non-@sc{ascii} strings are |
| 319 | multibyte strings should not be noticeable, since inserting them in |
| 320 | unibyte buffers converts them to unibyte automatically. However, if |
| 321 | this does make a difference, you can force a particular Lisp file to be |
| 322 | interpreted as unibyte by writing @samp{-*-unibyte: t;-*-} in a |
| 323 | comment on the file's first line. With that designator, the file will |
| 324 | unconditionally be interpreted as unibyte, even in an ordinary |
| 325 | multibyte Emacs session. This can matter when making keybindings to |
| 326 | non-@sc{ascii} characters written as @code{?v@var{literal}}. |
| 327 | |
| 328 | @node Autoload |
| 329 | @section Autoload |
| 330 | @cindex autoload |
| 331 | |
| 332 | The @dfn{autoload} facility allows you to make a function or macro |
| 333 | known in Lisp, but put off loading the file that defines it. The first |
| 334 | call to the function automatically reads the proper file to install the |
| 335 | real definition and other associated code, then runs the real definition |
| 336 | as if it had been loaded all along. |
| 337 | |
| 338 | There are two ways to set up an autoloaded function: by calling |
| 339 | @code{autoload}, and by writing a special ``magic'' comment in the |
| 340 | source before the real definition. @code{autoload} is the low-level |
| 341 | primitive for autoloading; any Lisp program can call @code{autoload} at |
| 342 | any time. Magic comments are the most convenient way to make a function |
| 343 | autoload, for packages installed along with Emacs. These comments do |
| 344 | nothing on their own, but they serve as a guide for the command |
| 345 | @code{update-file-autoloads}, which constructs calls to @code{autoload} |
| 346 | and arranges to execute them when Emacs is built. |
| 347 | |
| 348 | @defun autoload function filename &optional docstring interactive type |
| 349 | This function defines the function (or macro) named @var{function} so as |
| 350 | to load automatically from @var{filename}. The string @var{filename} |
| 351 | specifies the file to load to get the real definition of @var{function}. |
| 352 | |
| 353 | If @var{filename} does not contain either a directory name, or the |
| 354 | suffix @code{.el} or @code{.elc}, then @code{autoload} insists on adding |
| 355 | one of these suffixes, and it will not load from a file whose name is |
| 356 | just @var{filename} with no added suffix. |
| 357 | |
| 358 | The argument @var{docstring} is the documentation string for the |
| 359 | function. Normally, this should be identical to the documentation string |
| 360 | in the function definition itself. Specifying the documentation string |
| 361 | in the call to @code{autoload} makes it possible to look at the |
| 362 | documentation without loading the function's real definition. |
| 363 | |
| 364 | If @var{interactive} is non-@code{nil}, that says @var{function} can be |
| 365 | called interactively. This lets completion in @kbd{M-x} work without |
| 366 | loading @var{function}'s real definition. The complete interactive |
| 367 | specification is not given here; it's not needed unless the user |
| 368 | actually calls @var{function}, and when that happens, it's time to load |
| 369 | the real definition. |
| 370 | |
| 371 | You can autoload macros and keymaps as well as ordinary functions. |
| 372 | Specify @var{type} as @code{macro} if @var{function} is really a macro. |
| 373 | Specify @var{type} as @code{keymap} if @var{function} is really a |
| 374 | keymap. Various parts of Emacs need to know this information without |
| 375 | loading the real definition. |
| 376 | |
| 377 | An autoloaded keymap loads automatically during key lookup when a prefix |
| 378 | key's binding is the symbol @var{function}. Autoloading does not occur |
| 379 | for other kinds of access to the keymap. In particular, it does not |
| 380 | happen when a Lisp program gets the keymap from the value of a variable |
| 381 | and calls @code{define-key}; not even if the variable name is the same |
| 382 | symbol @var{function}. |
| 383 | |
| 384 | @cindex function cell in autoload |
| 385 | If @var{function} already has a non-void function definition that is not |
| 386 | an autoload object, @code{autoload} does nothing and returns @code{nil}. |
| 387 | If the function cell of @var{function} is void, or is already an autoload |
| 388 | object, then it is defined as an autoload object like this: |
| 389 | |
| 390 | @example |
| 391 | (autoload @var{filename} @var{docstring} @var{interactive} @var{type}) |
| 392 | @end example |
| 393 | |
| 394 | For example, |
| 395 | |
| 396 | @example |
| 397 | @group |
| 398 | (symbol-function 'run-prolog) |
| 399 | @result{} (autoload "prolog" 169681 t nil) |
| 400 | @end group |
| 401 | @end example |
| 402 | |
| 403 | @noindent |
| 404 | In this case, @code{"prolog"} is the name of the file to load, 169681 |
| 405 | refers to the documentation string in the |
| 406 | @file{emacs/etc/DOC-@var{version}} file (@pxref{Documentation Basics}), |
| 407 | @code{t} means the function is interactive, and @code{nil} that it is |
| 408 | not a macro or a keymap. |
| 409 | @end defun |
| 410 | |
| 411 | @cindex autoload errors |
| 412 | The autoloaded file usually contains other definitions and may require |
| 413 | or provide one or more features. If the file is not completely loaded |
| 414 | (due to an error in the evaluation of its contents), any function |
| 415 | definitions or @code{provide} calls that occurred during the load are |
| 416 | undone. This is to ensure that the next attempt to call any function |
| 417 | autoloading from this file will try again to load the file. If not for |
| 418 | this, then some of the functions in the file might be defined by the |
| 419 | aborted load, but fail to work properly for the lack of certain |
| 420 | subroutines not loaded successfully because they come later in the file. |
| 421 | |
| 422 | If the autoloaded file fails to define the desired Lisp function or |
| 423 | macro, then an error is signaled with data @code{"Autoloading failed to |
| 424 | define function @var{function-name}"}. |
| 425 | |
| 426 | @findex update-file-autoloads |
| 427 | @findex update-directory-autoloads |
| 428 | A magic autoload comment consists of @samp{;;;###autoload}, on a line |
| 429 | by itself, just before the real definition of the function in its |
| 430 | autoloadable source file. The command @kbd{M-x update-file-autoloads} |
| 431 | writes a corresponding @code{autoload} call into @file{loaddefs.el}. |
| 432 | Building Emacs loads @file{loaddefs.el} and thus calls @code{autoload}. |
| 433 | @kbd{M-x update-directory-autoloads} is even more powerful; it updates |
| 434 | autoloads for all files in the current directory. |
| 435 | |
| 436 | The same magic comment can copy any kind of form into |
| 437 | @file{loaddefs.el}. If the form following the magic comment is not a |
| 438 | function-defining form or a @code{defcustom} form, it is copied |
| 439 | verbatim. ``Function-defining forms'' include @code{define-skeleton}, |
| 440 | @code{define-derived-mode}, @code{define-generic-mode} and |
| 441 | @code{define-minor-mode} as well as @code{defun} and |
| 442 | @code{defmacro}. To save space, a @code{defcustom} form is converted to |
| 443 | a @code{defvar} in @file{loaddefs.el}, with some additional information |
| 444 | if it uses @code{:require}. |
| 445 | |
| 446 | You can also use a magic comment to execute a form at build time |
| 447 | @emph{without} executing it when the file itself is loaded. To do this, |
| 448 | write the form @emph{on the same line} as the magic comment. Since it |
| 449 | is in a comment, it does nothing when you load the source file; but |
| 450 | @kbd{M-x update-file-autoloads} copies it to @file{loaddefs.el}, where |
| 451 | it is executed while building Emacs. |
| 452 | |
| 453 | The following example shows how @code{doctor} is prepared for |
| 454 | autoloading with a magic comment: |
| 455 | |
| 456 | @smallexample |
| 457 | ;;;###autoload |
| 458 | (defun doctor () |
| 459 | "Switch to *doctor* buffer and start giving psychotherapy." |
| 460 | (interactive) |
| 461 | (switch-to-buffer "*doctor*") |
| 462 | (doctor-mode)) |
| 463 | @end smallexample |
| 464 | |
| 465 | @noindent |
| 466 | Here's what that produces in @file{loaddefs.el}: |
| 467 | |
| 468 | @smallexample |
| 469 | (autoload 'doctor "doctor" "\ |
| 470 | Switch to *doctor* buffer and start giving psychotherapy." |
| 471 | t) |
| 472 | @end smallexample |
| 473 | |
| 474 | @noindent |
| 475 | The backslash and newline immediately following the double-quote are a |
| 476 | convention used only in the preloaded uncompiled Lisp files such as |
| 477 | @file{loaddefs.el}; they tell @code{make-docfile} to put the |
| 478 | documentation string in the @file{etc/DOC} file. @xref{Building Emacs}. |
| 479 | See also the commentary in @file{lib-src/make-docfile.c}. |
| 480 | |
| 481 | @node Repeated Loading |
| 482 | @section Repeated Loading |
| 483 | @cindex repeated loading |
| 484 | |
| 485 | You can load a given file more than once in an Emacs session. For |
| 486 | example, after you have rewritten and reinstalled a function definition |
| 487 | by editing it in a buffer, you may wish to return to the original |
| 488 | version; you can do this by reloading the file it came from. |
| 489 | |
| 490 | When you load or reload files, bear in mind that the @code{load} and |
| 491 | @code{load-library} functions automatically load a byte-compiled file |
| 492 | rather than a non-compiled file of similar name. If you rewrite a file |
| 493 | that you intend to save and reinstall, you need to byte-compile the new |
| 494 | version; otherwise Emacs will load the older, byte-compiled file instead |
| 495 | of your newer, non-compiled file! If that happens, the message |
| 496 | displayed when loading the file includes, @samp{(compiled; note, source is |
| 497 | newer)}, to remind you to recompile it. |
| 498 | |
| 499 | When writing the forms in a Lisp library file, keep in mind that the |
| 500 | file might be loaded more than once. For example, think about whether |
| 501 | each variable should be reinitialized when you reload the library; |
| 502 | @code{defvar} does not change the value if the variable is already |
| 503 | initialized. (@xref{Defining Variables}.) |
| 504 | |
| 505 | The simplest way to add an element to an alist is like this: |
| 506 | |
| 507 | @example |
| 508 | (setq minor-mode-alist |
| 509 | (cons '(leif-mode " Leif") minor-mode-alist)) |
| 510 | @end example |
| 511 | |
| 512 | @noindent |
| 513 | But this would add multiple elements if the library is reloaded. |
| 514 | To avoid the problem, write this: |
| 515 | |
| 516 | @example |
| 517 | (or (assq 'leif-mode minor-mode-alist) |
| 518 | (setq minor-mode-alist |
| 519 | (cons '(leif-mode " Leif") minor-mode-alist))) |
| 520 | @end example |
| 521 | |
| 522 | To add an element to a list just once, you can also use @code{add-to-list} |
| 523 | (@pxref{Setting Variables}). |
| 524 | |
| 525 | Occasionally you will want to test explicitly whether a library has |
| 526 | already been loaded. Here's one way to test, in a library, whether it |
| 527 | has been loaded before: |
| 528 | |
| 529 | @example |
| 530 | (defvar foo-was-loaded nil) |
| 531 | |
| 532 | (unless foo-was-loaded |
| 533 | @var{execute-first-time-only} |
| 534 | (setq foo-was-loaded t)) |
| 535 | @end example |
| 536 | |
| 537 | @noindent |
| 538 | If the library uses @code{provide} to provide a named feature, you can |
| 539 | use @code{featurep} earlier in the file to test whether the |
| 540 | @code{provide} call has been executed before. |
| 541 | @ifnottex |
| 542 | @xref{Named Features}. |
| 543 | @end ifnottex |
| 544 | |
| 545 | @node Named Features |
| 546 | @section Features |
| 547 | @cindex features |
| 548 | @cindex requiring features |
| 549 | @cindex providing features |
| 550 | |
| 551 | @code{provide} and @code{require} are an alternative to |
| 552 | @code{autoload} for loading files automatically. They work in terms of |
| 553 | named @dfn{features}. Autoloading is triggered by calling a specific |
| 554 | function, but a feature is loaded the first time another program asks |
| 555 | for it by name. |
| 556 | |
| 557 | A feature name is a symbol that stands for a collection of functions, |
| 558 | variables, etc. The file that defines them should @dfn{provide} the |
| 559 | feature. Another program that uses them may ensure they are defined by |
| 560 | @dfn{requiring} the feature. This loads the file of definitions if it |
| 561 | hasn't been loaded already. |
| 562 | |
| 563 | To require the presence of a feature, call @code{require} with the |
| 564 | feature name as argument. @code{require} looks in the global variable |
| 565 | @code{features} to see whether the desired feature has been provided |
| 566 | already. If not, it loads the feature from the appropriate file. This |
| 567 | file should call @code{provide} at the top level to add the feature to |
| 568 | @code{features}; if it fails to do so, @code{require} signals an error. |
| 569 | @cindex load error with require |
| 570 | |
| 571 | For example, in @file{emacs/lisp/prolog.el}, |
| 572 | the definition for @code{run-prolog} includes the following code: |
| 573 | |
| 574 | @smallexample |
| 575 | (defun run-prolog () |
| 576 | "Run an inferior Prolog process, with I/O via buffer *prolog*." |
| 577 | (interactive) |
| 578 | (require 'comint) |
| 579 | (switch-to-buffer (make-comint "prolog" prolog-program-name)) |
| 580 | (inferior-prolog-mode)) |
| 581 | @end smallexample |
| 582 | |
| 583 | @noindent |
| 584 | The expression @code{(require 'comint)} loads the file @file{comint.el} |
| 585 | if it has not yet been loaded. This ensures that @code{make-comint} is |
| 586 | defined. Features are normally named after the files that provide them, |
| 587 | so that @code{require} need not be given the file name. |
| 588 | |
| 589 | The @file{comint.el} file contains the following top-level expression: |
| 590 | |
| 591 | @smallexample |
| 592 | (provide 'comint) |
| 593 | @end smallexample |
| 594 | |
| 595 | @noindent |
| 596 | This adds @code{comint} to the global @code{features} list, so that |
| 597 | @code{(require 'comint)} will henceforth know that nothing needs to be |
| 598 | done. |
| 599 | |
| 600 | @cindex byte-compiling @code{require} |
| 601 | When @code{require} is used at top level in a file, it takes effect |
| 602 | when you byte-compile that file (@pxref{Byte Compilation}) as well as |
| 603 | when you load it. This is in case the required package contains macros |
| 604 | that the byte compiler must know about. It also avoids byte-compiler |
| 605 | warnings for functions and variables defined in the file loaded with |
| 606 | @code{require}. |
| 607 | |
| 608 | Although top-level calls to @code{require} are evaluated during |
| 609 | byte compilation, @code{provide} calls are not. Therefore, you can |
| 610 | ensure that a file of definitions is loaded before it is byte-compiled |
| 611 | by including a @code{provide} followed by a @code{require} for the same |
| 612 | feature, as in the following example. |
| 613 | |
| 614 | @smallexample |
| 615 | @group |
| 616 | (provide 'my-feature) ; @r{Ignored by byte compiler,} |
| 617 | ; @r{evaluated by @code{load}.} |
| 618 | (require 'my-feature) ; @r{Evaluated by byte compiler.} |
| 619 | @end group |
| 620 | @end smallexample |
| 621 | |
| 622 | @noindent |
| 623 | The compiler ignores the @code{provide}, then processes the |
| 624 | @code{require} by loading the file in question. Loading the file does |
| 625 | execute the @code{provide} call, so the subsequent @code{require} call |
| 626 | does nothing when the file is loaded. |
| 627 | |
| 628 | @defun provide feature |
| 629 | This function announces that @var{feature} is now loaded, or being |
| 630 | loaded, into the current Emacs session. This means that the facilities |
| 631 | associated with @var{feature} are or will be available for other Lisp |
| 632 | programs. |
| 633 | |
| 634 | The direct effect of calling @code{provide} is to add @var{feature} to |
| 635 | the front of the list @code{features} if it is not already in the list. |
| 636 | The argument @var{feature} must be a symbol. @code{provide} returns |
| 637 | @var{feature}. |
| 638 | |
| 639 | @smallexample |
| 640 | features |
| 641 | @result{} (bar bish) |
| 642 | |
| 643 | (provide 'foo) |
| 644 | @result{} foo |
| 645 | features |
| 646 | @result{} (foo bar bish) |
| 647 | @end smallexample |
| 648 | |
| 649 | When a file is loaded to satisfy an autoload, and it stops due to an |
| 650 | error in the evaluating its contents, any function definitions or |
| 651 | @code{provide} calls that occurred during the load are undone. |
| 652 | @xref{Autoload}. |
| 653 | @end defun |
| 654 | |
| 655 | @defun require feature &optional filename noerror |
| 656 | This function checks whether @var{feature} is present in the current |
| 657 | Emacs session (using @code{(featurep @var{feature})}; see below). The |
| 658 | argument @var{feature} must be a symbol. |
| 659 | |
| 660 | If the feature is not present, then @code{require} loads @var{filename} |
| 661 | with @code{load}. If @var{filename} is not supplied, then the name of |
| 662 | the symbol @var{feature} is used as the base file name to load. |
| 663 | However, in this case, @code{require} insists on finding @var{feature} |
| 664 | with an added suffix; a file whose name is just @var{feature} won't be |
| 665 | used. |
| 666 | |
| 667 | If loading the file fails to provide @var{feature}, @code{require} |
| 668 | signals an error, @samp{Required feature @var{feature} was not |
| 669 | provided}, unless @var{noerror} is non-@code{nil}. |
| 670 | @end defun |
| 671 | |
| 672 | @defun featurep feature |
| 673 | This function returns @code{t} if @var{feature} has been provided in the |
| 674 | current Emacs session (i.e., if @var{feature} is a member of |
| 675 | @code{features}.) |
| 676 | @end defun |
| 677 | |
| 678 | @defvar features |
| 679 | The value of this variable is a list of symbols that are the features |
| 680 | loaded in the current Emacs session. Each symbol was put in this list |
| 681 | with a call to @code{provide}. The order of the elements in the |
| 682 | @code{features} list is not significant. |
| 683 | @end defvar |
| 684 | |
| 685 | @node Unloading |
| 686 | @section Unloading |
| 687 | @cindex unloading |
| 688 | |
| 689 | @c Emacs 19 feature |
| 690 | You can discard the functions and variables loaded by a library to |
| 691 | reclaim memory for other Lisp objects. To do this, use the function |
| 692 | @code{unload-feature}: |
| 693 | |
| 694 | @deffn Command unload-feature feature &optional force |
| 695 | This command unloads the library that provided feature @var{feature}. |
| 696 | It undefines all functions, macros, and variables defined in that |
| 697 | library with @code{defun}, @code{defalias}, @code{defsubst}, |
| 698 | @code{defmacro}, @code{defconst}, @code{defvar}, and @code{defcustom}. |
| 699 | It then restores any autoloads formerly associated with those symbols. |
| 700 | (Loading saves these in the @code{autoload} property of the symbol.) |
| 701 | |
| 702 | Before restoring the previous definitions, @code{unload-feature} runs |
| 703 | @code{remove-hook} to remove functions in the library from certain |
| 704 | hooks. These hooks include variables whose names end in @samp{hook} or |
| 705 | @samp{-hooks}, plus those listed in @code{loadhist-special-hooks}. This |
| 706 | is to prevent Emacs from ceasing to function because important hooks |
| 707 | refer to functions that are no longer defined. |
| 708 | |
| 709 | @vindex @var{feature}-unload-hook |
| 710 | If these measures are not sufficient to prevent malfunction, a library |
| 711 | can define an explicit unload hook. If @code{@var{feature}-unload-hook} |
| 712 | is defined, it is run as a normal hook before restoring the previous |
| 713 | definitions, @emph{instead of} the usual hook-removing actions. The |
| 714 | unload hook ought to undo all the global state changes made by the |
| 715 | library that might cease to work once the library is unloaded. |
| 716 | @code{unload-feature} can cause problems with libraries that fail to do |
| 717 | this, so it should be used with caution. |
| 718 | |
| 719 | Ordinarily, @code{unload-feature} refuses to unload a library on which |
| 720 | other loaded libraries depend. (A library @var{a} depends on library |
| 721 | @var{b} if @var{a} contains a @code{require} for @var{b}.) If the |
| 722 | optional argument @var{force} is non-@code{nil}, dependencies are |
| 723 | ignored and you can unload any library. |
| 724 | @end deffn |
| 725 | |
| 726 | The @code{unload-feature} function is written in Lisp; its actions are |
| 727 | based on the variable @code{load-history}. |
| 728 | |
| 729 | @defvar load-history |
| 730 | This variable's value is an alist connecting library names with the |
| 731 | names of functions and variables they define, the features they provide, |
| 732 | and the features they require. |
| 733 | |
| 734 | Each element is a list and describes one library. The @sc{car} of the |
| 735 | list is the name of the library, as a string. The rest of the list is |
| 736 | composed of these kinds of objects: |
| 737 | |
| 738 | @itemize @bullet |
| 739 | @item |
| 740 | Symbols that were defined by this library. |
| 741 | @item |
| 742 | Lists of the form @code{(require . @var{feature})} indicating |
| 743 | features that were required. |
| 744 | @item |
| 745 | Lists of the form @code{(provide . @var{feature})} indicating |
| 746 | features that were provided. |
| 747 | @end itemize |
| 748 | |
| 749 | The value of @code{load-history} may have one element whose @sc{car} is |
| 750 | @code{nil}. This element describes definitions made with |
| 751 | @code{eval-buffer} on a buffer that is not visiting a file. |
| 752 | @end defvar |
| 753 | |
| 754 | The command @code{eval-region} updates @code{load-history}, but does so |
| 755 | by adding the symbols defined to the element for the file being visited, |
| 756 | rather than replacing that element. @xref{Eval}. |
| 757 | |
| 758 | Preloaded libraries don't contribute initially to @code{load-history}. |
| 759 | Instead, preloading writes information about preloaded libraries into a |
| 760 | file, which can be loaded later on to add information to |
| 761 | @code{load-history} describing the preloaded files. This file is |
| 762 | installed in @code{exec-directory} and has a name of the form |
| 763 | @file{fns-@var{emacsversion}.el}. |
| 764 | |
| 765 | @findex symbol-file |
| 766 | See the source for the function @code{symbol-file}, for an example of |
| 767 | code that loads this file to find functions in preloaded libraries. |
| 768 | |
| 769 | @defvar loadhist-special-hooks |
| 770 | This variable holds a list of hooks to be scanned before unloading a |
| 771 | library, to remove functions defined in the library. |
| 772 | @end defvar |
| 773 | |
| 774 | @node Hooks for Loading |
| 775 | @section Hooks for Loading |
| 776 | @cindex loading hooks |
| 777 | @cindex hooks for loading |
| 778 | |
| 779 | You can ask for code to be executed if and when a particular library is |
| 780 | loaded, by calling @code{eval-after-load}. |
| 781 | |
| 782 | @defun eval-after-load library form |
| 783 | This function arranges to evaluate @var{form} at the end of loading the |
| 784 | library @var{library}, if and when @var{library} is loaded. If |
| 785 | @var{library} is already loaded, it evaluates @var{form} right away. |
| 786 | |
| 787 | The library name @var{library} must exactly match the argument of |
| 788 | @code{load}. To get the proper results when an installed library is |
| 789 | found by searching @code{load-path}, you should not include any |
| 790 | directory names in @var{library}. |
| 791 | |
| 792 | An error in @var{form} does not undo the load, but does prevent |
| 793 | execution of the rest of @var{form}. |
| 794 | @end defun |
| 795 | |
| 796 | In general, well-designed Lisp programs should not use this feature. |
| 797 | The clean and modular ways to interact with a Lisp library are (1) |
| 798 | examine and set the library's variables (those which are meant for |
| 799 | outside use), and (2) call the library's functions. If you wish to |
| 800 | do (1), you can do it immediately---there is no need to wait for when |
| 801 | the library is loaded. To do (2), you must load the library (preferably |
| 802 | with @code{require}). |
| 803 | |
| 804 | But it is OK to use @code{eval-after-load} in your personal |
| 805 | customizations if you don't feel they must meet the design standards for |
| 806 | programs meant for wider use. |
| 807 | |
| 808 | @defvar after-load-alist |
| 809 | This variable holds an alist of expressions to evaluate if and when |
| 810 | particular libraries are loaded. Each element looks like this: |
| 811 | |
| 812 | @example |
| 813 | (@var{filename} @var{forms}@dots{}) |
| 814 | @end example |
| 815 | |
| 816 | The function @code{load} checks @code{after-load-alist} in order to |
| 817 | implement @code{eval-after-load}. |
| 818 | @end defvar |
| 819 | |
| 820 | @c Emacs 19 feature |