Emacs Lisp has several interfaces for loading. For example, autoload creates a placeholder object for a function defined in a file; trying to call the autoloading function loads the file to get the function’s real definition (see Autoload). require loads a file if it isn’t already loaded (see Named Features). Ultimately, all these facilities call the load function to do the work.
This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file.
To find the file, load first looks for a file named filename.elc, that is, for a file whose name is filename with the extension ‘.elc’ appended. If such a file exists, it is loaded. If there is no file by that name, then load looks for a file named filename.el. If that file exists, it is loaded. If Emacs was compiled with support for dynamic modules (see Dynamic Modules), load next looks for a file named filename.ext, where ext is a system-dependent file-name extension of shared libraries. Finally, if neither of those names is found, load looks for a file named filename with nothing appended, and loads it if it exists. (The load function is not clever about looking at filename. In the perverse case of a file named foo.el.el, evaluation of (load "foo.el") will indeed find it.)
If Auto Compression mode is enabled, as it is by default, then if load can not find a file, it searches for a compressed version of the file before trying other file names. It decompresses and loads it if it exists. It looks for compressed versions by appending each of the suffixes in jka-compr-load-suffixes to the file name. The value of this variable must be a list of strings. Its standard value is (".gz").
If the optional argument nosuffix is non-nil, then load does not try the suffixes ‘.elc’ and ‘.el’. In this case, you must specify the precise file name you want, except that, if Auto Compression mode is enabled, load will still use jka-compr-load-suffixes to find compressed versions. By specifying the precise file name and using t for nosuffix, you can prevent file names like foo.el.el from being tried.
If the optional argument must-suffix is non-nil, then load insists that the file name used must end in either ‘.el’ or ‘.elc’ (possibly extended with a compression suffix) or the shared-library extension, unless it contains an explicit directory name.
If the option load-prefer-newer is non-nil, then when searching suffixes, load selects whichever version of a file (‘.elc’, ‘.el’, etc.) has been modified most recently.
If filename is a relative file name, such as foo or baz/foo.bar, load searches for the file using the variable load-path. It appends filename to each of the directories listed in load-path, and loads the first file it finds whose name matches. The current default directory is tried only if it is specified in load-path, where nil stands for the default directory. load tries all three possible suffixes in the first directory in load-path, then all three suffixes in the second directory, and so on. See Library Search.
Whatever the name under which the file is eventually found, and the directory where Emacs found it, Emacs sets the value of the variable load-file-name to that file’s name.
If you get a warning that foo.elc is older than foo.el, it means you should consider recompiling foo.el. See Byte Compilation.
When loading a source file (not compiled), load performs character set translation just as Emacs would do when visiting the file. See Coding Systems.
When loading an uncompiled file, Emacs tries to expand any macros that the file contains (see Macros). We refer to this as eager macro expansion. Doing this (rather than deferring the expansion until the relevant code runs) can significantly speed up the execution of uncompiled code. Sometimes, this macro expansion cannot be done, owing to a cyclic dependency. In the simplest example of this, the file you are loading refers to a macro defined in another file, and that file in turn requires the file you are loading. This is generally harmless. Emacs prints a warning (‘Eager macro-expansion skipped due to cycle…’) giving details of the problem, but it still loads the file, just leaving the macro unexpanded for now. You may wish to restructure your code so that this does not happen. Loading a compiled file does not cause macroexpansion, because this should already have happened during compilation. See Compiling Macros.
Messages like ‘Loading foo...’ and ‘Loading foo...done’ appear in the echo area during loading unless nomessage is non-nil.
Any unhandled errors while loading a file terminate loading. If the load was done for the sake of autoload, any function definitions made during the loading are undone.
If load can’t find the file to load, then normally it signals a file-error (with ‘Cannot open load file filename’). But if missing-ok is non-nil, then load just returns nil.
You can use the variable load-read-function to specify a function for load to use instead of read for reading expressions. See below.
load returns t if the file loads successfully.
This command loads the file filename. If filename is a relative file name, then the current default directory is assumed. This command does not use load-path, and does not append suffixes. However, it does look for compressed versions (if Auto Compression Mode is enabled). Use this command if you wish to specify precisely the file name to load.
This command loads the library named library. It is equivalent to load, except for the way it reads its argument interactively. See Lisp Libraries in The GNU Emacs Manual.
This variable is non-nil if Emacs is in the process of loading a file, and it is nil otherwise.
When Emacs is in the process of loading a file, this variable’s value is the name of that file, as Emacs found it during the search described earlier in this section.
This variable specifies an alternate expression-reading function for load and eval-region to use instead of read. The function should accept one argument, just as read does.
By default, this variable’s value is read. See Input Functions.
Instead of using this variable, it is cleaner to use another, newer feature: to pass the function as the read-function argument to eval-region. See Eval.
For information about how load is used in building Emacs, see Building Emacs.
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Licensed under the GNU GPL license.
https://www.gnu.org/software/emacs/manual/html_node/elisp/How-Programs-Do-Loading.html