How Programs Do Loading

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 Features). Ultimately, all these facilities call the load function to do the work.

Function: load filename &optional missing-ok nomessage nosuffix must-suffix

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, and Emacs was compiled with native-compilation support (see Compilation of Lisp to Native Code), load attempts to find a corresponding ‘.eln’ file, and if found, loads it instead of filename.elc. Otherwise, it loads filename.elc (and starts a background native compilation to produce the missing ‘.eln’ file, followed by loading that file). If there is no filename.elc, 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 Emacs Dynamic Modules), load next looks for a file named filename.ext, where ext is a system-dependent file-name extension of shared libraries (‘.so’ on GNU and Unix systems). 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. In this case, load doesn’t load the ‘.eln’ native-compiled file even if it exists.

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. Emacs will issue an error about (‘Eager macro-expansion skipped due to cycle…’) giving details of the problem. You have 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 Macros and Byte Compilation.

Messages like ‘Loading foo...’ and ‘Loading foo...done’ appear in the echo area during loading unless nomessage is non-nil. If a native-compiled ‘.eln’ file is loaded, the message says so.

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.

Command: load-file filename

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.

Command: load-library library

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.

Variable: load-in-progress

This variable is non-nil if Emacs is in the process of loading a file, and it is nil otherwise.

Variable: load-file-name

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.

Variable: load-read-function

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.

Function: load-path-filter-cache-directory-files path filename suffixes

This function filters path to remove any directories that could not hold filename with any of suffixes, and returns the filtered list of directories. The function caches the directories it scans and the files inside them, and uses the cache in subsequent calls, which speeds up repeated lookups of files in path.

Variable: load-path-filter-function

If this variable names a function, load will call that function when it scans load-path to find files. The function will be called with 3 arguments: the value of load-path, filename, the name of a file being looked up as passed to load, and a list of suffixes to append to filename. It should return a shorter list of directories where filename can reside, thus making the lookup faster. The function load-path-filter-cache-directory-files is a good candidate to be such a function.

For information about how load is used in building Emacs, see Building Emacs.