Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file.
The load functions evaluate all the expressions in a file just
as the eval-current-buffer function evaluates all the
expressions in a buffer. The difference is that the load functions
read and evaluate the text in the file as found on disk, not the text
in an Emacs buffer.
The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a top-level form. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions.
A file containing Lisp code is often called a library. Thus, the "Rmail library" is a file containing code for Rmail mode. Similarly, a "Lisp library directory" is a directory of files containing Lisp code.
load function and others.
Emacs Lisp has several interfaces for loading. For example,
autoload creates a placeholder object for a function in a file;
trying to call the autoloading function loads the file to get the
function's real definition (see section Autoload). require loads a
file if it isn't already loaded (see section Features). Ultimately, all
these facilities call the load function to do the work.
To find the file, load first looks for a file named
`filename.elc', that is, for a file whose name is
filename with `.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.
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 the optional argument nosuffix is non-nil, then the
suffixes `.elc' and `.el' are not tried. In this case, you
must specify the precise file name you want.
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.
If you get a warning that `foo.elc' is older than `foo.el', it means you should consider recompiling `foo.el'. See section Byte Compilation.
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 the
error file-error (with `Cannot open load file
filename'). But if missing-ok is non-nil, then
load just returns nil.
load returns t if the file loads successfully.
load. Each element is a string (which must be
a directory name) or nil (which stands for the current working
directory). The value of load-path is initialized from the
environment variable EMACSLOADPATH, if that exists; otherwise its
default value is specified in `emacs/src/paths.h' when Emacs is
built.
The syntax of EMACSLOADPATH is the same as used for PATH;
`:' separates directory names, and `.' is used for the current
default directory. Here is an example of how to set your
EMACSLOADPATH variable from a csh `.login' file:
setenv EMACSLOADPATH .:/user/bil/emacs:/usr/lib/emacs/lisp
Here is how to set it using sh:
export EMACSLOADPATH EMACSLOADPATH=.:/user/bil/emacs:/usr/local/lib/emacs/lisp
Here is an example of code you can place in a `.emacs' file to add
several directories to the front of your default load-path:
(setq load-path
(append (list nil "/user/bil/emacs"
"/usr/local/lisplib"
(expand-file-name "~/emacs"))
load-path))
In this example, the path searches the current working directory first, followed then by the `/user/bil/emacs' directory and then by the `/usr/local/lisplib' directory, which are then followed by the standard directories for Lisp code.
The command line options `-l' or `-load' specify a Lisp
library to load as part of Emacs startup. Since this file might be in
the current directory, Emacs 18 temporarily adds the current directory
to the front of load-path so the file can be found there. Newer
Emacs versions also find such files in the current directory, but
without altering load-path.
nil if Emacs is in the process of loading a
file, and it is nil otherwise. This is how defun and
provide determine whether a load is in progress, so that their
effect can be undone if the load fails.
To learn how load is used to build Emacs, see section Building Emacs.
The autoload facility allows you to make a function or macro available but put off loading its actual definition. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along.
There are two ways to set up an autoloaded function: by calling
autoload, and by writing a special "magic" comment in the
source before the real definition. autoload is the low-level
primitive for autoloading; any Lisp program can call autoload at
any time. Magic comments do nothing on their own; they serve as a guide
for the command update-file-autoloads, which constructs calls to
autoload and arranges to execute them when Emacs is built. Magic
comments are the most convenient way to make a function autoload, but
only for packages installed along with Emacs.
The argument docstring is the documentation string for the
function. Normally, this is the identical to the documentation string
in the function definition itself. Specifying the documentation string
in the call to autoload makes it possible to look at the
documentation without loading the function's real definition.
If interactive is non-nil, then the function can be called
interactively. This lets completion in M-x work without loading
the function's real definition. The complete interactive specification
need not be given here; it's not needed unless the user actually calls
function, and when that happens, it's time to load the real
definition.
You can autoload macros and keymaps as well as ordinary functions.
Specify type as macro if function is really a macro.
Specify type as keymap if function is really a
keymap. Various parts of Emacs need to know this information without
loading the real definition.
If function already has a non-void function definition that is not
an autoload object, autoload does nothing and returns nil.
If the function cell of function is void, or is already an autoload
object, then it is defined as an autoload object like this:
(autoload filename docstring interactive type)
For example,
(symbol-function 'run-prolog)
=> (autoload "prolog" 169681 t nil)
In this case, "prolog" is the name of the file to load, 169681
refers to the documentation string in the `emacs/etc/DOC' file
(see section Documentation Basics), t means the function is
interactive, and nil that it is not a macro or a keymap.
The autoloaded file usually contains other definitions and may require
or provide one or more features. If the file is not completely loaded
(due to an error in the evaluation of its contents), any function
definitions or provide calls that occurred during the load are
undone. This is to ensure that the next attempt to call any function
autoloading from this file will try again to load the file. If not for
this, then some of the functions in the file might appear defined, but
they might fail to work properly for the lack of certain subroutines
defined later in the file and not loaded successfully.
If the autoloaded file fails to define the desired Lisp function or
macro, then an error is signaled with data "Autoloading failed to
define function function-name".
A magic autoload comment looks like `;;;###autoload', on a line
by itself, just before the real definition of the function in its
autoloadable source file. The command M-x update-file-autoloads
writes a corresponding autoload call into `loaddefs.el'.
Building Emacs loads `loaddefs.el' and thus calls autoload.
M-x update-directory-autoloads is even more powerful; it updates
autoloads for all files in the current directory.
The same magic comment can copy any kind of form into
`loaddefs.el'. If the form following the magic comment is not a
function definition, it is copied verbatim. You can also use a magic
comment to execute a form at build time without executing it when
the file itself is loaded. To do this, write the form on the same
line as the magic comment. Since it is in a comment, it does nothing
when you load the source file; but update-file-autoloads copies
it to `loaddefs.el', where it is executed while building Emacs.
The following example shows how doctor is prepared for
autoloading with a magic comment:
;;;###autoload (defun doctor () "Switch to *doctor* buffer and start giving psychotherapy." (interactive) (switch-to-buffer "*doctor*") (doctor-mode))
Here's what that produces in `loaddefs.el':
(autoload 'doctor "doctor" "\ Switch to *doctor* buffer and start giving psychotherapy." t)
The backslash and newline immediately following the double-quote are a
convention used only in the preloaded Lisp files such as
`loaddefs.el'; they tell make-docfile to put the
documentation string in the `etc/DOC' file. See section Building Emacs.
You may load one file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.
When you load or reload files, bear in mind that the load and
load-library functions automatically load a byte-compiled file
rather than a non-compiled file of similar name. If you rewrite a file
that you intend to save and reinstall, remember to byte-compile it if
necessary; otherwise you may find yourself inadvertently reloading the
older, byte-compiled file instead of your newer, non-compiled file!
When writing the forms in a Lisp library file, keep in mind that the
file might be loaded more than once. For example, the choice of
defvar vs. defconst for defining a variable depends on
whether it is desirable to reinitialize the variable if the library is
reloaded: defconst does so, and defvar does not.
(See section Defining Global Variables.)
The simplest way to add an element to an alist is like this:
(setq minor-mode-alist
(cons '(leif-mode " Leif") minor-mode-alist))
But this would add multiple elements if the library is reloaded. To avoid the problem, write this:
(or (assq 'leif-mode minor-mode-alist)
(setq minor-mode-alist
(cons '(leif-mode " Leif") minor-mode-alist)))
Occasionally you will want to test explicitly whether a library has already been loaded. Here's one way to test, in a library, whether it has been loaded before:
(if (not (boundp 'foo-was-loaded))
execute-first-time-only)
(setq foo-was-loaded t)
If the library uses provide to provide a named feature, you can
use featurep to test whether the library has been loaded.
provide and require are an alternative to
autoload for loading files automatically. They work in terms of
named features. Autoloading is triggered by calling a specific
function, but a feature is loaded the first time another program asks
for it by name.
A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should provide the feature. Another program that uses them may ensure they are defined by requiring the feature. This loads the file of definitions if it hasn't been loaded already.
To require the presence of a feature, call require with the
feature name as argument. require looks in the global variable
features to see whether the desired feature has been provided
already. If not, it loads the feature from the appropriate file. This
file should call provide at the top level to add the feature to
features; if it fails to do so, require signals an error.
Features are normally named after the files that provide them, so that
require need not be given the file name.
For example, in `emacs/lisp/prolog.el',
the definition for run-prolog includes the following code:
(defun run-prolog () "Run an inferior Prolog process, input and output via buffer *prolog*." (interactive) (require 'comint) (switch-to-buffer (make-comint "prolog" prolog-program-name)) (inferior-prolog-mode))
The expression (require 'comint) loads the file `comint.el'
if it has not yet been loaded. This ensures that make-comint is
defined.
The `comint.el' file contains the following top-level expression:
(provide 'comint)
This adds comint to the global features list, so that
(require 'comint) will henceforth know that nothing needs to be
done.
When require is used at top level in a file, it takes effect
when you byte-compile that file (see section Byte Compilation) as well as
when you load it. This is in case the required package contains macros
that the byte compiler must know about.
Although top-level calls to require are evaluated during
byte compilation, provide calls are not. Therefore, you can
ensure that a file of definitions is loaded before it is byte-compiled
by including a provide followed by a require for the same
feature, as in the following example.
(provide 'my-feature) ; Ignored by byte compiler,
; evaluated by load.
(require 'my-feature) ; Evaluated by byte compiler.
The compiler ignores the provide, then processes the
require by loading the file in question. Loading the file does
execute the provide call, so the subsequent require call
does nothing while loading.
The direct effect of calling provide is to add feature to
the front of the list features if it is not already in the list.
The argument feature must be a symbol. provide returns
feature.
features
=> (bar bish)
(provide 'foo)
=> foo
features
=> (foo bar bish)
If the file isn't completely loaded, due to an error in the evaluating
its contents, any function definitions or provide calls that
occurred during the load are undone. See section Autoload.
(featurep feature); see below). If it
is not, then require loads filename with load. If
filename is not supplied, then the name of the symbol
feature is used as the file name to load.
If loading the file fails to provide feature, require
signals an error, `Required feature feature was not
provided'.
t if feature has been provided in the
current Emacs session (i.e., feature is a member of
features.)
provide. The order of the elements in the
features list is not significant.
You can discard the functions and variables loaded by a library to
reclaim memory for other Lisp objects. To do this, use the function
unload-feature:
defconst, defvar, defun,
defmacro, defsubst and defalias. It then restores
any autoloads formerly associated with those symbols.
The unload-feature function is written in Lisp; its actions are
based on the variable load-history.
Each element is a list and describes one library. The CAR of the list is the name of the library, as a string. The rest of the list is composed of these kinds of objects:
(require . feature) indicating
features that were required.
(provide . feature) indicating
features that were provided.
The value of load-history may have one element whose CAR is
nil. This element describes definitions made with
eval-buffer on a buffer that is not visiting a file.
The command eval-region updates load-history, but does so
by adding the symbols defined to the element for the file being visited,
rather than replacing that element.
You can ask for code to be executed if and when a particular library is
loaded, by calling eval-after-load.
The library name library must exactly match the argument of
load. To get the proper results when an installed library is
found by searching load-path, you should not include any
directory names in library.
An error in form does not undo the load, but does prevent execution of the rest of form.
(filename forms...)
The function load checks after-load-alist in order to
implement eval-after-load.