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[emacs.git] / lispref / loading.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
@c   Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/loading
@node Loading, Byte Compilation, Customization, Top
@chapter Loading
@cindex loading
@cindex library
@cindex Lisp library

  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 @code{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.

@cindex top-level form
  The loaded file must contain Lisp expressions, either as source code
or as byte-compiled code.  Each form in the file is called a
@dfn{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 @dfn{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.

@menu
* How Programs Do Loading::     The @code{load} function and others.
* Library Search::              Finding a library to load.
* Loading Non-ASCII::           Non-@sc{ascii} characters in Emacs Lisp files.
* Autoload::                    Setting up a function to autoload.
* Repeated Loading::            Precautions about loading a file twice.
* Named Features::              Loading a library if it isn't already loaded.
* Unloading::                   How to ``unload'' a library that was loaded.
* Hooks for Loading::           Providing code to be run when
                                  particular libraries are loaded.
@end menu

@node How Programs Do Loading
@section How Programs Do Loading

  Emacs Lisp has several interfaces for loading.  For example,
@code{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 (@pxref{Autoload}).  @code{require} loads a
file if it isn't already loaded (@pxref{Named Features}).  Ultimately,
all these facilities call the @code{load} function to do the work.

@defun 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, @code{load} first looks for a file named
@file{@var{filename}.elc}, that is, for a file whose name is
@var{filename} with @samp{.elc} appended.  If such a file exists, it is
loaded.  If there is no file by that name, then @code{load} looks for a
file named @file{@var{filename}.el}.  If that file exists, it is loaded.
Finally, if neither of those names is found, @code{load} looks for a
file named @var{filename} with nothing appended, and loads it if it
exists.  (The @code{load} function is not clever about looking at
@var{filename}.  In the perverse case of a file named @file{foo.el.el},
evaluation of @code{(load "foo.el")} will indeed find it.)

If the optional argument @var{nosuffix} is non-@code{nil}, then the
suffixes @samp{.elc} and @samp{.el} are not tried.  In this case, you
must specify the precise file name you want.  By specifying the precise
file name and using @code{t} for @var{nosuffix}, you can prevent
perverse file names such as @file{foo.el.el} from being tried.

If the optional argument @var{must-suffix} is non-@code{nil}, then
@code{load} insists that the file name used must end in either
@samp{.el} or @samp{.elc}, unless it contains an explicit directory
name.  If @var{filename} does not contain an explicit directory name,
and does not end in a suffix, then @code{load} insists on adding one.

If @var{filename} is a relative file name, such as @file{foo} or
@file{baz/foo.bar}, @code{load} searches for the file using the variable
@code{load-path}.  It appends @var{filename} to each of the directories
listed in @code{load-path}, and loads the first file it finds whose name
matches.  The current default directory is tried only if it is specified
in @code{load-path}, where @code{nil} stands for the default directory.
@code{load} tries all three possible suffixes in the first directory in
@code{load-path}, then all three suffixes in the second directory, and
so on.  @xref{Library Search}.

If you get a warning that @file{foo.elc} is older than @file{foo.el}, it
means you should consider recompiling @file{foo.el}.  @xref{Byte
Compilation}.

When loading a source file (not compiled), @code{load} performs
character set translation just as Emacs would do when visiting the file.
@xref{Coding Systems}.

Messages like @samp{Loading foo...} and @samp{Loading foo...done} appear
in the echo area during loading unless @var{nomessage} is
non-@code{nil}.

@cindex load errors
Any unhandled errors while loading a file terminate loading.  If the
load was done for the sake of @code{autoload}, any function definitions
made during the loading are undone.

@kindex file-error
If @code{load} can't find the file to load, then normally it signals the
error @code{file-error} (with @samp{Cannot open load file
@var{filename}}).  But if @var{missing-ok} is non-@code{nil}, then
@code{load} just returns @code{nil}.

You can use the variable @code{load-read-function} to specify a function
for @code{load} to use instead of @code{read} for reading expressions.
See below.

@code{load} returns @code{t} if the file loads successfully.
@end defun

@deffn Command load-file filename
This command loads the file @var{filename}.  If @var{filename} is a
relative file name, then the current default directory is assumed.
@code{load-path} is not used, and suffixes are not appended.  Use this
command if you wish to specify precisely the file name to load.
@end deffn

@deffn Command load-library library
This command loads the library named @var{library}.  It is equivalent to
@code{load}, except in how it reads its argument interactively.
@end deffn

@defvar load-in-progress
This variable is non-@code{nil} if Emacs is in the process of loading a
file, and it is @code{nil} otherwise.
@end defvar

@defvar load-read-function
This variable specifies an alternate expression-reading function for
@code{load} and @code{eval-region} to use instead of @code{read}.
The function should accept one argument, just as @code{read} does.

Normally, the variable's value is @code{nil}, which means those
functions should use @code{read}.

@strong{Note:} Instead of using this variable, it is cleaner to use
another, newer feature: to pass the function as the @var{read-function}
argument to @code{eval-region}.  @xref{Eval}.
@end defvar

  For information about how @code{load} is used in building Emacs, see
@ref{Building Emacs}.

@node Library Search
@section Library Search

  When Emacs loads a Lisp library, it searches for the library
in a list of directories specified by the variable @code{load-path}.

@defopt load-path
@cindex @code{EMACSLOADPATH} environment variable
The value of this variable is a list of directories to search when
loading files with @code{load}.  Each element is a string (which must be
a directory name) or @code{nil} (which stands for the current working
directory).
@end defopt

  The value of @code{load-path} is initialized from the environment
variable @code{EMACSLOADPATH}, if that exists; otherwise its default
value is specified in @file{emacs/src/paths.h} when Emacs is built.
Then the list is expanded by adding subdirectories of the directories
in the list.

  The syntax of @code{EMACSLOADPATH} is the same as used for @code{PATH};
@samp{:} (or @samp{;}, according to the operating system) separates
directory names, and @samp{.} is used for the current default directory.
Here is an example of how to set your @code{EMACSLOADPATH} variable from
a @code{csh} @file{.login} file:

@smallexample
setenv EMACSLOADPATH .:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp
@end smallexample

  Here is how to set it using @code{sh}:

@smallexample
export EMACSLOADPATH
EMACSLOADPATH=.:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp
@end smallexample

  Here is an example of code you can place in your init file (@pxref{Init
File}) to add several directories to the front of your default
@code{load-path}:

@smallexample
@group
(setq load-path
      (append (list nil "/user/bil/emacs"
                    "/usr/local/lisplib"
                    "~/emacs")
              load-path))
@end group
@end smallexample

@c Wordy to rid us of an overfull hbox.  --rjc 15mar92
@noindent
In this example, the path searches the current working directory first,
followed then by the @file{/user/bil/emacs} directory, the
@file{/usr/local/lisplib} directory, and the @file{~/emacs} directory,
which are then followed by the standard directories for Lisp code.

  Dumping Emacs uses a special value of @code{load-path}.  If the value of
@code{load-path} at the end of dumping is unchanged (that is, still the
same special value), the dumped Emacs switches to the ordinary
@code{load-path} value when it starts up, as described above.  But if
@code{load-path} has any other value at the end of dumping, that value
is used for execution of the dumped Emacs also.

  Therefore, if you want to change @code{load-path} temporarily for
loading a few libraries in @file{site-init.el} or @file{site-load.el},
you should bind @code{load-path} locally with @code{let} around the
calls to @code{load}.

  The default value of @code{load-path}, when running an Emacs which has
been installed on the system, includes two special directories (and
their subdirectories as well):

@smallexample
"/usr/local/share/emacs/@var{version}/site-lisp"
@end smallexample

@noindent
and

@smallexample
"/usr/local/share/emacs/site-lisp"
@end smallexample

@noindent
The first one is for locally installed packages for a particular Emacs
version; the second is for locally installed packages meant for use with
all installed Emacs versions.

  There are several reasons why a Lisp package that works well in one
Emacs version can cause trouble in another.  Sometimes packages need
updating for incompatible changes in Emacs; sometimes they depend on
undocumented internal Emacs data that can change without notice;
sometimes a newer Emacs version incorporates a version of the package,
and should be used only with that version.

  Emacs finds these directories' subdirectories and adds them to
@code{load-path} when it starts up.  Both immediate subdirectories and
subdirectories multiple levels down are added to @code{load-path}.

  Not all subdirectories are included, though.  Subdirectories whose
names do not start with a letter or digit are excluded.  Subdirectories
named @file{RCS} or @file{CVS} are excluded.  Also, a subdirectory which
contains a file named @file{.nosearch} is excluded.  You can use these
methods to prevent certain subdirectories of the @file{site-lisp}
directories from being searched.

  If you run Emacs from the directory where it was built---that is, an
executable that has not been formally installed---then @code{load-path}
normally contains two additional directories.  These are the @code{lisp}
and @code{site-lisp} subdirectories of the main build directory.  (Both
are represented as absolute file names.)

@deffn Command locate-library library &optional nosuffix path interactive-call
This command finds the precise file name for library @var{library}.  It
searches for the library in the same way @code{load} does, and the
argument @var{nosuffix} has the same meaning as in @code{load}: don't
add suffixes @samp{.elc} or @samp{.el} to the specified name
@var{library}.

If the @var{path} is non-@code{nil}, that list of directories is used
instead of @code{load-path}.

When @code{locate-library} is called from a program, it returns the file
name as a string.  When the user runs @code{locate-library}
interactively, the argument @var{interactive-call} is @code{t}, and this
tells @code{locate-library} to display the file name in the echo area.
@end deffn

@node Loading Non-ASCII
@section Loading Non-@sc{ascii} Characters

  When Emacs Lisp programs contain string constants with non-@sc{ascii}
characters, these can be represented within Emacs either as unibyte
strings or as multibyte strings (@pxref{Text Representations}).  Which
representation is used depends on how the file is read into Emacs.  If
it is read with decoding into multibyte representation, the text of the
Lisp program will be multibyte text, and its string constants will be
multibyte strings.  If a file containing Latin-1 characters (for
example) is read without decoding, the text of the program will be
unibyte text, and its string constants will be unibyte strings.
@xref{Coding Systems}.

  To make the results more predictable, Emacs always performs decoding
into the multibyte representation when loading Lisp files, even if it
was started with the @samp{--unibyte} option.  This means that string
constants with non-@sc{ascii} characters translate into multibyte
strings.  The only exception is when a particular file specifies no
decoding.

  The reason Emacs is designed this way is so that Lisp programs give
predictable results, regardless of how Emacs was started.  In addition,
this enables programs that depend on using multibyte text to work even
in a unibyte Emacs.  Of course, such programs should be designed to
notice whether the user prefers unibyte or multibyte text, by checking
@code{default-enable-multibyte-characters}, and convert representations
appropriately.

  In most Emacs Lisp programs, the fact that non-@sc{ascii} strings are
multibyte strings should not be noticeable, since inserting them in
unibyte buffers converts them to unibyte automatically.  However, if
this does make a difference, you can force a particular Lisp file to be
interpreted as unibyte by writing @samp{-*-unibyte: t;-*-} in a
comment on the file's first line.  With that designator, the file will
unconditionally be interpreted as unibyte, even in an ordinary
multibyte Emacs session.  This can matter when making keybindings to
non-@sc{ascii} characters written as @code{?v@var{literal}}.

@node Autoload
@section Autoload
@cindex autoload

  The @dfn{autoload} facility allows you to make a function or macro
known in Lisp, but put off loading the file that defines it.  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
@code{autoload}, and by writing a special ``magic'' comment in the
source before the real definition.  @code{autoload} is the low-level
primitive for autoloading; any Lisp program can call @code{autoload} at
any time.  Magic comments are the most convenient way to make a function
autoload, for packages installed along with Emacs.  These comments do
nothing on their own, but they serve as a guide for the command
@code{update-file-autoloads}, which constructs calls to @code{autoload}
and arranges to execute them when Emacs is built.

@defun autoload function filename &optional docstring interactive type
This function defines the function (or macro) named @var{function} so as
to load automatically from @var{filename}.  The string @var{filename}
specifies the file to load to get the real definition of @var{function}.

If @var{filename} does not contain either a directory name, or the
suffix @code{.el} or @code{.elc}, then @code{autoload} insists on adding
one of these suffixes, and it will not load from a file whose name is
just @var{filename} with no added suffix.

The argument @var{docstring} is the documentation string for the
function.  Normally, this should be identical to the documentation string
in the function definition itself.  Specifying the documentation string
in the call to @code{autoload} makes it possible to look at the
documentation without loading the function's real definition.

If @var{interactive} is non-@code{nil}, that says @var{function} can be
called interactively.  This lets completion in @kbd{M-x} work without
loading @var{function}'s real definition.  The complete interactive
specification is not given here; it's not needed unless the user
actually calls @var{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 @var{type} as @code{macro} if @var{function} is really a macro.
Specify @var{type} as @code{keymap} if @var{function} is really a
keymap.  Various parts of Emacs need to know this information without
loading the real definition.

An autoloaded keymap loads automatically during key lookup when a prefix
key's binding is the symbol @var{function}.  Autoloading does not occur
for other kinds of access to the keymap.  In particular, it does not
happen when a Lisp program gets the keymap from the value of a variable
and calls @code{define-key}; not even if the variable name is the same
symbol @var{function}.

@cindex function cell in autoload
If @var{function} already has a non-void function definition that is not
an autoload object, @code{autoload} does nothing and returns @code{nil}.
If the function cell of @var{function} is void, or is already an autoload
object, then it is defined as an autoload object like this:

@example
(autoload @var{filename} @var{docstring} @var{interactive} @var{type})
@end example

For example, 

@example
@group
(symbol-function 'run-prolog)
     @result{} (autoload "prolog" 169681 t nil)
@end group
@end example

@noindent
In this case, @code{"prolog"} is the name of the file to load, 169681
refers to the documentation string in the
@file{emacs/etc/DOC-@var{version}} file (@pxref{Documentation Basics}),
@code{t} means the function is interactive, and @code{nil} that it is
not a macro or a keymap.
@end defun

@cindex autoload errors
  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 @code{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 be defined by the
aborted load, but fail to work properly for the lack of certain
subroutines not loaded successfully because they come later in the file.

  If the autoloaded file fails to define the desired Lisp function or
macro, then an error is signaled with data @code{"Autoloading failed to
define function @var{function-name}"}.

@findex update-file-autoloads
@findex update-directory-autoloads
  A magic autoload comment consists of @samp{;;;###autoload}, on a line
by itself, just before the real definition of the function in its
autoloadable source file.  The command @kbd{M-x update-file-autoloads}
writes a corresponding @code{autoload} call into @file{loaddefs.el}.
Building Emacs loads @file{loaddefs.el} and thus calls @code{autoload}.
@kbd{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
@file{loaddefs.el}.  If the form following the magic comment is not a
function-defining form or a @code{defcustom} form, it is copied
verbatim.  ``Function-defining forms'' include @code{define-skeleton},
@code{define-derived-mode}, @code{define-generic-mode} and
@code{define-minor-mode} as well as @code{defun} and
@code{defmacro}.  To save space, a @code{defcustom} form is converted to
a @code{defvar} in @file{loaddefs.el}, with some additional information
if it uses @code{:require}.

  You can also use a magic comment to execute a form at build time
@emph{without} executing it when the file itself is loaded.  To do this,
write the form @emph{on the same line} as the magic comment.  Since it
is in a comment, it does nothing when you load the source file; but
@kbd{M-x update-file-autoloads} copies it to @file{loaddefs.el}, where
it is executed while building Emacs.

  The following example shows how @code{doctor} is prepared for
autoloading with a magic comment:

@smallexample
;;;###autoload
(defun doctor ()
  "Switch to *doctor* buffer and start giving psychotherapy."
  (interactive)
  (switch-to-buffer "*doctor*")
  (doctor-mode))
@end smallexample

@noindent
Here's what that produces in @file{loaddefs.el}:

@smallexample
(autoload 'doctor "doctor" "\
Switch to *doctor* buffer and start giving psychotherapy."
  t)
@end smallexample

@noindent
The backslash and newline immediately following the double-quote are a
convention used only in the preloaded uncompiled Lisp files such as
@file{loaddefs.el}; they tell @code{make-docfile} to put the
documentation string in the @file{etc/DOC} file.  @xref{Building Emacs}.
See also the commentary in @file{lib-src/make-docfile.c}.

  If you write a function definition with an unusual macro that is not
one of the known and recognized function definition methods, use of an
ordinary magic autoload comment would copy the whole definition into
@code{loaddefs.el}.  That is not desirable.  You can put the desired
@code{autoload} call into @code{loaddefs.el} instead by writing this:

@smallexample
;;;###autoload (autoload 'foo "myfile")
(mydefunmacro foo
  ...)
@end smallexample

@node Repeated Loading
@section Repeated Loading
@cindex repeated loading

  You can load a given 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 @code{load} and
@code{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, you need to byte-compile the new
version; otherwise Emacs will load the older, byte-compiled file instead
of your newer, non-compiled file!  If that happens, the message
displayed when loading the file includes, @samp{(compiled; note, source is
newer)}, to remind you to recompile it.

  When writing the forms in a Lisp library file, keep in mind that the
file might be loaded more than once.  For example, think about whether
each variable should be reinitialized when you reload the library;
@code{defvar} does not change the value if the variable is already
initialized.  (@xref{Defining Variables}.)

  The simplest way to add an element to an alist is like this:

@example
(setq minor-mode-alist
      (cons '(leif-mode " Leif") minor-mode-alist))
@end example

@noindent
But this would add multiple elements if the library is reloaded.
To avoid the problem, write this:

@example
(or (assq 'leif-mode minor-mode-alist)
    (setq minor-mode-alist
          (cons '(leif-mode " Leif") minor-mode-alist)))
@end example

  To add an element to a list just once, you can also use @code{add-to-list}
(@pxref{Setting Variables}).

  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:

@example
(defvar foo-was-loaded nil)

(unless foo-was-loaded
  @var{execute-first-time-only}
  (setq foo-was-loaded t))
@end example

@noindent
If the library uses @code{provide} to provide a named feature, you can
use @code{featurep} earlier in the file to test whether the
@code{provide} call has been executed before.
@ifnottex
@xref{Named Features}.
@end ifnottex

@node Named Features
@section Features
@cindex features
@cindex requiring features
@cindex providing features

  @code{provide} and @code{require} are an alternative to
@code{autoload} for loading files automatically.  They work in terms of
named @dfn{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 @dfn{provide} the
feature.  Another program that uses them may ensure they are defined by
@dfn{requiring} the feature.  This loads the file of definitions if it
hasn't been loaded already.

  To require the presence of a feature, call @code{require} with the
feature name as argument.  @code{require} looks in the global variable
@code{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 @code{provide} at the top level to add the feature to
@code{features}; if it fails to do so, @code{require} signals an error.
@cindex load error with require

  For example, in @file{emacs/lisp/prolog.el}, 
the definition for @code{run-prolog} includes the following code:

@smallexample
(defun run-prolog ()
  "Run an inferior Prolog process, with I/O via buffer *prolog*."
  (interactive)
  (require 'comint)
  (switch-to-buffer (make-comint "prolog" prolog-program-name))
  (inferior-prolog-mode))
@end smallexample

@noindent
The expression @code{(require 'comint)} loads the file @file{comint.el}
if it has not yet been loaded.  This ensures that @code{make-comint} is
defined.  Features are normally named after the files that provide them,
so that @code{require} need not be given the file name.

The @file{comint.el} file contains the following top-level expression:

@smallexample
(provide 'comint)
@end smallexample

@noindent
This adds @code{comint} to the global @code{features} list, so that
@code{(require 'comint)} will henceforth know that nothing needs to be
done.

@cindex byte-compiling @code{require}
  When @code{require} is used at top level in a file, it takes effect
when you byte-compile that file (@pxref{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.  It also avoids byte-compiler
warnings for functions and variables defined in the file loaded with
@code{require}.

  Although top-level calls to @code{require} are evaluated during
byte compilation, @code{provide} calls are not.  Therefore, you can
ensure that a file of definitions is loaded before it is byte-compiled
by including a @code{provide} followed by a @code{require} for the same
feature, as in the following example.

@smallexample
@group
(provide 'my-feature)  ; @r{Ignored by byte compiler,}
                       ;   @r{evaluated by @code{load}.}
(require 'my-feature)  ; @r{Evaluated by byte compiler.}
@end group
@end smallexample

@noindent
The compiler ignores the @code{provide}, then processes the
@code{require} by loading the file in question.  Loading the file does
execute the @code{provide} call, so the subsequent @code{require} call
does nothing when the file is loaded.

@defun provide feature &optional subfeatures
This function announces that @var{feature} is now loaded, or being
loaded, into the current Emacs session.  This means that the facilities
associated with @var{feature} are or will be available for other Lisp
programs.

The direct effect of calling @code{provide} is to add @var{feature} to
the front of the list @code{features} if it is not already in the list.
The argument @var{feature} must be a symbol.  @code{provide} returns
@var{feature}.

If provided, @var{subfeatures} should be a list of symbols indicating
a set of specific subfeatures provided by this version of @var{feature}.

@smallexample
features
     @result{} (bar bish)

(provide 'foo)
     @result{} foo
features
     @result{} (foo bar bish)
@end smallexample

When a file is loaded to satisfy an autoload, and it stops due to an
error in the evaluating its contents, any function definitions or
@code{provide} calls that occurred during the load are undone.
@xref{Autoload}.
@end defun

@defun require feature &optional filename noerror
This function checks whether @var{feature} is present in the current
Emacs session (using @code{(featurep @var{feature})}; see below).  The
argument @var{feature} must be a symbol.

If the feature is not present, then @code{require} loads @var{filename}
with @code{load}.  If @var{filename} is not supplied, then the name of
the symbol @var{feature} is used as the base file name to load.
However, in this case, @code{require} insists on finding @var{feature}
with an added suffix; a file whose name is just @var{feature} won't be
used.

If loading the file fails to provide @var{feature}, @code{require}
signals an error, @samp{Required feature @var{feature} was not
provided}, unless @var{noerror} is non-@code{nil}.
@end defun

@defun featurep feature &optional subfeature
This function returns @code{t} if @var{feature} has been provided in the
current Emacs session (i.e.@:, if @var{feature} is a member of
@code{features}.)  If @var{subfeature} is non-nil, then the function
returns @code{t} only if that subfeature is provided as well (i.e.@:
if @var{subfeature} is a member of the @code{subfeature} property
of the @var{feature} symbol.)
@end defun

@defvar features
The value of this variable is a list of symbols that are the features
loaded in the current Emacs session.  Each symbol was put in this list
with a call to @code{provide}.  The order of the elements in the
@code{features} list is not significant.
@end defvar

@node Unloading
@section Unloading
@cindex unloading

@c Emacs 19 feature
  You can discard the functions and variables loaded by a library to
reclaim memory for other Lisp objects.  To do this, use the function
@code{unload-feature}:

@deffn Command unload-feature feature &optional force
This command unloads the library that provided feature @var{feature}.
It undefines all functions, macros, and variables defined in that
library with @code{defun}, @code{defalias}, @code{defsubst},
@code{defmacro}, @code{defconst}, @code{defvar}, and @code{defcustom}.
It then restores any autoloads formerly associated with those symbols.
(Loading saves these in the @code{autoload} property of the symbol.)

Before restoring the previous definitions, @code{unload-feature} runs
@code{remove-hook} to remove functions in the library from certain
hooks.  These hooks include variables whose names end in @samp{hook} or
@samp{-hooks}, plus those listed in @code{loadhist-special-hooks}.  This
is to prevent Emacs from ceasing to function because important hooks
refer to functions that are no longer defined.

@vindex @var{feature}-unload-hook
If these measures are not sufficient to prevent malfunction, a library
can define an explicit unload hook.  If @code{@var{feature}-unload-hook}
is defined, it is run as a normal hook before restoring the previous
definitions, @emph{instead of} the usual hook-removing actions.  The
unload hook ought to undo all the global state changes made by the
library that might cease to work once the library is unloaded.
@code{unload-feature} can cause problems with libraries that fail to do
this, so it should be used with caution.

Ordinarily, @code{unload-feature} refuses to unload a library on which
other loaded libraries depend.  (A library @var{a} depends on library
@var{b} if @var{a} contains a @code{require} for @var{b}.)  If the
optional argument @var{force} is non-@code{nil}, dependencies are
ignored and you can unload any library.
@end deffn

  The @code{unload-feature} function is written in Lisp; its actions are
based on the variable @code{load-history}.

@defvar load-history
This variable's value is an alist connecting library names with the
names of functions and variables they define, the features they provide,
and the features they require.

Each element is a list and describes one library.  The @sc{car} of the
list is the name of the library, as a string.  The rest of the list
elements have these forms:

@table @code
@item @var{fun}
The function @var{fun} was defined by this library.
@item (t . @var{fun})
The function @var{fun} was previously an autoload before this library
redefined it as a function.  The following element is always the
symbol @var{fun}, which signifies that the library defined @var{fun}
as a function.
@item (autoload . @var{fun})
The function @var{fun} was defined as an autoload.
@item (defvar . @var{var})
The symbol @var{var} was defined as a variable.
@item (require . @var{feature})
The feature @var{feature} was required.
@item (provide . @var{feature})
The feature @var{feature} was provided.
@end table

The value of @code{load-history} may have one element whose @sc{car} is
@code{nil}.  This element describes definitions made with
@code{eval-buffer} on a buffer that is not visiting a file.
@end defvar

  The command @code{eval-region} updates @code{load-history}, but does so
by adding the symbols defined to the element for the file being visited,
rather than replacing that element.  @xref{Eval}.

  Preloaded libraries don't contribute initially to @code{load-history}.
Instead, preloading writes information about preloaded libraries into a
file, which can be loaded later on to add information to
@code{load-history} describing the preloaded files.  This file is
installed in @code{exec-directory} and has a name of the form
@file{fns-@var{emacsversion}.el}.

@findex symbol-file
  See the source for the function @code{symbol-file}, for an example of
code that loads this file to find functions in preloaded libraries.

@defvar loadhist-special-hooks
This variable holds a list of hooks to be scanned before unloading a
library, to remove functions defined in the library.
@end defvar

@node Hooks for Loading
@section Hooks for Loading
@cindex loading hooks
@cindex hooks for loading

You can ask for code to be executed if and when a particular library is
loaded, by calling @code{eval-after-load}.

@defun eval-after-load library form
This function arranges to evaluate @var{form} at the end of loading the
library @var{library}, if and when @var{library} is loaded.  If
@var{library} is already loaded, it evaluates @var{form} right away.

If @var{library} is a string, it must exactly match the argument of
@code{load} used to load the library.  To get the proper results when an
installed library is found by searching @code{load-path}, you should not
include any directory names in @var{library}.

@var{library} can also be a feature (i.e.@: a symbol), in which case
@var{form} is evaluated when @code{(provide @var{library})} is called.

An error in @var{form} does not undo the load, but does prevent
execution of the rest of @var{form}.
@end defun

In general, well-designed Lisp programs should not use this feature.
The clean and modular ways to interact with a Lisp library are (1)
examine and set the library's variables (those which are meant for
outside use), and (2) call the library's functions.  If you wish to
do (1), you can do it immediately---there is no need to wait for when
the library is loaded.  To do (2), you must load the library (preferably
with @code{require}).

But it is OK to use @code{eval-after-load} in your personal
customizations if you don't feel they must meet the design standards for
programs meant for wider use.

@defvar after-load-alist
This variable holds an alist of expressions to evaluate if and when
particular libraries are loaded.  Each element looks like this:

@example
(@var{filename} @var{forms}@dots{})
@end example

The function @code{load} checks @code{after-load-alist} in order to
implement @code{eval-after-load}.
@end defvar

@c Emacs 19 feature