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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 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.
This function announces that feature is now loaded, or being loaded, into the current SXEmacs session. This means that the facilities associated with feature are or will be available for other Lisp programs.
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)
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
provide
calls that occurred during the load are undone.
See Autoload.
This function checks whether feature is present in the current
SXEmacs session (using (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’.
This function returns t
if feature fexp is present in this
SXEmacs. Use this to conditionalize execution of lisp code based on the
presence or absence of emacs or environment extensions.
fexp can be a symbol, a number, or a list.
If fexp is a symbol, it is looked up in the features
variable,
and t
is returned if it is found, nil
otherwise.
If fexp is a number, the function returns t
if this SXEmacs
has an equal or greater number than fexp, nil
otherwise.
Note that minor SXEmacs version is expected to be 2 decimal places wide,
so (featurep 22.1)
will return nil
on SXEmacs 22.1—you
must write (featurep 22.01)
, unless you wish to match for SXEmacs
22.10.
If fexp is a list whose car is the symbol and
, the function
returns t
if all the features in its cdr are present, nil
otherwise.
If fexp is a list whose car is the symbol or
, the function
returns t
if any the features in its cdr are present, nil
otherwise.
If fexp is a list whose car is the symbol not
, the function
returns t
if the feature is not present, nil
otherwise.
Examples:
(featurep 'sxemacs) ⇒ ; t on SXEmacs. (featurep '(and sxemacs gnus)) ⇒ ; t on SXEmacs with Gnus loaded. (featurep '(or tty-frames (and emacs 19.30))) ⇒ ; t if this Emacs supports TTY frames. (featurep '(or (and sxemacs 22.01) (and xemacs 21.04) (and emacs 21.3))) ⇒ ; t on SXEmacs >=22.1, XEmacs >=21.4, or FSF Emacs >=21.3.
Please note: The advanced arguments of this function (anything other than a symbol) are not yet supported by FSF Emacs. If you feel they are useful for supporting multiple Emacs variants, lobby Richard Stallman at ‘<bug-gnu-emacs@prep.ai.mit.edu>’.
The value of this variable is a list of symbols that are the features
loaded in the current SXEmacs session. Each symbol was put in this list
with a call to provide
. The order of the elements in the
features
list is not significant.
Next: Unloading, Previous: Repeated Loading, Up: Loading [Contents][Index]