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We usually give a name to a function when it is first created. This is
called defining a function, and it is done with the defun
special form.
defun is the usual way to define new Lisp functions. It defines the
symbol name as a function that looks like this:
(lambda argument-list . body-forms) |
defun stores this lambda expression in the function cell of
name. It returns the value name, but usually we ignore this
value.
As described previously, argument-list is a list of argument names and
may include the keywords &optional and &rest (voir la section Lambda Expressions). Also, the first two of the body-forms may be a
documentation string and an interactive declaration.
There is no conflict if the same symbol name is also used as a variable, since the symbol's value cell is independent of the function cell. @xref{Symbol Components}.
Here are some examples:
(defun foo () 5)
⇒ foo
(foo)
⇒ 5
(defun bar (a &optional b &rest c)
(list a b c))
⇒ bar
(bar 1 2 3 4 5)
⇒ (1 2 (3 4 5))
(bar 1)
⇒ (1 nil nil)
(bar)
error--> Wrong number of arguments.
(defun capitalize-backwards ()
"Upcase the last letter of a word."
(interactive)
(backward-word 1)
(forward-word 1)
(backward-char 1)
(capitalize-word 1))
⇒ capitalize-backwards
|
Be careful not to redefine existing functions unintentionally. defun
redefines even primitive functions such as car without any hesitation
or notification. Redefining a function already defined is often done
deliberately, and there is no way to distinguish deliberate redefinition
from unintentional redefinition.
This special form defines the symbol name as a function, with definition definition (which can be any valid Lisp function). It returns definition.
If docstring is non-nil, it becomes the function documentation
of name. Otherwise, any documentation provided by definition is
used.
The proper place to use defalias is where a specific function name is
being defined—especially where that name appears explicitly in the source
file being loaded. This is because defalias records which file
defined the function, just like defun (voir la section Unloading).
By contrast, in programs that manipulate function definitions for other
purposes, it is better to use fset, which does not keep such
records. Voir la section Accessing Function Cell Contents.
You cannot create a new primitive function with defun or
defalias, but you can use them to change the function definition of
any symbol, even one such as car or x-popup-menu whose normal
definition is a primitive. However, this is risky: for instance, it is next
to impossible to redefine car without breaking Lisp completely.
Redefining an obscure function such as x-popup-menu is less
dangerous, but it still may not work as you expect. If there are calls to
the primitive from C code, they call the primitive's C definition directly,
so changing the symbol's definition will have no effect on them.
See also defsubst, which defines a function like defun and
tells the Lisp compiler to open-code it. Voir la section Inline Functions.
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Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.