12.8 Accessing Function Cell Contents

The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.

See also the function indirect-function. Voir Definition of indirect-function.

Function: symbol-function symbol

This returns the object in the function cell of symbol. If the symbol's function cell is void, a void-function error is signaled.

This function does not check that the returned object is a legitimate function.

(defun bar (n) (+ n 2)) ⇒ bar (symbol-function 'bar) ⇒ (lambda (n) (+ n 2)) (fset 'baz 'bar) ⇒ bar (symbol-function 'baz) ⇒ bar

If you have never given a symbol any function definition, we say that that symbol's function cell is void. In other words, the function cell does not have any Lisp object in it. If you try to call such a symbol as a function, it signals a void-function error.

Note that void is not the same as nil or the symbol void. The symbols nil and void are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with defun). A void function cell contains no object whatsoever.

You can test the voidness of a symbol's function definition with fboundp. After you have given a symbol a function definition, you can make it void once more using fmakunbound.

Function: fboundp symbol

This function returns t if the symbol has an object in its function cell, nil otherwise. It does not check that the object is a legitimate function.

Function: fmakunbound symbol

This function makes symbol's function cell void, so that a subsequent attempt to access this cell will cause a void-function error. It returns symbol. (See also makunbound, in When a Variable is “Void”.)

(defun foo (x) x) ⇒ foo (foo 1) ⇒1 (fmakunbound 'foo) ⇒ foo (foo 1) error--> Symbol's function definition is void: foo

Function: fset symbol definition

This function stores definition in the function cell of symbol. The result is definition. Normally definition should be a function or the name of a function, but this is not checked. The argument symbol is an ordinary evaluated argument.

There are three normal uses of this function:

• Copying one symbol's function definition to another—in other words, making an alternate name for a function. (If you think of this as the definition of the new name, you should use defalias instead of fset; see Definition of defalias.)
• Giving a symbol a function definition that is not a list and therefore cannot be made with defun. For example, you can use fset to give a symbol s1 a function definition which is another symbol s2; then s1 serves as an alias for whatever definition s2 presently has. (Once again use defalias instead of fset if you think of this as the definition of s1.)
• In constructs for defining or altering functions. If defun were not a primitive, it could be written in Lisp (as a macro) using fset.

Here are examples of these uses:

;; Save foo's definition in old-foo. (fset 'old-foo (symbol-function 'foo)) ;; Make the symbol car the function definition of xfirst. ;; (Most likely, defalias would be better than fset here.) (fset 'xfirst 'car) ⇒ car (xfirst '(1 2 3)) ⇒ 1 (symbol-function 'xfirst) ⇒ car (symbol-function (symbol-function 'xfirst)) ⇒ #<subr car> ;; Define a named keyboard macro. (fset 'kill-two-lines "\^u2\^k") ⇒ "\^u2\^k" ;; Here is a function that alters other functions. (defun copy-function-definition (new old) "Define NEW with the same function definition as OLD." (fset new (symbol-function old)))

fset is sometimes used to save the old definition of a function before redefining it. That permits the new definition to invoke the old definition. But it is unmodular and unclean for a Lisp file to redefine a function defined elsewhere. If you want to modify a function defined by another package, it is cleaner to use defadvice (voir la section Advising Emacs Lisp Functions).

Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.