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' in Note: Function Indirection. - 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. (See also `makunbound', in Note: Void Variables.) (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 Note: Defining Functions.) * 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))) When writing a function that extends a previously defined function, the following idiom is sometimes used: (fset 'old-foo (symbol-function 'foo)) (defun foo () "Just like old-foo, except more so." (old-foo) (more-so)) This does not work properly if `foo' has been defined to autoload. In such a case, when `foo' calls `old-foo', Lisp attempts to define `old-foo' by loading a file. Since this presumably defines `foo' rather than `old-foo', it does not produce the proper results. The only way to avoid this problem is to make sure the file is loaded before moving aside the old definition of `foo'. But it is unmodular and unclean, in any case, for a Lisp file to redefine a function defined elsewhere. It is cleaner to use the advice facility (Note: Advising Functions).
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