Function Cells - SXEmacs Lisp Reference Manual

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.

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

— Function: fset symbol object

This function stores object in the function cell of symbol. The result is object. Normally object should be a function or the name of a function, but this is not checked.
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.)
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 symbol1 a function definition which is another symbol symbol2; then symbol1 serves as an alias for whatever definition symbol2 presently has.
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 the first two uses:
          ;; Give first the same definition car has.
          (fset 'first (symbol-function 'car))
               ⇒ #<subr car>
          (first '(1 2 3))
               ⇒ 1
          
          ;; Make the symbol car the function definition of xfirst.
          (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"
See also the related functions define-function and defalias, in Defining Functions.

When writing a function that extends a previously defined function, the following idiom is sometimes used:

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.

17.8 Accessing Function Cell Contents