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(elisp)Function Cells

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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).