If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called symbol function indirection, is repeated until it obtains a non-symbol. See Function Names, for more information about symbol function indirection.
One possible consequence of this process is an infinite loop, in the
event that a symbol's function cell refers to the same symbol. Or a
symbol may have a void function cell, in which case the subroutine
symbol-function signals a
void-function error. But if
neither of these things happens, we eventually obtain a non-symbol,
which ought to be a function or other suitable object.
More precisely, we should now have a Lisp function (a lambda
expression), a byte-code function, a primitive function, a Lisp macro,
a special form, or an autoload object. Each of these types is a case
described in one of the following sections. If the object is not one
of these types, Emacs signals an
The following example illustrates the symbol indirection process. We
fset to set the function cell of a symbol and
symbol-function to get the function cell contents
(see Function Cells). Specifically, we store the symbol
into the function cell of
first, and the symbol
the function cell of
;; Build this function cell linkage: ;; ------------- ----- ------- ------- ;; | #<subr car> | <-- | car | <-- | first | <-- | erste | ;; ------------- ----- ------- -------
(symbol-function 'car) ⇒ #<subr car> (fset 'first 'car) ⇒ car (fset 'erste 'first) ⇒ first (erste '(1 2 3)) ; Call the function referenced by
erste. ⇒ 1
By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol.
((lambda (arg) (erste arg)) '(1 2 3)) ⇒ 1
Executing the function itself evaluates its body; this does involve
symbol function indirection when calling
The built-in function
indirect-function provides an easy way to
perform symbol function indirection explicitly.
This function returns the meaning of function as a function. If function is a symbol, then it finds function's function definition and starts over with that value. If function is not a symbol, then it returns function itself.
This function signals a
void-functionerror if the final symbol is unbound and optional argument noerror is
nilor omitted. Otherwise, if noerror is non-
nil, it returns
nilif the final symbol is unbound.
It signals a
cyclic-function-indirectionerror if there is a loop in the chain of symbols.
Here is how you could define
indirect-functionin Lisp:(defun indirect-function (function) (if (symbolp function) (indirect-function (symbol-function function)) function))