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 Naming a Function, 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. Otherwise, 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.
fset to set the function cell of a symbol and
symbol-function to get the function cell contents
(see Accessing Function Cell Contents). Specifically, we store the symbol
car into the function cell of
first, and the symbol
first into 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
This form is rarely used and is now deprecated. Instead, you should write it as:
(funcall (lambda (arg) (erste arg)) '(1 2 3))
(let ((arg '(1 2 3))) (erste arg))
The built-in function
indirect-function provides an easy way to
perform symbol function indirection explicitly.
indirect-functionfunction &optional noerror ¶
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 returns
nil if the final symbol is unbound. It
cyclic-function-indirection error if there is a loop
in the chain of symbols.
The optional argument noerror is obsolete, kept for backward compatibility, and has no effect.
Here is how you could define
indirect-function in Lisp:
(defun indirect-function (function) (if (and function (symbolp function)) (indirect-function (symbol-function function)) function))