9.2.4 Symbol Function Indirection

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. Voir la section Naming a Function, for more information about using a symbol as a name for a function stored in the function cell of the symbol.

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, the error invalid-function is signaled.

The following example illustrates the symbol indirection process. We use fset to set the function cell of a symbol and symbol-function to get the function cell contents (voir la section 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 erste.

;; 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 erste.

The built-in function indirect-function provides an easy way to perform symbol function indirection explicitly.

Function: indirect-function function &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 signals a void-function error if the final symbol is unbound and optional argument noerror is nil or omitted. Otherwise, if noerror is non-nil, it returns nil if the final symbol is unbound.

It signals a cyclic-function-indirection error if there is a loop in the chain of symbols.

Here is how you could define indirect-function in Lisp:

(defun indirect-function (function) (if (symbolp function) (indirect-function (symbol-function function)) function))

Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.