11.9.2 Extent

Extent refers to the time during program execution that a variable name is valid. In Emacs Lisp, a variable is valid only while the form that bound it is executing. This is called dynamic extent. “Local” or “automatic” variables in most languages, including C and Pascal, have dynamic extent.

One alternative to dynamic extent is indefinite extent. This means that a variable binding can live on past the exit from the form that made the binding. Common Lisp and Scheme, for example, support this, but Emacs Lisp does not.

To illustrate this, the function below, make-add, returns a function that purports to add n to its own argument m. This would work in Common Lisp, but it does not do the job in Emacs Lisp, because after the call to make-add exits, the variable n is no longer bound to the actual argument 2.

(defun make-add (n)
    (function (lambda (m) (+ n m))))  ; Return a function.
     ⇒ make-add
(fset 'add2 (make-add 2))  ; Define function add2
                           ;   with (make-add 2).
     ⇒ (lambda (m) (+ n m))
(add2 4)                   ; Try to add 2 to 4.
error--> Symbol's value as variable is void: n

Some Lisp dialects have “closures,” objects that are like functions but record additional variable bindings. Emacs Lisp does not have closures.

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