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Evaluating forms in the order they appear is the most common way control
passes from one form to another. In some contexts, such as in a function
body, this happens automatically. Elsewhere you must use a control
structure construct to do this: progn, the simplest control construct
of Lisp.
A progn special form looks like this:
(progn a b c …) |
and it says to execute the forms a, b, c, and so on, in
that order. These forms are called the body of the progn
form. The value of the last form in the body becomes the value of the
entire progn. (progn) returns nil.
In the early days of Lisp, progn was the only way to execute two or
more forms in succession and use the value of the last of them. But
programmers found they often needed to use a progn in the body of a
function, where (at that time) only one form was allowed. So the body of a
function was made into an “implicit progn”: several forms are
allowed just as in the body of an actual progn. Many other control
structures likewise contain an implicit progn. As a result,
progn is not used as much as it was many years ago. It is needed now
most often inside an unwind-protect, and, or, or in the
then-part of an if.
This special form evaluates all of the forms, in textual order, returning the result of the final form.
(progn (print "The first form")
(print "The second form")
(print "The third form"))
-| "The first form"
-| "The second form"
-| "The third form"
⇒ "The third form"
|
Two other control constructs likewise evaluate a series of forms but return a different value:
This special form evaluates form1 and all of the forms, in textual order, returning the result of form1.
(prog1 (print "The first form")
(print "The second form")
(print "The third form"))
-| "The first form"
-| "The second form"
-| "The third form"
⇒ "The first form"
|
Here is a way to remove the first element from a list in the variable
x, then return the value of that former element:
(prog1 (car x) (setq x (cdr x))) |
This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2.
(prog2 (print "The first form")
(print "The second form")
(print "The third form"))
-| "The first form"
-| "The second form"
-| "The third form"
⇒ "The second form"
|
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Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.