《通链参考手册》


Special Operator PROGN

●语法设定:

progn form* => result*

●参数和值:

forms---an implicit progn.

results---the values of the forms.

●详情:

progn evaluates forms, in the order in which they are given.

The values of each form but the last are discarded.

If progn appears as a top level form, then all forms within that progn are considered by the compiler to be top level forms.

●例子:

 (progn) =>  NIL
 (progn 1 2 3) =>  3
 (progn (values 1 2 3)) =>  1, 2, 3
 (setq a 1) =>  1
 (if a
      (progn (setq a nil) 'here)
      (progn (setq a t) 'there)) =>  HERE
 a =>  NIL

●受制于: 无。

●例外情况: 无。

●更多信息:

prog1, prog2,

Section 3.1 (Evaluation)

●说明:

Many places in Common Lisp involve syntax that uses implicit progns. That is, part of their syntax allows many forms to be written that are to be evaluated sequentially, discarding the results of all forms but the last and returning the results of the last form. Such places include, but are not limited to, the following: the body of a lambda expression; the bodies of various control and conditional forms (e.g., case, catch, progn, and when).


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