Contents [HideShow] 1 顺序结构： 2 选择结构

顺序结构：

顺序结构主要用在需要顺序form组的地方， 有些结构已经隐含有了form组链结的功能：

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'.

顺序结构有以下几类：

• Special Form: progn
• Special Form: prog1
• Special Form: prog2

一般形式:

顺序执行所包含的form，返回最后一个form的返回值：

(progn (print "The first form")
       (print "The second form")
       (print "The third form"))

输出的结果为：

 -| "The first form"
 -| "The second form"
 -| "The third form"
=> "The third form"

变体1:

(prog1 (print "The first form")
       (print "The second form")
       (print "The third form"))

返回第一个form的返回值：

 -| "The first form"
 -| "The second form"
 -| "The third form"
=> "The first form"

变体2:

(prog2 (print "The first form")
       (print "The second form")
       (print "The third form"))

返回第二个form的返回值：

 -| "The first form"
 -| "The second form"
 -| "The third form"
=> "The second form"

选择结构

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