04.09 赋值：set!

4.9 赋值：set!

使用set!赋值给变量：

(set! id expr)

一个set!表达式对expr求值并改变id（它必须限制在闭括号的环境内）为结果值。set!表达式自己返回的结果是#<void>。

例如：

(define greeted null)

(define (greet name)
  (set! greeted (cons name greeted))
  (string-append "Hello, " name))

> (greet "Athos")
"Hello, Athos"
> (greet "Porthos")
"Hello, Porthos"
> (greet "Aramis")
"Hello, Aramis"
> greeted
'("Aramis" "Porthos" "Athos")

(define (make-running-total)
  (let ([n 0])
    (lambda ()
      (set! n (+ n 1))
      n)))
(define win (make-running-total))
(define lose (make-running-total))

> (win)
1
> (win)
2
> (lose)
1
> (win)
3