为scheme添加for循环语句

有三个辅助关键字in, from和to
能够 break和continue
语法有三种
(for n in '(1 2 3 4 5 6 7 8 9 10)
  (if (> n 8) (break #f))
  (if (even? n) (continue #f))
  (display n)
  (display " "))
=> 1 3 5 7 #f
(for n from 1 to 100
  (if (> n 10) (break #f))
  (display n)
  (display " "))
=> 1 2 3 4 5 6 7 8 9 10 #f
(for n from 100 99 to 1
  (if (< n 90) (break #f))
  (display n)
  (display " "))
=> 100 99 98 97 96 95 94 93 92 91 90 #f

(define (product nums)
  (define p 1)
  (for n in nums
    (when (zero? n)
      (set! p 0)
      (break #f))
    (set! p (* p n)))
  p)

(product '(1 2 3 0 2)) ;=> 0                                                                                                                          
(product