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 '(1 2 3 2)) ;=> 12
因为要捕捉可能的break和continue，用syntax-case编写。
(define-syntax for
(lambda (x)
(syntax-case x (in from to)
((k e in l c c* ...)
(with-syntax ((break (datum->syntax (syntax k) 'break))
(continue (datum->syntax (syntax k) 'continue)))
(syntax
(call/cc
(lambda (break)
(do ((ns l (cdr ns)))
((null? ns))
(let ((e (car ns)))
(call/cc
(lambda (continue)
c
c* ...)))))))))
((k i from a to z c c* ...)
(with-syntax ((break (datum->syntax (syntax k) 'break))
(continue (datum->syntax (syntax k) 'continue)))
(syntax
(let ((a* a)
(z* z))
(call/cc
(lambda (break)
(do ((i a* (+ i 1)))
((> i z*))
(call/cc
(lambda (continue)
c
c* ...)))))))))
((k i from a b to z c c* ...)
(with-syntax ((break (datum->syntax (syntax k) 'break))
(continue (datum->syntax (syntax k) 'continue)))
(syntax
(let* ((a* a)
(b* b)
(z* z)
(s (- b* a*))
(p (if (> b* a*) > <)))
(call/cc
(lambda (break)
(do ((i a* (+ i s)))
((p i z*))
(call/cc
(lambda (continue)
c
c* ...))))))))))))
