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
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* ...))))))))))))

为scheme添加for循环语句