scheme tinkering

generator.sch

(define (f yield)
  (yield 1)
  (yield 2)
  (yield 3))

(define gen
  (letrec ((pause '())
           (resume '())
           (yield (lambda (x)
                    (call/cc (lambda (k)
                               (set! resume k)
                               (pause x))))))
    (lambda ()
      (call/cc
        (lambda (k)
          (set! pause k)
          (if (null? resume)
            (f yield)
            (resume '())))))))