angels-and-devils.scm

(define-module (angels-and-devils)
  #:export (milestone devil angel))

;; See the paper "call-with-current-continuation patterns"

(define future '())
(define past '())

(define-syntax-rule (push var val)
  (set! var (cons val var)))

(define-syntax-rule (pop var)
  (if (null? var)
      (lambda (x) x)
      (let ((result (car var)))
        (set! var (cdr var))
        result)))

(define (milestone x)
  (call/cc
   (lambda (k)
     (push future k)
     x)))

(define (devil x)
  (call/cc
   (lambda (k)
     (push future k)
     ((pop past) x))))

(define (angel x)
  ((pop future) x))