amb operator

gistfile1.scm

;;; amb stuff


;; stack of cc.
(define fail '())

;;; nondeterminsm macro operator
(define-syntax amb
  (syntax-rules ()
    ((_) ((pop! fail)))
    ((_ a) a)
    ((_ a b ...)
     (call/cc
      (lambda (cc)
        (push! fail (lambda ()
                      (cc (amb b ...))))
        a)))))

(call/cc
 (lambda (cc)
   ;; initial value for fail
   (push! fail
	  (lambda ()
	    (cc 'no-choise)))))



;;; test code

(define (divisible? n m)
  (zero? (mod n m)))

(define (square n)
  (* n n))

(define (*prime? n i)
  (if (< n (square i))
      #t
      (if (divisible? n i)
	  #f
	  (*prime? n (+ i 1)))))

(define (prime? n)
  (*prime? n 2))

(define (solve)   
  (let ((i (amb 4 6 7))
        (j (amb 5 8 11)))
    (if (prime? (+ i j))
        (list i j)
        (amb))))

(print (solve))