; send + more = money

(define (fail)
  (error 'amb "tree exhausted"))

(define-syntax amb
  (syntax-rules ()
    ((amb) (fail))
    ((amb expr) expr)
    ((amb expr ...)
      (let ((prev-fail fail))
        ((call-with-current-continuation
          (lambda (success)
            (call-with-current-continuation
              (lambda (failure)
                (set! fail failure)
                (success (lambda () expr))))
            ...
            (set! fail prev-fail)
            prev-fail)))))))

(define (require condition)
  (if (not condition) (amb)))

(define (distinct? xs)
  (cond ((null? xs) #t)
        ((member (car xs) (cdr xs)) #f)
        (else (distinct? (cdr xs)))))

(define (send-more-money)
  (let ((s (amb 1 2 3 4 5 6 7 8 9))
        (e (amb 0 1 2 3 4 5 6 7 8 9))
        (n (amb 0 1 2 3 4 5 6 7 8 9))
        (d (amb 0 1 2 3 4 5 6 7 8 9))
        (m (amb 1 2 3 4 5 6 7 8 9))
        (o (amb 0 1 2 3 4 5 6 7 8 9))
        (r (amb 0 1 2 3 4 5 6 7 8 9))
        (y (amb 0 1 2 3 4 5 6 7 8 9)))
    (require (distinct? (list s e n d m o r y)))
    (require (= (modulo (+ s m (amb 0 1)) 10) o))
    (require (= (modulo (+ e o (amb 0 1)) 10) n))
    (require (= (modulo (+ n r (amb 0 1)) 10) e))
    (require (= (modulo (+ d e (amb 0 1)) 10) y))
    (require (= (+ (* 1000 s) (* 100 e) (* 10 n) d
                   (* 1000 m) (* 100 o) (* 10 r) e)
                (+ (* 10000 m) (* 1000 o) (* 100 n) (* 10 e) y)))
    (list (list s e n d) (list m o r e) (list m o n e y))))

(send-more-money)

Timeout