; 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)