Implement a new kind of assignment called
that is not undone upon failure. For example, we can choose two distinct elements from a list and count the number of trials required to make a successful choice as follows:
(define count 0) (let ((x (an-element-of '(a b c))) (y (an-element-of '(a b c)))) (permanent-set! count (+ count 1)) (require (not (eq? x y))) (list x y count)) ;;; Starting a new problem ;;; Amb-Eval value: (a b 2) ;;; Amb-Eval input: try-again ;;; Amb-Eval value: (a c 3)
What values would have been displayed if we had used
here rather than
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