Non-determinism implemented with the call/cc operator in Racket

nondet-callcc.rkt

#lang racket

(define *remaining-choices* '())

(define (choose choices)
  (call/cc
   (λ (k)
     (for ([i (cdr choices)])
       (set! *remaining-choices* (cons (cons k i) *remaining-choices*)))
     (k (car choices)))))

(define (fail)
  (if (empty? *remaining-choices*)
      'fail
      (let ([next-choice (car *remaining-choices*)])
        ;; Pop off that choice we just saved
        (set! *remaining-choices* (cdr *remaining-choices*))
        (match next-choice
          [(cons k chosen-one)
           (k chosen-one)]))))

(define (find-factor n)
  (let ([the-factor (choose (range 2 n))])
    (if (= 0 (modulo n the-factor))
        the-factor
        (fail))))