4つの数字から10を作る問題のやつ

make10.rkt

#lang racket
(require racket/control)

(define (amb args)
  (shift k
         (for ([v args])
           (k v))))

(define (perm li)
  (if (null? (cdr li))
      (list li)
      (append-map (lambda (hd)
             (map (lambda (l) (cons hd l)) (perm (remq hd li))
             )) li)))
 
(define (minians a res)
  (append-map (lambda (exp)
                  (list (list '+ a exp)
                        (list '- a exp)
                        (list '* a exp)
                        (list '/ a exp))
                ) res))

(define (make-expr li)
  (foldl minians (list (car li)) (cdr li)))
  
(define (safe-eval cand)
  (with-handlers ([exn:fail:contract:divide-by-zero?
                   (lambda (exn) +inf.0)])
    (eval cand (make-base-namespace))))

(define (print-make10 question)
  (reset 
   (let* ([q    (perm question)]
          [now  (amb q)]
          [cand (amb (make-expr now))]
          [ev   (safe-eval cand)])
     (cond [(eq? ev 10)
            (printf "~a: ~a\n" cand ev)]))))

(print-make10 '(1 1 5 8))