solve24.rkt

#lang racket

(require (only-in srfi/26 cute))

(define-syntax t>=
  (syntax-rules ()
    ((_ x) x)
    ((_ x f g ...) (t>= (f x) g ...))))

(define (solve24 n)
  (t>= "9876543210" string->list
       permutations
       (cute list-ref <> (sub1 n))
       reverse list->string))

(define (solve24-seq n)
  (t>= "0123456789" string->list
       in-permutations
       (cute sequence-ref <> (sub1 n))
       reverse list->string))

(define N 1000000)
(time (solve24 N))
(time (solve24-seq N))