#lang racket

(define (partition good? list return)
    (match list
        [(list)
            (return `() `())]

        [(list head tail ...)
            (partition good? tail (λ (accepted rejected)
            (if (good? head)
                (return (cons head accepted) rejected)
                (return accepted (cons head rejected)))))]))

(partition (curry < 4) `(1 9 2 8 3 7 4 6 5) (λ (good bad)
(printf "good: ~a; bad ~a~%" good bad)))

(define (split-at n list return)
    (if ((zero? n) . or . (null? list))
        (return `() list)
    ; else
        (match list [(list head tail ...)
            (split-at (n . - . 1) tail
                (λ (before after)
                    (return (cons head before) after)))])))

(split-at 5 `(1 2 3 4 5 6 7 8 9) (λ (before after)
(printf "before: ~a; after ~a~%" before after)))