exercise 3.22

実際できるだろうと分かっても、書いて動かすと何か楽しかった。

(define (make-queue)
  (let ((front-ptr '())
        (rear-ptr '()))
    (define (set-front-ptr! item)
      (set! front-ptr item))
    (define (set-rear-ptr! item)
      (set! rear-ptr item))
    (define (empty-queue?)
      (null? front-ptr))
    (define (front-queue)
      (if (empty-queue?)
          (error "FRONT called with an empty queue" (cons front-ptr rear-ptr))
          (car front-ptr)))
    (define (insert-queue! item)
      (let ((new-pair (cons item '())))
        (cond ((empty-queue?)
               (set-front-ptr! new-pair)
               (set-rear-ptr! new-pair)
               (cons front-ptr rear-ptr))
              (else
               (set-cdr! rear-ptr new-pair)
               (set-rear-ptr! new-pair)
               (cons front-ptr rear-ptr)))))
    (define (delete-queue!)
      (cond ((empty-queue?)
             (error "DELETE called with an empty queue"
                    (cons front-ptr rear-ptr)))
            (else
             (set-front-ptr! (cdr front-ptr))
             (cons front-ptr rear-ptr))))
    (define (print-queue)
      front-ptr)
    (define (dispatch m)
      (cond ((eq? m 'insert-queue!) insert-queue!)
            ((eq? m 'delete-queue!) (delete-queue!))
            ((eq? m 'front-queue) (front-queue))
            ((eq? m 'empty-queue?) (empty-queue?))
            ((eq? m 'print-queue) (print-queue))
            (else
             (error "Undefined operation make-queue" m))))
    dispatch))

(define (empty-queue? z) (z 'empty-queue?))
(define (delete-queue! z) (z 'delete-queue!))
(define (insert-queue! z item) ((z 'insert-queue!) item))
(define (front-queue z) (z 'front-queue))
(define (print-queue z) (z 'print-queue))