#lang r5rs

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

(define (empty-queue? queue)
  ((queue 'empty-queue?) queue))

(define (front-queue queue)
  ((queue 'front-queue) queue))

(define (insert-queue! queue item)
  ((queue 'insert-queue!) queue item))

(define (delete-queue! queue)
  ((queue 'delete-queue!) queue))