sicp-3-3-2

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

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