Ex-3.22

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

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