3_21.scm

(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 (print-queue queue)
  (front-ptr queue))

(define (print-queue2 queue)
  (define (print-list L)
    (if (not (null? L))
      (begin (display (car L))
             (display " ")
       (print-list (cdr L)))))
  (newline)
  (print-list (front-ptr queue)))
  ;(display (front-ptr queue)))

(define q1 (make-queue))
(insert-queue! q1 'a)
(insert-queue! q1 'b)
(print-queue q1)
(delete-queue! q1)
(delete-queue! q1)