M

(([[[(((()))))]](())])).rkt

(define (make-queue)
  (let ([front-ptr '()]
	[rear-ptr '()])
    (define (dispatch m)
      (cond ([eq? m 'empty-queue?]
	     (null? front-ptr))
	    ([eq? m 'front-queue]
	     (if (null? front-ptr)
		 (error "FRONT called with an empty queue")
		 (mcar front-ptr)))
	    ([eq? m 'insert-queue!]
	     (lambda (item)
	       (let ([new-item (mcons item '())])
		 (if (null? front-ptr)
		     (begin
		       (set! front-ptr new-item)
		       (set! rear-ptr new-item)
		       #t)
		     (begin
		       (set-mcdr! rear-ptr new-item)
		       (set! rear-ptr new-item)
		       #t)))))
	    ([eq? m 'delete-queue!]
	     (if (null? front-ptr)
		 (error "DELETE! called with an empty queue")
		 (set! front-ptr (mcdr front-ptr))))
	    ([eq? m 'print-queue]
	     (letrec ([queue-to-list (lambda (queue)
				       (if (null? queue)
					   '()
					   (cons (mcar queue) (queue-to-list (mcdr queue)))))])
	       (display (queue-to-list front-ptr))))
	    (#t (error "Unknown operation on queue" m))
	    ))
    dispatch))