1 ;; weiqun zhang's solution
2 
3 (define (make-queue)
4   (let ((front-ptr '())
5 	(rear-ptr '()))
6     (define (prt-q)
7       (newline)
8       (display front-ptr))
9     (define (emp-q?)
10       (null? front-ptr))
11     (define (fro-q)
12       (if (emp-q?)
13 	  (error "FRONT called with an empty queue")
14 	  (car front-ptr)))
15     (define (ins-q! item)
16       (let ((new-pair (cons item '())))
17 	(cond ((emp-q?)
18 	       (set! front-ptr new-pair)
19 	       (set! rear-ptr new-pair))
20 	      (else
21 	       (set-cdr! rear-ptr new-pair)
22 	       (set! rear-ptr new-pair)))))
23     (define (del-q!)
24       (cond ((emp-q?)
25 	     (error "DELETE! called with an empty queue"))
26 	    (else
27 	     (set! front-ptr (cdr front-ptr)))))
28     (define (dispach m)
29       (cond ((eq? m 'insert-queue!) ins-q!)
30 	    ((eq? m 'delete-queue!) (del-q!))
31 	    ((eq? m 'front-queue) (fro-q))
32 	    ((eq? m 'empty-queue?) (emp-q?))
33 	    ((eq? m 'print-queue) (prt-q))
34 	    (else (error "Undefined operation -- MAKE-QUEUE" m))))
35     dispatch))
36 (define (insert-queue! queue item)
37   ((queue 'insert-queue!) item)
38   queue)
39 (define (delete-queue! queue)
40   (queue 'delete-queue!)
41   queue)
42 (define (empty-queue? queue)
43   (queue 'empty-queue?))
44 (define (front-queue queue)
45   (queue 'front-queue))
46 (define (print-queue queue)
47   (queue 'print-queue))