scheme 模擬queue

[code 1] shows a implementation of queue. The function enqueue! returns a queue in that the obj is added at the last of the queue. The function dequeue!removes the first item from the queue and return the first item.

[code 1]

(define (make-queue)
  (cons '() '()))

(define (enqueue! queue obj)
  (let ((lobj (cons obj '())))
    (if (null? (car queue))
	(begin
	  (set-car! queue lobj)
	  (set-cdr! queue lobj))
	(begin
	  (set-cdr! (cdr queue) lobj)
	  (set-cdr! queue lobj)))
    (car queue)))

(define (dequeue! queue)
  (let ((obj (car (car queue))))
    (set-car! queue (cdr (car queue)))
    obj))

(define q (make-queue))
;Value: q

(enqueue! q 'a)
;Value 12: (a)

(enqueue! q 'b)
;Value 12: (a b)

(enqueue! q 'c)
;Value 12: (a b c)

(dequeue! q)
;Value: a

q
;Value 13: ((b c) c)

實際上
(make-queue q 'a)
後輸出q爲:
((a) a)

(enqueue! q 'b)
(a b)
> q
((a b) b)

http://www.shido.info/lisp/scheme_asg_e.html