Lisp学习笔记（四）

带有局部状态的队列实现：

(define (make-queue)
  (let ((front-ptr '())
        (rear-ptr '())
        ) 
    (define (empty-queue?)
      (eq? front-ptr '())
        )
    (define (front-queue)
      (if (empty-queue?) (error "called with an empty queue")
          (car front-ptr)
          )
      )
    (define (print)
      front-ptr
      )
    (define (insert-queue item)
      (let ((new-pair (cons item '())))
      (cond ((empty-queue?)
             (set! front-ptr new-pair)
             (set! rear-ptr new-pair)
             (print))
            (else (set-cdr! rear-ptr new-pair) (set! rear-ptr new-pair) (print)))
      ))
    (define (delete-queue)
      (cond ((empty-queue?) (display "called with an empty queue"))
          (else (set! front-ptr (cdr front-ptr)) (print))
          )
      )
    (define (dispatch m)
      (cond ((eq? m 'empty?) (empty-queue?))
            ((eq? m 'front)  (front-queue))
            ((eq? m 'insert) insert-queue)
            ((eq? m 'delete) (delete-queue))
            (else (error "Unknown operation for queue" m))
        )
      )    
    dispatch)
  )

使用的编译器是DrScheme，语言是R5RS。
使用这种消息分发的实现方式，封装性更强，扩展起来也比较容易。