sicp-ex-3.22

  
  
  
 (define (make-queue) 
   (let ((front-ptr '()) 
         (rear-ptr '())) 
         (define (emtpy-queue?) (null? front-ptr)) 
         (define (set-front-ptr! item) (set! front-ptr item)) 
         (define (set-rear-ptr! item) (set! rear-ptr item)) 
         (define (front-queue)  
           (if (emtpy-queue?) 
             (error "FRONT called with an empty queue") 
             (car front-ptr))) 
         (define (insert-queue! item) 
           (let ((new-pair (cons item '()))) 
             (cond ((emtpy-queue?) 
                     (set-front-ptr! new-pair) 
                     (set-rear-ptr! new-pair)) 
                   (else  
                     (set-cdr! rear-ptr new-pair) 
                     (set-rear-ptr! new-pair))))) 
         (define (delete-queue!) 
           (cond ((emtpy-queue?) 
                   (error "DELETE called with an emtpy queue")) 
                 (else (set-front-ptr! (cdr front-ptr))))) 
      
         (define (print-queue) front-ptr) 
  
         (define (dispatch m) 
           (cond ((eq? m 'empty-queue) emtpy-queue?) 
                 ((eq? m 'front-queue) front-queue) 
                 ((eq? m 'insert-queue!) insert-queue!) 
                 ((eq? m 'delete-queue!) delete-queue!) 
                 ((eq? m 'print-queue) print-queue) 
                 (else (error "undefined operation -- QUEUE" m)))) 
         dispatch)) 
  
 }}}}