#lang sicp

(define (make-queue)
  (let ((front-ptr '())
        (rear-ptr '()))
    (define (front)
      (cond ((empty?)
             (error "FRONT called with empty queue"))
            (else
             (car front-ptr))))
    (define (insert! item)
      (let ((new-pair (cons item '())))
        (cond ((empty?) 
               (set! front-ptr new-pair)
               (set! rear-ptr new-pair))
              (else
               (set-cdr! rear-ptr new-pair)
               (set! rear-ptr new-pair)))))
    (define (delete!)
      (cond ((empty?)
             (error "DELETE! called with empty queue"))
            (else
             (set! front-ptr (cdr front-ptr)))))
    (define (dispatch m)
      (cond ((eq? m 'empty?)
             (null? front-ptr))
            ((eq? m 'front) (front))
            ((eq? m 'insert!) insert!)
            ((eq? m 'delete!) (delete!))
            ((eq? m 'print)
             (display front-ptr)
             (newline))
            (else
             (error "Unknown method: " m))))
    dispatch))
    
(define q1 (make-queue))
(q1 'print)
((q1 'insert!) 'a)
(q1 'print)
((q1 'insert!) 'b)
(q1 'print)
(q1 'front)
(q1 'print)
(q1 'delete!)
(q1 'print)