Queue Concurrency

queue.scm

; 3.22

(define (make-queue)
  (let ((front-ptr '())
        (rear-ptr '()))
    
    (define (empty?)
      (null? front-ptr))
    
    (define (insert! item)
      (let ((new-pair (mcons item '())))
        (cond ((empty?)
               (set! front-ptr new-pair)
               (set! rear-ptr new-pair)
               )
              (else
               (set-mcdr! rear-ptr new-pair)
               (set! rear-ptr new-pair)))))
    
    (define (front)
      (cond ((empty?) (error "Empty queue"))
            (else (mcar front-ptr))))
    
    (define (rear)
      (cond ((empty?) (error "Empty queue"))
            (else (mcar rear-ptr))))
    
    (define (delete!)
      (cond ((empty?) (error "Empty queue"))
            (else (set! front-ptr (mcar front-ptr)))))
    
    (define (dispatch msg)
      (cond ((eq? msg 'insert!) insert!)
            ((eq? msg 'delete!) (delete!))
            ((eq? msg 'empty?) (empty?))
            ((eq? msg 'front) (front))
            ((eq? msg 'rear) (rear))
            (else (error "Bad message"))))
    dispatch
    ))