3.21.scm

(define (front-ptr q) 
  (car q))

(define (rear-ptr q)
  (cdr q))

(define (set-front-ptr! q item)
  (set-car! q item))

(define (set-rear-ptr! q item)
  (set-cdr! q item))

(define (empty-queue? queue) (null? (front-ptr queue)))


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

(define (front-queue queue)
  (if (empty-queue? queue)
      (error "FRONT вызвана с пустой очередью" queue)
      (car (front-ptr queue))))

(define (insert-queue! queue item)
  (let ((new-pair (cons item '())))
    (cond ((empty-queue? queue)
           (set-front-ptr! queue new-pair)
           (set-rear-ptr! queue new-pair)
           queue)
          (else
           (set-cdr! (rear-ptr queue) new-pair)
           (set-rear-ptr! queue new-pair)
           queue))))

(define (delete-queue! queue)
  (cond ((empty-queue? queue)
         (error "DELETE! вызвана с пустой очередью" queue))
        (else
         (set-front-ptr! queue (cdr (front-ptr queue)))
         queue)))

(define (print-queue queue)
  (display (front-ptr queue)))