sicp deque implementation

deque.rkt

#lang racket
(define (make-deque) (mcons null null))

(define (front deque) (mcar deque))
(define (rear deque) (mcdr deque))

(define (set-front! deque item) (set-mcar! deque item))
(define (set-rear! deque item) (set-mcdr! deque item))

(define (empty? deque) (null? (front deque)))

(define (print deque)
  (define (collect deque result)
    (if (null? deque)
        result
        (collect (mcdr deque)
                 (cons (mcar (mcar deque)) result))))
  (collect (rear deque) null))

(define (insert-front! deque item)
  (let ([new-pair (mcons (mcons item null) null)])
    (cond ((empty? deque)
           (set-front! deque new-pair)
           (set-rear! deque new-pair))
          (else
           (set-mcdr! (mcar new-pair) (front deque))           
           (set-mcdr! (front deque) new-pair)
           (set-front! deque new-pair))))
  (print deque))

(define (insert-rear! deque item)
  (let ([new-pair (mcons (mcons item null) null)])
    (cond ((empty? deque)
           (set-front! deque new-pair)
           (set-rear! deque new-pair))
          (else
           (set-mcdr! (mcar (rear deque)) new-pair)
           (set-mcdr! new-pair (rear deque))
           (set-rear! deque new-pair))))
  (print deque))

(define (delete-rear! deque)
  (if (empty? deque)
      (error 'empty)
      (begin
        (set-mcdr! (mcar (mcdr (rear deque))) null)
        (set-rear! deque (mcdr (rear deque)))
        (print deque))))

(define (delete-front! deque)
  (if (empty? deque)
      (error 'empty)
      (begin
        (set-mcdr! (mcdr (mcar (front deque))) null)
        (set-front! deque (mcdr (mcar (front deque))))
        (print deque)))

Exercise 3.23: A deque (“double-ended queue”) is a sequence in which items can be inserted and deleted at either the front or the rear. Operations on deques are the constructor make-deque, the predicate empty-deque?, selectors front-deque and rear-deque, mutators front-insert-deque!, rear-insert-deque!, front-delete-deque!, and rear-delete-deque!. Show how to represent deques using pairs, and
give implementations of the operations.* All operations should be accomplished in Θ(1) steps.
* Be careful not to make the interpreter try to print a structure that contains cycles.