On this page:
make-queue
enqueue!
dequeue!
queue-empty?
queue?
queue/ c
nonempty-queue/ c
Version: 5.0.1

17 Imperative Queues

Carl Eastlund <cce@racket-lang.org>

 (require unstable/queue)

This library is unstable; compatibility will not be maintained. See Unstable for more information.

This module provides a simple mutable queue representation, first-in/first-out only. Operations on queues mutate it in a thread-unsafe way.

Produces an empty queue.

(enqueue! q v)  void?
  q : queue/c
  v : any/c
Adds an element to the back of a queue.

(dequeue! q)  any/c
  q : nonempty-queue/c
Removes an element from the front of a nonempty queue, and returns that element.

Examples:

  (define q (make-queue))
  > (enqueue! q 1)
  > (dequeue! q)

  1

  > (enqueue! q 2)
  > (enqueue! q 3)
  > (dequeue! q)

  2

  > (dequeue! q)

  3

(queue-empty? q)  boolean?
  q : queue/c
Recognizes whether a queue is empty or not.

Examples:

  (define q (make-queue))
  > (queue-empty? q)

  #t

  > (enqueue! q 1)
  > (queue-empty? q)

  #f

  > (dequeue! q)

  1

  > (queue-empty? q)

  #t

(queue? v)  boolean?
  v : any/c
This predicate recognizes queues.

Examples:

  > (queue? (make-queue))

  #t

  > (queue? 'not-a-queue)

  #f

These contracts recognize queues; the latter requires the queue to contain at least one value.