4 Imperative Queues

(require (planet cce/scheme:3:0/queue))

This module provides a mutable queue representation.

(make-queue) → queue/c

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

; attempt to remove from empty queue

> (dequeue! q)

'program broke the contract (-> "nonempty-queue" any/c) on

dequeue!; expected <nonempty-queue>, given: #<queue>

> (enqueue! q 2)

> (enqueue! q 3)

> (dequeue! q)

2

> (dequeue! q)

3

; attempt to remove from empty queue

> (dequeue! q)

'program broke the contract (-> "nonempty-queue" any/c) on

dequeue!; expected <nonempty-queue>, given: #<queue>

(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

queue/c : flat-contract?

nonempty-queue/c : flat-contract?

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