4 Purely functional queues

This module implements a First–In First–Out functional queue:

(let* ((Q (make-queue))
       (Q (enqueue Q 1))
       (Q (enqueue Q 2))
       (Q (enqueue Q 3)))
  (receive (A Q)
      (dequeue Q)
    (receive (B Q)
        (dequeue Q)
      (receive (C Q)
          (dequeue Q)
        (values A B C)))))
⇒ 1 2 3

by applying repeatedly dequeue to the same <queue> object, we extract the same item from the queue:

(let* ((Q (make-queue))
       (Q (enqueue Q 1))
       (Q (enqueue Q 2))
       (Q (enqueue Q 3)))
  (receive (A Q1)
      (dequeue Q)
    (receive (B Q1)
        (dequeue Q)
      (receive (C Q1)
          (dequeue Q)
        (values A B C)))))
⇒ 1 1 1

Function: make-queue

Return a queue containing no items.

Function: queue? OBJ

Return #t if OBJ is a queue, #f otherwise.

Function: queue-length queue

Return a non–negative integer representing the number of items in queue.

Function: queue-empty? queue

Return #t if there are no items in queue, #f otherwise.

Function: enqueue queue item

Return a new queue with the enqueued item at the end.

Function: dequeue queue

Return 2 values: the item at the front of queue, and a new queue containing the all the other items. Raise a condition with kind pfds-queue-empty-condition condition if queue is empty.

Function: queue-empty-condition? OBJ

Return #t if OBJ is a condition object with kind pfds-queue-empty-condition; otherwise return #f.

Function: queue->list queue

Return a list containing all the items in queue. The order of the item in the list is the same as the order of the elements in the queue.

Function: list->queue list-of-items

Return a queue containing all the items in the given list. The order of the items in the queue is the same as the order in the list.

This document describes version 0.5.0-devel.1 of MMCK PFDS.