Title

Queues based on lists

Author

John Cowan <cowan@ccil.org>

Status

This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see here. To provide input on this SRFI, please mail to <srfi minus 117 at srfi dot schemers dot org>. See instructions here to subscribe to the list. You can access previous messages via the archive of the mailing list.

Abstract

List queues are mutable ordered collections that can contain any Scheme object. Each list queue is based on an ordinary Scheme list containing the elements of the list queue by maintaining pointers to the first and last pairs of the list. It's cheap to add or remove elements from the front of the list or to add elements to the back, but not to remove elements from the back. List queues are disjoint from other types of Scheme objects.

Issues

No issues at present.

Rationale

The list queues described in this SRFI are objects of a disjoint type that provide an ordered sequence of arbitrary Scheme objects. Like lists, they provide sequential access to their elements; unlike lists, they normally should not share storage with other list queues, unless special precautions have been taken.

Technically, a queue is an object that makes it efficient to remove elements from the front and to add elements to the back. List queues also make it efficient to add elements to the front, but removing elements from the back is inefficient. Therefore, because these objects do not provide the normal behavioral guarantees of deques, they are not referred to as deques. True deques will be provided in a future SRFI.

Historically, objects of this form were called "tconc structures" (where "tconc" is short for "tail concatenate"), and Lispers have tended to roll their own using pairs. This version uses a record to hold the first-pair and last-pair pointers rather than a pair, but uses pairs and the empty list internally.

Because the representation of list queues as lists is exposed by the implementation, it's not necessary to provide a comprehensive API for list queues. Instead, SRFI 1 and other list APIs can serve the same purpose, using the conversion procedures to convert between list queues and lists fairly cheaply. Consequently, the API provided here over and above the bare necessities of queueing and dequeueing elements is roughly analogous to the R7RS-small API for lists. It also subsumes the Chicken and ​SLIB APIs.

Specification

Constructors

(make-list-queue list [ last ])

Returns a newly allocated list queue containing the elements of list in order. The result shares storage with list. If the last argument is not provided, this operation is O(n) where n is the length of list.

However, if last is provided, make-list-queue returns a newly allocated list queue containing the elements of the list whose first pair is first and whose last pair is last. It is an error if the pairs do not belong to the same list. Alternatively, both first and last can be the empty list. In either case, the operation is O(1).

Note: To apply a non-destructive list procedure to a list queue and return a new list queue, use (make-list-queue (proc (list-queue-list list-queue))).

(list-queue element ...)

Returns a newly allocated list queue containing the elements. This operation is O(n) where n is the number of elements.

(list-queue-copy list-queue)

Returns a newly allocated list queue containing the elements of list-queue. This operation is O(n) where n is the length of list-queue

(list-queue-unfold stop? mapper successor seed [ queue ])

Performs the following algorithm:

If the result of applying the predicate stop? to seed is true, return queue. Otherwise, apply the procedure mapper to seed, returning a value which is added to the front of queue. Then get a new seed by applying the procedure successor to seed, and repeat this algorithm.

If queue is omitted, a newly allocated list queue is used.

(list-queue-unfold-right stop? mapper successor seed [ queue ])

Performs the following algorithm:

If the result of applying the predicate stop? to seed is true, return the list queue. Otherwise, apply the procedure mapper to seed, returning a value which is added to the back of the list queue. Then get a new seed by applying the procedure successor to seed, and repeat this algorithm.

If queue is omitted, a newly allocated list queue is used.

Predicates

(list-queue? obj)

Returns #t if obj is a list queue, and #f otherwise. This operation is O(1).

(list-queue-empty? list-queue)

Returns #t if list-queue has no elements, and #f otherwise. This operation is O(1).

Accessors

(list-queue-front list-queue)

Returns the first element of list-queue. If the list queue is empty, it is an error. This operation is O(1).

(list-queue-back list-queue)

Returns the last element of list-queue. If the list queue is empty, it is an error. This operation is O(1).

(list-queue-list list-queue)

Returns the list that contains the members of list-queue in order. The result shares storage with list-queue. This operation is O(1).

(list-queue-first-last list-queue)

Returns two values, the first and last pairs of the list that contains the members of list-queue in order. If list-queue is empty, returns two empty lists. The results share storage with list-queue. This operation is O(1).

Mutators

(list-queue-add-front! list-queue element)

Adds element to the beginning of list-queue. Returns an unspecified value. This operation is O(1).

(list-queue-add-back! list-queue element)

Adds element to the end of list-queue. Returns an unspecified value. This operation is O(1).

(list-queue-remove-front! list-queue)

Removes the first element of list-queue and returns it. If the list queue is empty, it is an error. This operation is O(1).

(list-queue-remove-back! list-queue)

Removes the last element of list-queue and returns it. If the list queue is empty, it is an error. This operation is O(n) where n is the length of list-queue, because queues do not not have backward links.

(list-queue-remove-all! list-queue)

Removes all the elements of list-queue and returns them in order as a list. This operation is O(1).

(list-queue-set-list! list-queue list [ last ])

Replaces the list associated with list-queue with list, effectively discarding all the elements of list-queue in favor of those in list. Returns an unspecified value. This operation is O(n) where n is the length of list. If last is provided, it is treated in the same way as in make-list-queue, and the operation is O(1).

Note: To apply a destructive list procedure to a list queue, use (list-queue-set-list! (proc (list-queue-list list-queue))).

(list-queue-append list-queue ...)

Returns a list queue which contains all the elements in front-to-back order from all the list-queues in front-to-back order. The result does not share storage with any of the arguments. This operation is O(n) in the total number of elements in all queues.

(list-queue-append! list-queue ...)

Returns a list queue which contains all the elements in front-to-back order from all the list-queues in front-to-back order. It is an error to assume anything about the contents of the list-queues after the procedure returns. This operation is O(n) in the total number of queues, not elements. It is not part of the R7RS-small list API, but is included here for efficiency when pure functional append is not required.

(list-queue-concatenate list-of-list-queues)

Returns a list queue which contains all the elements in front-to-back order from all the list queues which are members of list-of-list-queues in front-to-back order. The result does not share storage with any of the arguments. This operation is O(n) in the total number of elements in all queues. It is not part of the R7RS-small list API, but is included here to make appending a large number of queues possible in Schemes that limit the number of arguments to apply.

Mapping

(list-queue-map proc list-queue)

Applies proc to each element of list-queue in unspecified order and returns a newly allocated list queue containing the results. This operation is O(n) where n is the length of list-queue.

(list-queue-map! proc list-queue)

Applies proc to each element of list-queue in front-to-back order and modifies list-queue to contain the results. This operation is O(n) in the length of list-queue. It is not part of the R7RS-small list API, but is included here to make transformation of a list queue by mutation more efficient.

(list-queue-for-each proc list-queue)

Applies proc to each element of list-queue in front-to-back order, discarding the returned values. Returns an unspecified value. This operation is O(n) where n is the length of list-queue.

Implementation

The sample implementation places the identifiers defined above into the list-queue library.

The sample implementation contains the following files:

Copyright

Copyright © John Cowan, 2014. All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.


Editor: Michael Sperber
Last modified: Tue Jun 9 08:44:01 MST 2015