---

jupyter: jupytext: text_representation: extension: .md format_name: markdown format_version: '1.3' jupytext_version: 1.16.7 kernelspec: display_name: Logtalk language: logtalk name: logtalk_kernel ---

<!--

This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2025 Paulo Moura <pmoura@logtalk.org> SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. -->

engines - pmq

This folder contains an implementation of a message priority queue using a perpetual threaded engine holding the priority queue. At any moment, we ask for a list of the pending messages ordered by priority.

A variant is also provided that splits top messages from normal messages into separate queues. In this case, asking for a list of the pending messages returns a list with top messages before the normal messages but keeping the message sent order otherwise.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

%versions

Start by loading the example and the required libraries:

logtalk_load(pmq(loader)).

<!-- true. -->

Send some messages to the priority queue:

pmq::(send(13-let), send(5-out), send(11-the), send(17-who), send(7-dogs)).

<!-- true -->

Retrieve the current messages sorted by priority:

pmq::messages(Messages).

<!-- Messages = [who, let, the, dogs, out]. -->

After retrieving the messages the priority queue is empty until new messages are received:

pmq::messages(Messages).

<!-- Messages = []. -->

Send the next batch of messages:

pmq::(send(8-fun), send(11-have), send(3-':-)')).

Retrieve the current messages sorted by priority:

pmq::messages(Messages).

<!-- Messages = [have, fun, ':-)']. -->

Same messages but to an alternative implementation that splits top messages from normal messages instead of sorting all messages by priority:

split::(send(13-let), send(5-out), send(11-the), send(17-who), send(7-dogs)).

<!-- true. -->

split::messages(List).

<!-- List = [let, the, who, out, dogs]. -->

split::messages(List).

<!-- List = []. -->

split::(send(8-fun), send(11-have), send(3-':-)')).

<!-- true. -->

split::messages(List).

<!-- List = [have, fun, ':-)']. -->