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This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 2020 Paulo Moura <pmoura@logtalk.org> SPDX-FileCopyrightText: 2020 Michael T. Richter 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.

To load this example and for sample queries, please see the SCRIPT.txt file.

This example illustrates how to use SWI-Prolog Pengines from objects. It is based on simple example provided in the Pengines documentation:

https://www.swi-prolog.org/pldoc/man?section=pengine-examples

The main issue when reusing the original example code from within an object is that the pengines:pengine_create/1 meta-predicate template is ambiguous due to the use of : as the meta-predicate argument specifier (Logtalk is not based on a predicate-prefixing mechanism as used by most Prolog module systems). Thus, we must override the template using the following directive to avoid a compilation error:

:- meta_predicate(pengines:pengine_create(*)).

Two object versions are provided. The first version, dumper, uses the original example code plus the overriding directive above to write all the pengine answers to the current output.

The second version, engines, uses a threaded engine to provide an interface to the pengine in order to access the answers on demand with (1) separate predicates for creating the pengine and for querying its answers (including easily collecting all query answers in a list) and (2) asking the pengine to start computing the next solution when the current solution is retrieved.

The minimal pengine_server Prolog module code is also based on the pengines documentation available at:

https://www.swi-prolog.org/pldoc/doc_for?object=section(%27packages/pengines.html%27)