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. -->

questions

This example illustrates the question asking mechanism, which complements the message printing mechanism to allow questions to be abstracted and intercepted. The question used in this example is taken from Douglas Adams book "The Hitchhiker's Guide to the Galaxy".

The loader.lgt file loads code that asks the question in the top-level interpreter.

The loader_gui.lgt file loads code that asks the question using a Java GUI dialog. It can be used with JIProlog, SWI-Prolog, or YAP. Running on SWI-Prolog or YAP requires a functional JPL library installation.

When running the GUI dialog on the macOS Terminal application, you may get a Java error saying that the AWT cannot be started. In alternative, try to run the GUI version using either JIProlog or from within the SWI-Prolog macOS application instead of using the shell integration script. This issue is due to a macOS Java issue that's orthogonal to both backends and Logtalk.

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

%versions

Start by loading the example:

logtalk_load(questions(loader)).

We can now ask the ultimate question (skip if running as a notebook):

(current_object(jupyter) -> true; logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N)).

<!-- The answer to the ultimate question of Life, The Universe and Everything is? > 42.

N = 42. -->

Note that the fourth argument of the `logtalk::ask_question/5` predicate is a closure that is used to type-check the answer (skip if running as a notebook):

(current_object(jupyter) -> true; logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N)).

<!-- The answer to the ultimate question of Life, The Universe and Everything is? > icecream. > tea. > 42.

N = 42. -->

If running using JIProlog, SWI-Prolog, or YAP as the backend, asking the question again after the goal will present a GUI dialog:

logtalk_load(questions(loader_gui)).

Skip if running as a notebook:

(current_object(jupyter) -> true; logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N)).