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# 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):

```logtalk
%versions
```

Start by loading the example:

```logtalk
logtalk_load(questions(loader)).
```

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

```logtalk
(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):

```logtalk
(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
logtalk_load(questions(loader_gui)).
```

<!--
true.
-->

 Skip if running as a notebook:

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

<!--
true.
-->
