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attvars

This an example of using attributed variables within Logtalk objects and categories. It requires the use of a backend Prolog compiler with support for attributed variables. Tested with B-Prolog, SWI-Prolog, XSB, and YAP. The individual examples are adaptations of code found on the SWI-Prolog documentation.

When using SWI-Prolog or YAP, the attvars_hook hook object works by creating a shadow module (with the same name as the object or category internal prefix) containing the attribute_goals//1 and attr_unify_hook/2 hooks that are required by the attributed variables implementation. These hooks are compiled as module multifile predicates in order to avoid spurious compiler warning messages (note that the hook clause bodies are compiled within the context of the container object).

When using XSB, the attvars_hook hook object works by generating the install_verify_attribute_handler/4 and install_attribute_portray_hook/3 XSB directives, invoked using Logtalk-compiled arguments. These directives allow the use of common code for SWI-Prolog, XSB, and YAP.

When using B-Prolog, the attribute_goals//1 hook grammar rule is discarded.

Caveats:

The supported attribute variables built-in predicates (get_attr/3, put_attr/3, and del_attr/3) can only be called from within objects (or categories).
Calls to the supported attribute variables built-in predicates with a second argument not corresponding to the enclosing entity are compiled as references to other Logtalk entities. If you need to refer to a Prolog module instead, wrap the predicate call using the {}/1 control construct.
Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):
```
%versions
```
Start by loading the example and the required library files:
```
logtalk_load(attvars(loader)).
```
Simple finite, typed, domain reasoner:
```
domain(atom)::domain(X, [a,b]), X = c.
```

```
domain(integer)::domain(X, [1,2]), domain(integer)::domain(X, [1,3]).
```

```
domain(integer)::domain(X, [1,2,3]), domain(integer)::domain(X, [1,3]).
```