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# dynpred

This folder contains examples of using the built-in database handling
methods with object and categories. Two object hierarchies are provided,
one prototype-based, and the other class-based, in order to illustrate
the differences between asserting predicates in a class and in a prototype:

The following objects are defined:

- `root`  
	root of the prototype hierarchy; declares and defines a public,
	dynamic predicate
- `descendant`  
	simple prototype extending the root prototype

- `class`  
	root of the class hierarchy; declares and defines a public predicate
- `metaclass`  
	class metaclass
- `instance`  
	simple instance of class class

- `prototype`  
	simple prototype used to illustrate how the scope of asserted 
	predicates depends on the target object (this, self, or an explicit 
	object)

In addition, the file `categories.lgt` illustrates how to define category
predicates that handle dynamic predicates in the context of _this_ and in
the context of _self_.

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

```logtalk
%versions
```

Start by loading the example:

```logtalk
logtalk_load(dynpred(loader)).
```

Sending to descendant the message `p/1`, returns the definition in root:

```logtalk
descendant::p(Value).
```

<!--
Value = root.
-->

Asserting a local definition for `p/1` in descendant overrides the inherited 
definition:

```logtalk
descendant::(assertz(p(descendant)), p(Value)).
```

<!--
Value = descendant.
-->

If we retract the local definition, again the definition inherited from root
will be used:

```logtalk
descendant::(retractall(p(_)), p(Value)).
```

<!--
Value = root.
-->

The object `class` does not understand the message `p1/1` (the predicate is
declared only for the `class` descendant instances):

```logtalk
catch(class::p1(X), Error, true).
```

<!--
Error = error(existence_error(predicate_declaration,p1/1),logtalk(class::p1(_948),c(user,user,r(user,class,[],[])))).
-->

The same message is valid for the `class` instances:

```logtalk
instance::p1(X).
```

<!--
X = class.
-->

If we assert a clause for a new predicate, `p2/1`, in `class`
(a side-effect being a dynamic declaration of the predicate):

```logtalk
class::assertz(p2(class)).
```

<!--
true.
-->

The new predicate, like p1/1, is not available for `class`:

```logtalk
catch(class::p2(Value), Error, true).
```

<!--
catch(instance::p2(_), Error, true).
-->

But is available for the `class` instances, the same way as `p1/1`:

```logtalk
instance::p2(X).
```

<!--
X = class.
-->

If we change our mind and abolish the new predicate:

```logtalk
class::abolish(p2/1).
```

<!--
true.
-->

Then `instance` will no longer accept the `p2/1` message:

```logtalk
catch(instance::p2(_), Error, true).
```

<!--
Error = error(existence_error(predicate_declaration,p2/1),logtalk(instance::p2(_1718),c(user,user,r(user,instance,[],[])))).
-->

Using a prototype, assert three new predicates (the method `object_assert/0`
asserts the predicate `public_predicate/0` from outside the prototype; the 
method `self_assert/0` asserts the predicate `protected_predicate/0` in _self_; 
the method `this_assert/0` asserts the predicate `private_predicate/0` in _this_):

```logtalk
prototype::(object_assert, self_assert, this_assert).
```

<!--
true
-->

Check the resulting scope of each predicate:

```logtalk
prototype::dynamic_predicates.
```

<!--
public_predicate/0 - public
protected_predicate/0 - protected
private_predicate/0 - private
true.
-->
