Object oriented programming is useful to encapsulate internal state in predicates and manipulate the state.

MINERVA allows to define classes and their methods. An object of a class has an internal state that can be accessed like blackboard data, i.e. using bb_* predicates.

Example:

:- class(counter).

Counter :: init :-
	bb_put(Counter, index, 0).

Counter :: init(InitialValue) :-
	bb_put(Counter, index, InitialValue).

Counter :: get(Value) :-
	bb_get(Counter, index, Value).

Counter :: set(Value) :-
	bb_put(Counter, index, Value).

Counter :: next(Value) :-
	Counter :: get(OldValue),
	NewValue is OldValue + 1,
	Counter :: set(NewValue),
	Value = NewValue.

:- endclass.

The MINERVA compiler processes these predicates and in addition generates predicates create_CLASS/1/2 to produce a new object.

Resulting predicates are

	Object::Method(Arg1,..,ArgN)

For the example above in classic Prolog:

	create_counter(10,Counter),
	Counter :: next(Index)

For the example above with function notation:

	Counter <- create_counter(10),
	Index <- Counter::next,
	...