Object Oriented Programming

Another typical way to use the module system is for defining classes within an object oriented paradigm. The class structure and the methods of a class can be defined in a module and the explicit module-boundary overruling describes in section can by used by the message passing code to invoke the behaviour. An outline of this mechanism is given below.

%	Define class point

:- module(point, []).		% class point, no exports

%	 name		type,		default	access
%					value

variable(x,		integer,	0,	both).
variable(y,		integer,	0,	both).

%	  method name	predicate name	arguments

behaviour(mirror,	mirror,		[]).

mirror(P) :-
	fetch(P, x, X),
	fetch(P, y, Y),
	store(P, y, X),
	store(P, x, Y).

The predicates fetch/3 and store/3 are predicates that change instance variables of instances. The figure below indicates how message passing can easily be implemented:

%	invoke(+Instance, +Selector, ?ArgumentList)
%	send a message to an instance

invoke(I, S, Args) :-
	class_of_instance(I, Class),
	Class:behaviour(S, P, ArgCheck), !,
	convert_arguments(ArgCheck, Args, ConvArgs),
	Goal =.. [P|ConvArgs],
	Class:Goal.

The construct `Module:Goal' explicitely calls Goal in module Module. It is discussed in more detail in section .