Include Paulo Moura's Logtalk OO LP system

git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@53 b08c6af1-5177-4d33-ba66-4b1c6b8b522a

Logtalk/examples/classvars/NOTES

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+=================================================================
+Logtalk - Object oriented extension to Prolog
+Release 2.8.4
+
+Copyright (c) 1998-2001 Paulo Moura.  All Rights Reserved.
+=================================================================
+
+To load all objects in this example consult the classvars.loader utility
+file (note that the *.loader files are Prolog files).
+
+This folder contains an example that shows how to implement class variables 
+as defined in Smalltalk. The name shared instance variables is however much
+more accurate. In systems like Logtalk that enable the use of explicit 
+metaclasses, true class variables are just the class (as an object) own 
+instance variables!

Logtalk/examples/classvars/SCRIPT

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+=================================================================
+Logtalk - Object oriented extension to Prolog
+Release 2.8.4
+
+Copyright (c) 1998-2001 Paulo Moura.  All Rights Reserved.
+=================================================================
+
+
+% get the value of the class variable for each instance:
+
+| ?- instance1::cv(Value1), instance2::cv(Value2), instance3::cv(Value3).
+
+Value1 = 0
+Value2 = 0
+Value3 = 0
+yes
+
+
+% change the value of the class variable via instance1:
+
+| ?- instance1::set_cv(1).
+
+yes
+
+
+% get the value of the class variable for the other instances:
+
+| ?- instance2::cv(Value2), instance3::cv(Value3).
+
+Value2 = 1
+Value3 = 1
+yes

Logtalk/examples/classvars/classvars.loader

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+
+:- initialization(
+	logtalk_load([
+		root,
+		instance1,
+		instance2,
+		instance3])).

Logtalk/examples/classvars/instance1.lgt

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+
+:- object(instance1,
+	instantiates(root)).
+
+
+:- end_object.

Logtalk/examples/classvars/instance2.lgt

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+
+:- object(instance2,
+	instantiates(root)).
+
+
+:- end_object.

Logtalk/examples/classvars/instance3.lgt

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+
+:- object(instance3,
+	instantiates(root)).
+
+
+:- end_object.

Logtalk/examples/classvars/root.lgt

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+
+:- object(root,
+	instantiates(root)).
+
+
+	:- private(cv_/1).
+	:- dynamic(cv_/1).
+	:- mode(cv_(?integer), zero_or_one).
+
+	:- public(cv/1).
+	:- mode(cv(?integer), zero_or_one).
+
+	:- public(set_cv/1).
+	:- mode(set_cv(+integer), one).
+
+
+	cv_(0).						% cv value is stored locally, in this class
+
+
+	cv(Value) :-
+		cv_(Value).				% retrive cv value, shared for all instances
+
+
+	set_cv(Value) :-
+		retractall(cv_(_)),		% retract old cv value from this class
+		asserta(cv_(Value)).	% assert the new value in this class
+
+
+:- end_object.