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Logtalk - Object oriented extension to Prolog
Release 2.10.0

Copyright (c) 1998-2002 Paulo Moura.  All Rights Reserved.
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% we can start by asking joe its age:

| ?- joePerson::age(Age).

Age = 30
yes


% the same question could be made via any of its viewpoints:

| ?- joeSportsman::age(Age).

Age = 30
yes


% now let's tell joe to grow old:

| ?- joePerson::growOld.

yes


% we can verify the effect of the above message from any of the viewpoints:

| ?- joeFilmEnthusiast::age(Age).

Age = 31
yes


% because the growOld/0 and the age/1 predicates are implemented using 
% property sharing, we can send the growOld message to any viewpoint:

| ?- joeEmployee::growOld.

yes


% we can check this by asking joe its age:


| ?- joePerson::age(Age).

Age = 32
yes


% as you can see, although the modification message have been sent to a 
% descendant, its the predicate age/1 in the parent that got updated


% to illustrate value sharing we use a couple of predicates, counter/1 and
% incCounter/0, defined in joePerson:


| ?- joePerson::counter(Counter).

Counter = 0
yes


% initially, counter/1 is only defined for joePerson, so every descendant 
% or viewpoint will share its value/definition:

| ?- joeSportsman::counter(Counter).

Counter = 0
yes


% but if we decide to increment the counter by sending the incCounter/0 
% message to a descendant:

| ?- joeFilmEnthusiast::incCounter.

yes

| ?- joeFilmEnthusiast::counter(Counter).

Counter = 1
yes


% then the descendant will now have a local definition for counter/1,
% independent of the definition in its parent, joePerson:

| ?- joePerson::counter(Counter).

Counter = 0
yes


% the other descendants/viewpoints will continue to share the definition 
% in joePerson:

| ?- joeSportsman::counter(Counter).

Counter = 0
yes