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viewpoints

Example adapted from the chapter "Classifying Prototype-Based Programming Languages" by Christophe Dony, Jacques Malenfant, and Daniel Bardou, found on the book "Prototype-Based Programming - Concepts, Languages, and Applications" published by Springer.

This prototype programming example illustrates how we can do both property sharing and value sharing in Logtalk by calling the built-in predicate modification methods asserta/1, assertz/1, and retract/1 either in the context of this or in the context of self.

In this example we have a prototype, joe_person, containing general data on Joe such as its age, name, or address, and four descendant prototypes or viewpoints, joe_sportsman, joe_employee, joe_chess_player, and joe_film_enthusiast. Each descendant contains data related to a particular viewpoint about Joe.

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

%versions

Start by loading the example:

logtalk_load(viewpoints(loader)).

<!-- true. -->

We can start by asking joe its age:

joe_person::age(Age).

<!-- Age = 30. -->

The same question could be made via any of its viewpoints:

joe_sportsman::age(Age).

<!-- Age = 30 -->

Now let's tell joe to get older:

joe_person::grow_older.

<!-- true. -->

We can verify the effect of the above message from any of the viewpoints:

joe_chess_player::age(Age).

<!-- Age = 31 -->

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

joe_employee::grow_older.

<!-- true. -->

We can check this by asking joe its age:

joe_person::age(Age).

<!-- Age = 32 -->

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, score/1 and set_score/0, defined in joe_person:

joe_person::score(Score).

<!-- Score = 0. -->

Initially, score/1 is only defined for joe_person, so every descendant or viewpoint will share its value/definition:

joe_employee::score(Score).

<!-- Score = 0. -->

But if we decide to increment the counter by sending the set_score/0 message to a descendant (don't use message broadcasting syntax in order to workaround a XSB parser bug):

joe_chess_player::set_score(2200), joe_chess_player::score(Score).

<!-- Score = 2200. -->

Then the descendant will now have a local definition for counter/1, independent of the definition in its parent, joe_person:

joe_person::score(Score).

<!-- Score = 0. -->

The other descendants/viewpoints will continue to share the definition in joe_person:

joe_sportsman::score(Score).

<!-- Score = 0. -->