viernes, septiembre 26, 2008

(Thinking in Objects)

Henry Story, en su blog The Sun BabelFish, plantea un problema que mide los límites de la programación orientada a objetos, y la contrapone a las herramientas usuales en la Web Semantica. Con un ejemplo basado en el mundo del autismo, Story presenta un mundo de vistas personales, no de objetos reales. Probablemente el ejemplo usado no fue muy afortunado, particularmente en cómo implementarlo en OOP y en su contrapartida con RDF. Henry fue muy criticado, y en general con acierto, tanto en su expresión del problema en OOP como en la correlación parcial entre ambos ejemplos. Sin embargo, la presentación del problema es estimulante para medir OOP y sus límites:

In order to be able to have a mental theory one needs to be able to understand that other people may have a different view of the world. On a narrow three dimensional understanding of 'view', this reveals itself in that people at different locations in a room will see different things. One person may be able to see a cat behind a tree that will be hidden to another. In some sense though these two views can easily be merged into a coherent description. They are not contradictory. But we can do the same in higher dimensions. We can think of people as believing themselves to be in one of a number of possible worlds. Sally believes she is in a world where the ball is in the basket, whereas Ann believes she is in a world where the ball is in the box. Here the worlds are contradictory. They cannot both be true of the actual world.

To be able to make this type of statement one has to be able to do at least the following things:

  • Speak of ways the world could be
  • Refer to objects across these worlds
  • Compare these worlds
Henry propone un ejemplo (imperfecto y criticado por varios de sus lectores):

Let us illustrate this with a simple example. Let us see how one could naively program the puppet play in Java. Let us first create the objects we will need:

Person sally = new Person("Sally");
Person ann = new Person("Ann");
Container basket = new Container("Basket");
Container box = new Container("Box");
Ball ball = new Ball("b1");
Container room = new Container("Room");
So far so good. We have all the objects. We can easily imagine code like the following to add the ball into the basket, and the basket into the room.
Perhaps we have methods whereby the objects can ask what their container is. This would be useful for writing code to make sure that a thing could not be in two different places at once - in the basket and in the box, unless the basket was in the box.
Container c = ball.getImmediateContainer();
Assert.true(c == basket);
try {
} catch (InTwoPlacesException e) {
All that is going to be tedious coding, full of complicated issues of their own, but it's the usual stuff. Now what about the beliefs of Sally and Ann? How do we specify those? Perhaps we can think of sally and ann as being small databases of objects they are conscious of. Then one could just add them like this:
But the problem should be obvious now. If we move the ball from the basket to the box, the state of the objects in sally and ann's database will be exactly the same! After all they are the same objects!
Ball sb = sally.get(Ball.class,"b1");
//that is because
Ball ab = ann.get(Ball.class,"b1");
There is really no way to change the state of the ball for one person and not for the other,... unless perhaps we give both people different objects. This means that for each person we would have to make a copy of all the objects that they could think of. But then we would have a completely different problem: namely deciding when these two objects were the same. For it is usually understood that the equality of two objects depends on their state. So one usually would not think that an physical object could be the same if it was in two different physical places. Certainly if we had a ball b1 in a box, and another ball b2 in a basket, then what on earth would allow us to say we were speaking of the same ball? Perhaps their name, if it we could guarantee that we had unique names for things. But we would still have some pretty odd things going on then, we would have objects that would somehow be equal, but would be in completely different states! And this is just the beginning of our problems. Just think of the dangers involved here in taking an object from ann's belief database, and how easy it would be to by mistake allow it to be added to sally's belief store.
Henry aboga por el uso de RDF (Resource Description Framework) como herramienta capaz de expresar adecuadamente las distintas vistas de un problema, en un universo de vistas en lugar de objetos. Pero sin duda, esto merece otro espacio.
De los comentarios que siguen al artículo, particularmente abre un poco más el que hiciera Ryan, y la respuesta de Henry
I really like your analysis of OOP and it's relation to autism. I have never thought of it in such a way, but it does make a lot of sense (when I am able to remove my preconceptions). However, why is autism bad in this scenario (if you are even implying it)? I can understand that if our perspective is not an omniscient one then this can fail us. Would you please provide a applied example of the problem at hand with relation to your point on Semantic Web? Thanks a lot!

Thanks for asking Ryan. Yes there are a lot of examples. The following article "Extending the Representational State Transfer (REST) Architectural Style for Decentralized Systems" which you can find here
makes the point about how the distance between the source of a message and the recipient of a message makes perfect immediate communication impossible, if you think of it as resources having the same access to an object. But if you think of it as message passing then you can do some interesting things... Ok I read that quickly, but that is what made me decide to write this article out today.

In the AddressBook I am writing, which I describe in an audio slide cast here:
I need to get data from distributed places around the web. This can only be done seriously if you accept that there will be spammers, liars, and just simply wrong data out there. So though you may by default merge data, you may want to make it easy to unmerge it too. I wrote about that in more detail here:

As I said, if you are writing tools, that you can think of as physical, mechanical objects, that don't have to have points of view on the universe, say if you are writing a web browser, a calculator, or some such thing, then this is not important. But as soon as you want to mesh the information on the web, you will need to take the opinion of others into account. We are fast moving to a world where this is going to become more and more important.

In any case it is good to know the limitations of your tools. :-)

En el mismo sentido, apunta Benjamin Damman:

Hmmmm. Parts of your intriguing article made me think of erlang.

"When one fetches information from a remote server one just has to take into account the fact that the server's view of the world may be different and incompatible in some respects with one's own. One cannot in an open world just assume that every body agrees with everything. One is forced to develop languages that enable a theory of mind. A lot of failures in distributed programming can probably be traced down to working with tools that don't."

Erlang was created for coding highly fault-tolerant (and distributed) systems; characteristics stemming this fact might make it an example of a language that 'enables a theory of mind.'

Un papel sugerente de ideas, mas allá de los puntos observados por su crítica.

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