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Pharo Object Model

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Oleksandr Zaitsev

Pharo Object Model

Hello,

I'm trying to understand the logic behind The Pharo Object Model.

Rule 3 states that every class in Pharo has a subclass. I was expecting some cool loop, like in the case of Metaclass, which is an instance and also a class of Metaclass class. But ProtoObject, which is the root of an inheritance hierarchy does not seem to have a superclass (ProtoObject superclass answers nil, which is not a class).

Does this mean that Pharo Object Model is inconsistent? Is ProtoObject an exception of the third rule?

If you think about it, an inheritance hierarchy can't have a loop, because otherwise, when looking for a nonexistent method in superclasses, Pharo would be stuck in that loop.

So why is there even a loop in instance hierarchy (Metaclass class)? Does it have a purpose, or is it there only to satisfy the second rule of the Object Model? Because if the consistency of third rule can not be satisfied, wouldn't it be better to add two exceptions to the model?

Please correct me if my understanding or reasoning is wrong.

Thanks!

Henrik Sperre Johansen

Re: Pharo Object Model

> On 10 Feb 2017, at 13:04 , Oleksandr Zaytsev <[hidden email]> wrote:
>
> Hello,
>
> I'm trying to understand the logic behind The Pharo Object Model.
>
> Rule 3 states that every class in Pharo has a subclass. I was expecting some cool loop, like in the case of Metaclass, which is an instance and also a class of Metaclass class. But ProtoObject, which is the root of an inheritance hierarchy does not seem to have a superclass (ProtoObject superclass answers nil, which is not a class).
>
> Does this mean that Pharo Object Model is inconsistent? Is ProtoObject an exception of the third rule?
>
> If you think about it, an inheritance hierarchy can't have a loop, because otherwise, when looking for a nonexistent method in superclasses, Pharo would be stuck in that loop.
>
> So why is there even a loop in instance hierarchy (Metaclass class)? Does it have a purpose, or is it there only to satisfy the second rule of the Object Model? Because if the consistency of third rule can not be satisfied, wouldn't it be better to add two exceptions to the model?
>
> Please correct me if my understanding or reasoning is wrong.
>
> Thanks!

Rule 3 says every class has a superclass; it does not say the superclass must be a class. (but it must be an object, as per #1)
Hence a superclass of nil isn't inconsistent (as nil is an object), and is a valid way to terminate the inheritance hierarchy.

Metaclass class class = Metaclass is necessary not to satisfy #2, but to satisfy both #1 and #2.
Rule 2, states that every object is an instance of a class.
Classes are also objects according to #1, and as such are instances of a class (Metaclass), so the circular definition when it comes to Metaclass is needed to satisfy both.

Cheers,
Henry

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