Basically here we discuss how to introduce in Pharo stateful class extension and stateful traits.�I like these features a lot because it means I would be able to reuse the same class differently depending on the context and the surrounding classes. But if we overuse that we will definitely go into chaos.�Martin tried in Mist and he concluded that he should have class composition instead of stateful traits / class extension.�

For now the common way to add extra instance variable is: you add an extra instance variable named 'properties' in the main class which needs to be committed on the other package, then in the class extension you initialize this varable lazily to a dictionary, and to use extra instance variable, you store/read them in the properties dictionary by overriding the getter/setter method. You can look at the refactoring browser AST which is implemented like that. This way is nice because it is easily optimizable by slots (even if we have not done it yet) and you only add 1 instance variable to the main class whereas you can add an infinite number of instance variables.

Best,


2013/10/31 Tudor Girba <tudor@tudorgirba.com>
I completely disagree with this point of view :).

We should assume an open world, not a close one. From this point of view, any part of the system should be extensible by anyone. In most other languages I know, it is not even possible to extend easily a class with new functionality. In Pharo we can, and we know it is a powerful mechanism. It is not the responsibility of the base class to know what extensions are out there and protect against them. Just like with subclassing, It is in the responsibility of the extender.

We should be able to do the same with state as well. Without this mechanism, we are forced to put in place clunky dictionary-based mechanism to support state extension. Essentially, any white-box framework does that. For example, Morphic does that, FAMIX and Roassal do that, too (and yes, this is not a bad thing).

We need this mechanism in the environment, and if I understand Slots correctly, now we have first class support for it. This also means that overrides will be easier to deal with, too. Of course, overrides can induce headaches from time to time, but we should treat these headaches with proper tools, not by forbidding the world to extend.

And if we are at it, we should also be able to extend a class with Traits, too.

Cheers,
Doru




On Wed, Oct 30, 2013 at 10:54 PM, Camillo Bruni <camillobruni@gmail.com> wrote:

On 2013-10-30, at 22:36, Igor Stasenko <siguctua@gmail.com> wrote:

> I don't think there's something to fix.
> You cannot 'extend' classes belonging to other package in any other way
> than adding extension methods.
> Allowing extension of ivars or any other vars by foreign package
> is road to nowhere.
>
> I would not like if shape of my kernel classes depends on what packages i load
> or in what order i loaded them.
> To me it is clear that if one needs to add/remove/modify instance variables
> of some class, those changes should belong to the package containing that class,
> not some random package.

Exactly, it would cause the same problem as we have with overrides in monticello



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