[Pharo-project] syntax highlighting for (stateful) traits (non-local methods)

Toon Verwaest toon.verwaest at gmail.com
Sun Apr 3 17:38:32 CEST 2011

On 04/03/2011 04:58 PM, Stéphane Ducasse wrote:
> Thanks I should digest it.
> Now what I did not like in our stateful model was that
> 	- in introduced too many operators
> 	- I'm not sure I like trait state to be private because it changes a lot the smalltalk model
I'm not sure if it changes smalltalk too much! I'd rather say that it 
stays very much the same. When you weave in a stateful trait it's like 
weaving in a stateless trait, only behavior is added (from the point of 
view of the class). As for the extra operators, I'm not sure how weighty 
the aliasing of state from traits will be... I hope we can limit it.
> 	- the initialization is not modular so you end up write a lot of pattern initializeTrait and calling it.
> How you model deal with initialization beside using initform a la clos (because you have first class slots ... that I want in pharo -- see the subliminal message).
> Stef
The idea is that slots define how they should be initialized. This could 
be just a value like in CLOS, but is preferably something a lot more 

Another very cool thing that we can do now (which would require 3 lines 
of change in my system though), is that the slot can also be responsible 
for instance migration. There are four cases:
1- the instance variable didn't exist and now it does
2- the instance variable existed but disappeared
3- the metaobject changed (new semantics)
4- the metaobject stayed the same

Basically the only case that is "handled" by standard Pharo is case 1; 
and what you do there is putting nil in the field. In our model the slot 
could do something more interesting when it's migrating the instances. 
And we could tackle the other cases too. I think it's starting to get 
ideal to implement an active context on top ;-)


> On Apr 3, 2011, at 12:23 PM, Toon Verwaest wrote:
>>> How are you dealing with the fact that the application of trait with state may change the layout of the class user and that you should recompile
>>> all the class method to deal with that. And if you have two traits having state you should do the same but for the traits themselves.
>>> So this means that the method in the traits cannot be reused (ok now we do not reuse them anymore sniff it was a nice model - reuse without cost of duplication).
>>> How your layout object helps you for that?
>>> This is why I want first class slot :)
>>> Stef
>> I don't think I fully understand what you are saying...
>> The model is like this at the moment:
>> Every class has a layout attached to it. Layouts that have slots have LayoutScopes. For example, if you have
>> Class A super: Object slots: #(a b c)
>> Class B super: A slots: #(d e)
>> Then you get
>> Class A<->  PointerLayout ->  LayoutClassScope #(a b c) ->  LayoutClassScope #() ->  LayoutEmptyScope
>> Class B<->  PointerLayout ->  LayoutClassScope #(d e) ->  LayoutClassScope #(a b c) ->  LayoutClassScope #() ->  LayoutEmptyScope
>> where LayoutClassScope #(a b c) is shared between the scope of B and the layout of A. The empty LayoutClassScope comes from Object and is shared as well.
>> Now if you get a stateful trait, a stateful trait C with slots #(f) looks like this:
>> StatefulTrait C<->  PointerLayout ->  LayoutTraitScope #(f) ->  LayoutEmptyScope
>> If you were to install the trait C on B, it would become:
>> Class B<->  PointerLayout ->  LayoutClassScope #(d e) ->  LayoutForkScope ->  LayoutClassScope #(a b c) ->  LayoutClassScope #() ->  LayoutEmptyScope
>> where the LayoutForkScope would have sidescopes:
>> LayoutForkScope sideScopes: { LayoutTraitScope #(f) ->  LayoutEmptyScope }
>> Then the classbuilder will build classes by always following the public path. Sidescopes aren't public. When you compile methods on the trait, its scopes are public; but when they are installed, they aren't public since they are sidescopes.
>> However, every method is compiled on the trait or class that provided the selector, so when you install the trait-related method, it will see the state related to the trait. And when the trait is installed, the sidescopes are actually copies of the original traitscope, so the actual fieldindices are updated in the LayoutTraitScope when it's installed.
>> Then how methods get updated based on state changes is at the moment completely unrelated to the trait implementation, since methods are already updated in my class builder based on a MethodModificationModel that knows how the fields have changed. This will use the decompiler/bytecode modification/recompiler to update the methods in place.
>> The only thing that I forgot to do until now is to actually modify all the classes that use a trait, every time the state of a trait changes... But that's straightforward. We just have to ask for the users of the stateful trait and reapply their class modification. That's all nicely modeled already.
>> As for overlapping state from multiple stateful traits.... there is no overlapping state since all state is private to the trait! You can use 2 traits with same slot names. This is no problem at all since the state is only seen by that trait. And their methods are only compiled on that trait, so the methods will always know exactly which of the slots you are referring to.
>> I hope this helps somehow :) Otherwise ... wait for the paper ;)
>> cheers,
>> Toon

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