On 04/03/2011 02:29 PM, Alexandre Bergel wrote:
I guess that what Stef meant is the following:
One of the problems when sharing code among mixin and stateful-trait
application is that the physical layout of instances varies between mixin
applications. (Section 6.5 of
http://bergel.eu/download/papers/Berg07e-StatefulTraits.pdf)
We have T.y and C.x
T has a method #getY, for which its bytecode returns the first slot (i.e., y).
If C uses T, the #getY needs to be adapted to return the second slot right?
Cheers,
Alexandre
Indeed. And this is not different from changing a layout of a class
having as impact that you have to update the methods.
The default traits implementation already recompiles all methods anyway
whenever it installs the trait. What I do is, I just let the trait
compile itself on the subpart of the class that originally defined the
trait.
Since this subpart is a copy of the original trait (just like the
default traits does), it also has a COPY of the original layout. In this
copy, the indices of the slots are updated when they are mixed with the
class that is going to use the trait. So it's being compiled in the
scope of the installed version of the trait-layout. So that easily works.
cheers
On 3 Apr 2011, at 06:23, 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
