Hi $Larry et al,

I brought this up as a question at YAPC::EU and found to my

surprise that no one seems to have thought of it yet. This is

the mail I said I'd write. (And apologies, Larry. ­ )

Consider the classic example of roles named Dog and Tree which

both have a `bark` method. Then there is a class that for some

inexplicable reason, assumes both roles. Maybe it is called

Mutant. This is standard fare so far: the class resolves the

conflict by renaming Dog's `bark` to `yap` and all is well.

But now consider that Dog has a method `on_sound_heard` that

calls `bark`.

You clearly don't want that to suddenly call Tree's `bark`.

Unless, of course, you actually do.

It therefore seems necessary to me to specify dispatch such that

method calls in the Dog role invoke the original Dog role methods

where such methods exist. There also needs to be a way for a

class that assumes a role to explicitly declare that it wants

to override that decision. Thus, by default, when you say that

Mutant does both Dog and Tree, Dog's methods do not silently

mutate their semantics. You can cause them to do so, but you

should have to ask for that.

I am, as I mentioned initially, surprised that no one seems to

have considered this issue, because I always thought this is what

avoiding the False Cognate problem of mixins, as chromatic likes

to call it, ultimately implies at the deepest level: that roles

provide scope for their innards that preserves their identity and

integrity (unless, of course, you explicitly stick your hands in),

kind of like the safety that hygienic macros provide.

It therefore seems necessary to me to specify dispatch such that

method calls in the Dog role invoke the original Dog role methods

where such methods exist. There also needs to be a way for a

class that assumes a role to explicitly declare that it wants

to override that decision. Thus, by default, when you say that

Mutant does both Dog and Tree, Dog’s methods do not silently

mutate their semantics. You can cause them to do so, but you

should have to ask for that.