[DPP-55] Unify Async APIs

[qnikst]

[Imported from JIRA. Reported by Tim Watson [Administrator] @hyperthunk) as DPP-55 on 2013-01-12 18:03:03]
Trying to unify the APIs for AsyncChan and AsyncSTM in d-p-platform. I've thought about doing this with type (or data) families but ... we need to handle AsyncChan a or AsyncSTM a so...

class Async a where
  type AsyncHandle a :: *
  poll :: AsyncHandle a -> Process (AsyncResult b)
  wait :: AsyncHandle a -> Process (AsyncResult b)
  ... etc

but ...

instance Async (STM.AsyncSTM a) where
  type AsyncHandle (STM.AsyncSTM a) = STM.AsyncSTM a
  poll = STM.poll
  wait = STM.wait

this isn't quite what I want, as the types cannot be inferred properly. And I couldn't quite figure out whether declaring data AsyncHandle a in the type class and having the instance as data AsyncHandle (STM.AsyncSTM a) = HStm (STM.AsyncSTM a) was the right approach either, but of course you can't leave a gap in the type signature without telling the compiler what the relation is between the two:

instance Async (STM.AsyncSTM a) where
  data AsyncHandle (STM.AsyncSTM a) = HStm (STM.AsyncSTM a)
  poll (HStm h) = STM.poll h  -- illegal because we don't know that the `a` in the associated type is related to the `a` in the instance declaration. 

I wondered about functional dependencies here, but couldn't see how to apply them. Anyhow, I realise this is probably a pretty basic question, but I don't see a neat way to handle it. Should I just live with having the two APIs without a common method of interchanging between them? At the moment, for the most part all you need to do to switch from AsyncChan to AsyncSTM is change your imports.