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haskell-distributed / distributed-process-platform

DEPRECATED (Cloud Haskell Platform) in favor of distributed-process-extras, distributed-process-async, distributed-process-client-server, distributed-process-registry, distributed-process-supervisor, distributed-process-task and distributed-process-execution
http://haskell-distributed.github.com
BSD 3-Clause "New" or "Revised" License
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Unify Async APIs #55

Closed hyperthunk closed 11 years ago

hyperthunk commented 11 years ago

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.

hyperthunk commented 11 years ago

This is done for a subset of the operations and I'm inclined to close it until some more specific requirement comes up.