Idea: Give Async a Monad and MonadPlus instance

[ChristopherKing42]

We would need to add a Pure :: a -> Async a data constructor to Async, but other than that, the current Async type supports the Monad and MonadPlus typeclasses. This is because STM is a Monad and MonadPlus.

return would equal Pure, obviously. Pure a >>= f = f a. For x >>= f where x isn't Pure, we give the resulting async x's asyncThreadId, and use the STM monad to generate the resulting _asyncWait.

The MonadPlus instance would be similar, with mzero corresponding to STM's mzero (and some junk ThreadId) and mplus corresponding to STM's mplus (and choosing the left ThreadId (or mzero's ThreadId).

If you don't want to add a new data constructor to Async, you can also define an Applicative and Alternative instance without changing the type at all. pure a would have a junk asyncThreadId and would use STM's pure for asyncThreadId. <*> would use STM's <*> for _asyncWait, and would choose the left-most non-junk-asyncThreadId for the resulting asyncThreadId. Alternative would work similarly. (If you do not like using an invalid ThreadId for asyncThreadId, you could slightly change asyncThreadId's type and use Nothing instead. Or you could make it a [ThreadId] of all the threads involved.)

[simonmar]

See discussion in #71