Add Monad instance for Concurrently

[TravisWhitaker]

In light of the ApplicativeDo extension made available in GHC 8.0.1, I'd like to propose the following Monad instance for Concurrently.

instance Monad Concurrently where
    return = pure
    x >>= f = Concurrently $ runConcurrently x >>= (runConcurrently . f)

Where bind simply runs two Concurrently actions sequentially. This should allow one to write things of the form:

do x <- a
   y <- b
   f x y

The desugared expression will require join, and should allow f to wait on a and b, which may run concurrently.

[simonmar]

Isn't this the same as the Monad instance that was removed in bb1d405? It didn't satisfy the Monad laws so it was removed.

[TravisWhitaker]

Hmm, that's tricky indeed; I definitely would've been tripped up by that at some point.

For anyone else who wonders why this isn't possible #26 discusses the implications of (<*>) /= ap and #38 from either includes discussion of how Haxl deals with this issue.