I have released a new version of the conceit package with the following changes:
The Applicative instance now doesn't require those ugly (Show e, Applicative e) constraints on the error type.
There's a Monad instance for Conceit. I initially made the >> operator concurrent, like *>, but that was a bad idea. In the end, the Monad instance has sequential operations and the Applicative concurrent ones, like it happens in Haxl.
There are also MonadThrow, MonadCatch and MonadError instances. The first two work throwing and catching exceptions from IO, the last one works more like ExceptT.
A special run function was added for when the error is "impossible": This makes easier to use Conceit in place of Concurrently.
The internals of this new version of conceit have been copied from those of Concurrently in Simon Marlow's async package, with modifications to support the new behaviour.