Re: [Haskell-cafe] Lifting IO actions into Applicatives

On Tue, 1 Oct 2013 02:21:13 -0500, John Lato wrote:

It's not a solution per se, but it seems to me that there's no need for the Monad superclass constraint on MonadIO. If that were removed, we could just have

class LiftIO t where liftIO :: IO a -> t a

and it would Just Work.

One concern with this is that it's not exactly clear what the semantics are on LiftIO (is liftIO a >> liftIO b equal to liftIO (a >> b) or not?) and the interaction between LiftIO and Applicative/Monad would have to be some sort of ugly ad-hoc law like we have with Bounded/Enum etc. Furthermore, you might end up specifying a lot of ‘redundant’ constraints like (Monad m, LiftIO m) that way, just a thing to keep in mind.