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

On Tue, Oct 01, 2013 at 09:29:00AM +0200, Niklas Haas wrote:

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.

Shouldn't it be an *Applicative* constraint? class Applicative t => ApplicativeIO t where liftIO :: IO a -> t a and require that liftIO (pure x) = pure x liftIO (f <*> x) = liftIO f <*> liftIO x Seems like ApplicativeIO makes more sense than MonadIO, which is unnecessarily restrictive. With planned Functor/Applicative/Monad shuffle, the former could completely replace the latter. Tom