[Haskell-cafe] Lifting IO actions into Applicatives

[Tom Ellis]

On Tue, Oct 01, 2013 at 09:29:00AM +0200, Niklas Haas wrote:
> On Tue, 1 Oct 2013 02:21:13 -0500, John Lato <jwlato at gmail.com> 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