From what you've said, it sounds like you can already write:

    serverSide :: IO a -> Form a

This seems elegant enough to me for your needs. Just encourage it as an idiom specific to Forms.

    myBlogForm = Blog <$> titleForm <*> serverSide getCurrentTime <*> contentsForm

Could you abstract `serverSide` out into a typeclass, such as ApplicativeIO? Sure. but why bother? The point is, you've got the specialization you need already.

-- Dan Burton


On Tue, Oct 1, 2013 at 1:20 AM, Tom Ellis <tom-lists-haskell-cafe-2013@jaguarpaw.co.uk> wrote:
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@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
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