>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-2...@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 <jwl...@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 > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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