[Haskell-cafe] Lifting IO actions into Applicatives

[Dan Burton]

>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 at 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 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
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