[Haskell-cafe] GHC 7.4: Expected behavior or bug?

[José Pedro Magalhães]

Hi,

This is a change in behavior. Previously GHC was more liberal than Haskell
98 prescribed, and would not default the kind of otherwise unconstrained
type variables to *. 7.4 does default to *, so you have to provide kind
signatures when you want another kind (particularly in phantom type
variables).


Cheers,
Pedro

On Tue, Dec 27, 2011 at 16:47, Bas van Dijk <v.dijk.bas at gmail.com> wrote:

> On 27 December 2011 17:38, Michael Snoyman <michael at snoyman.com> wrote:
> > Thanks to Mark Wright for pointing this out[1].
> >
> > We have the equivalent of the following code in persistent:
> >
> > {-# LANGUAGE MultiParamTypeClasses #-}
> > data Key backend entity = Key
> >
> > class Monad (b m) => Foo b m where
> >    func :: b m (Key b m)
> >
> > This code works fine with GHC 7.0, but I get the following message from
> GHC 7.4:
> >
> >    Expecting two more arguments to `b'
> >    In the type `b m (Key b m)'
> >    In the class declaration for `Foo'
> >
> > Is this expected behavior, or a bug? If the former, what would be a
> > possible workaround?
> >
> > Thanks,
> > Michael
> >
> > [1] https://github.com/yesodweb/persistent/issues/31
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
> I fixed a similar breakage in the hmatrix library:
>
>
> https://github.com/AlbertoRuiz/hmatrix/commit/a4f38eb196209436f72b938f6355f6e28474bef3
>
> I don't know if it's a bug in GHC, but the workaround is to add an
> explicit kind signature:
>
> {-# LANGUAGE KindSignatures, MultiParamTypeClasses #-}
> data Key (backend :: * -> * -> *) entity = Key
>
> class Monad (b m) => Foo b m where
>   func :: b m (Key b m)
>
> Cheers,
>
> Bas
>
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> Haskell-Cafe at haskell.org
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>
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