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

[Bas van Dijk]

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
>
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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