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

Thanks for the explanation. 2011/12/27 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 wrote:

...

On 27 December 2011 17:38, Michael Snoyman 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/a4f38eb196209436f72b938f6355f6...

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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