Re: ANNOUNCE: GHC 7.4.1 Release Candidate 1

On 22 December 2011 00:10, José Pedro Magalhães wrote:

Hi Bas,

On Wed, Dec 21, 2011 at 23:02, Bas van Dijk wrote:

...

On 21 December 2011 19:29, Ian Lynagh wrote:

...

 * There is a new feature constraint kinds (-XConstraintKinds):

 http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/constraint-kind...

I'm trying to run the ConstraintKinds example from the documentation:

{-# LANGUAGE ConstraintKinds, TypeFamilies #-} type family Typ a b :: Constraint type instance Typ Int  b = Show b type instance Typ Bool b = Num b

But GHC complains:    Not in scope: type constructor or class `Constraint'

Do I have to import some GHC module for this?

Yes: GHC.Prim.

(Which reminds me, Simon, we had agreed to re-export Constraint from GHC.Exts, but it wasn't clear to me how to do this, since Constraint is a kind, not a type, so it can't just go on the export list of the module...)

Cheers, Pedro

...

Cheers,

Bas

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Thanks Pedro, that works. I already read through the docs of GHC.Prim but could not find it: http://www.haskell.org/ghc/dist/stable/docs/html/libraries/ghc-prim-0.2.0.0/... I guess haddock is not smart enough to understand kinds yet...