Re: ANNOUNCE: GHC 7.4.1 Release Candidate 1

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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