On Wed, Dec 21, 2011 at 23:02, Bas van Dijk <v.dijk.bas@gmail.com> wrote:

On 21 December 2011 19:29, Ian Lynagh <igloo@earth.li> wrote:

> * There is a new feature constraint kinds (-XConstraintKinds):
> http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/constraint-kind.html

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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