What about
class Functor f where
type C f :: * -> Constraint
type C f = ()
After all, just as (Ord a, Show a) is a contraint, so is ().
Simon
| -----Original Message-----
| From: glasgow-haskell-users-boun...@haskell.org [mailto:glasgow-haskell-
| users-boun...@haskell.org] On Behalf Of Bas van Dijk
| Sent: 21 December 2011 23:46
| To: glasgow-haskell-users@haskell.org
| Subject: ConstraintKinds and default associated empty constraints
|
| I'm playing a bit with the new ConstraintKinds feature in GHC
| 7.4.1-rc1. I'm trying to give the Functor class an associated
| constraint so that we can make Set an instance of Functor. The
| following code works but I wonder if the trick with: class Empty a;
| instance Empty a, is the recommended way to do this:
|
| {-# LANGUAGE ConstraintKinds, TypeFamilies, FlexibleInstances #-}
|
| import GHC.Prim (Constraint)
|
| import Prelude hiding (Functor, fmap)
|
| import Data.Set (Set)
| import qualified Data.Set as S (map, fromList)
|
| class Functor f where
| type C f :: * -> Constraint
| type C f = Empty
|
| fmap :: (C f a, C f b) => (a -> b) -> f a -> f b
|
| class Empty a; instance Empty a
|
| instance Functor Set where
| type C Set = Ord
| fmap = S.map
|
| instance Functor [] where
| fmap = map
|
| testList = fmap (+1) [1,2,3]
| testSet = fmap (+1) (S.fromList [1,2,3])
|
| Cheers and thanks for a great new feature!
|
| Bas
|
| _______________________________________________
| Glasgow-haskell-users mailing list
| Glasgow-haskell-users@haskell.org
| http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
