On Thu, Dec 22, 2011 at 12:45 AM, Bas van Dijk
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
This is the same solution I ended up with, while of course that doesn't prove there's no better one.