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
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