On Thu, Dec 22, 2011 at 12:45 AM, Bas van Dijk <v.dijk....@gmail.com> wrote: > 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. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users