ConstraintKinds and default associated empty constraints

[Simon Peyton-Jones]

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-bounces at haskell.org [mailto:glasgow-haskell-
| users-bounces at haskell.org] On Behalf Of Bas van Dijk
| Sent: 21 December 2011 23:46
| To: glasgow-haskell-users at 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
| 
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