Functor instance for Set?
Bas van Dijk
v.dijk.bas at gmail.com
Thu Mar 1 09:31:00 CET 2012
On 29 February 2012 19:54, Daniel Gorín <dgorin at dc.uba.ar> wrote:
> I was always under the impression that the fact that Data.Set.Set can not be made an instance of Functor was a sort of unavoidable limitation.
I guess the way forward is to start using ConstraintKinds and TypeFamilies:
{-# LANGUAGE ConstraintKinds, TypeFamilies, FlexibleInstances #-}
import Prelude hiding (Functor, fmap)
import Data.Set (Set)
import qualified Data.Set as Set (map)
import GHC.Exts (Constraint)
class Functor f where
type FunctorConstraint f :: * -> Constraint
type FunctorConstraint f = Empty
fmap :: (FunctorConstraint f b) => (a -> b) -> f a -> f b
class Empty a
instance Empty a
instance Functor Set where
type FunctorConstraint Set = Ord
fmap = Set.map
-- Note that the unnecessary 'Ord a' constraint needs to be dropped from:
-- Set.map :: (Ord a, Ord b) => (a -> b) -> Set a -> Set b
Regards,
Bas
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