Am Donnerstag 04 März 2010 00:17:09 schrieb Tyson Whitehead:
The following code
{-# LANGUAGE FlexibleInstances, TypeFamilies #-}
import Control.Applicative
class Z t where type W t z :: t -> W t
instance Z (a -> b) where type W (a -> b) = a -> b z = id
instance Z (IO (a -> b)) where type W (IO (a -> b)) = IO a -> IO b z = (<*>)
works fine, but if I try and generalize to from IO to the Applicative classes
instance (Applicative m) => Z (m (a -> b)) where type W (m (a -> b)) = m a -> m b z = (<*>)
I get the following error
Temp.hs:10:9: Conflicting family instance declarations: type instance W (a -> b) -- Defined at Temp.hs:10:9 type instance W (m (a -> b)) -- Defined at Temp.hs:14:9 Failed, modules loaded: none.
unless I remove one of the instances, and then it is happy.
Is this correct? I don't claim to really understand the rules regarding type classes, but I can't see why these are overlapping.
Thanks! -Tyson
Because: instance Applicative ((->) a) -- Defined in Control.Applicative so, from the instance Z (a -> b), with b == c -> d, we have an instance Z (a -> (b -> c)) and from instance Z (m (u -> v)), we have, with m == ((->) x), an instance Z (x -> (u -> v))