At Tue, 14 Jun 2011 10:40:48 -0400, Dan Doel wrote:
1. As things stand in GHC you can do some things with functional dependencies that you can't do with type families. The archetypical example is type equality. We cannot write type family Eq a b :: * type instance Eq k k = True type instance Eq j k = False because the two instances overlap. But you can with fundeps class Eq a b c | a b -> c instance Eq k k True instance Eq j k False As Alexey mentioned, fundeps have other disadvantages, so it's reasonable to ask whether type families could extend to handle cases like this.
Fundeps no longer allow this as of GHC 7, it seems. It causes the same fundep violation as the case:
class C a b | a -> b instance C a R instance C T U
You absolutely still can use FunctionalDependencies to determine type equality in GHC 7. For example, I just verified the code below with GHC 7.02: *Main> typeEq True False HTrue *Main> typeEq (1 :: Int) (2 :: Int) HTrue *Main> typeEq (1 :: Int) False HFalse As always, you have to make sure one of the overlapping instances is more specific than the other, which you can do by substituting a parameter c for HFalse in the false case and fixing c to HFalse using another class like TypeCast in the context. (As contexts play no role in instance selection, they don't make the instance any more specific.) While I don't have convenient access to GHC 6 right this second, I'm pretty sure there has been no change for a while, as the HList paper discussed this topic in 2004. David {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE UndecidableInstances #-} class TypeCast a b | a -> b where typeCast :: a -> b instance TypeCast a a where typeCast = id data HTrue = HTrue deriving (Show) data HFalse = HFalse deriving (Show) class TypeEq a b c | a b -> c where typeEq :: a -> b -> c instance TypeEq a a HTrue where typeEq _ _ = HTrue instance (TypeCast HFalse c) => TypeEq a b c where typeEq _ _ = typeCast HFalse