On 6/26/05, Henning Thielemann
On Sun, 26 Jun 2005, Daniel Fischer wrote:
[...] I don't know, why the tyvars must be distinct in Haskell 98,
This is certainly to prevent from overlapping instances. An implementation for general (Either a b) could also be invoked when (Either String String) is requested.
If it is really necessary to make the Either type an instance of something better use a data or a newtype definition.
newtype EitherString = EitherString (Either String String)
and declare an instance for EitherString.
I see why there is ambiguity between (Either a b) and (Either b b). In fact, it seems rather obvious now. Thanks for your help. Just to make sure I understand, and hopefully instructive for others who run into the same difficulty: class Foo a where frob :: a -> String {- Illegal: instance Foo (Either String String) where frob (Right x) = x frob (Left y) = y -} -- Instead: -- Option a: -- Generic implementation in terms of other classes instance (Show a, Show b) => Foo (Either a b) where frob (Right x) = show x frob (Left y) = show y -- Option b: -- Let the type system know that this is a specific case by defining a type newtype EitherString = EitherString (Either String String) instance Foo EitherString where frob (EitherString (Right x)) = x frob (EitherString (Left y)) = y