On Sun, 26 Jun 2005, Daniel Fischer wrote:
Am Samstag, 25. Juni 2005 21:22 schrieb Josh Hoyt:
Hello,
I'm a new Haskeller, and I'm running into a problem attempting to declare certain types as instances. I was attempting something that's
effectively equivalent to:
class Foo a
instance Foo (Either b b)
My question is, why this restriction that the types must be distinct?
In particular, I'd like to declare a very specific type (Either String String) as an instance. What techniques can I use to accomplish this?
Josh Hoyt
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.