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)
but GHC complains:
Illegal instance declaration for `Foo (Either b b)' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Foo (Either b b)'
and so I looked in the report
(http://www.haskell.org/onlinereport/decls.html#sect4.3.2) and found:
The general form of the corresponding instance declaration is:
instance cx' => C (T u1 ... uk) where { d }
where k>=0. The type (T u1 ... uk) must take the form of a type constructor T applied to simple type variables u1, ... uk; furthermore, T must not be a type synonym, and *the ui must all be distinct*. (emphasis mine)
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, but if you use extensions (hugs -98; -fglasgow-exts for ghc(i)), you can declare instance Foo (Either b b) where ... or instance Foo (Either String String) where ... without problems. HTH, Daniel