Re: [Haskell-cafe] Why distinct tyvars in instance declarations?

Hi there,

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)'

unless I'm totally mistaken, your problem isn't the distinction thingy, but rather an error like supplying an Int for where you need (Int -> Int -> Int). That is, you're trying make (Either String String) an instance of Foo, (Either String String) already being a fully constructed type; Foo, on the other hand, seems to require a type constructor that is yet to parameterize over three more types (e.g. StateT). Greets, Frank-Andre Riess