[Haskell-cafe] an instance in other than the last type parameters

[Petr Pudlak]

Hi, I have probably a very simple question, but I wasn't able to figure it out
myself.

Consider a two-parameter data type:

> data X a b = X a b

If I want to make it a functor in the last type variable (b), I can just define

> instance Functor (X a) where
>   fmap f (X a b) = X a (f b)

But how do I write it if I want X to be a functor in its first type variable?
Is that possible at all?
Something like:

> instance Functor ??? where
>     fmap f (X a b) = X (f a) b

Thanks in advance,
  Petr