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

Petr,

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?

Short answer: you can't. Easiest way to workaround is to define a newtype wrapper around your original datatype: newtype X' b a = X' {unX' :: X a b} instance Functor (X' b) where fmap g (X' (X a b)) = X' (X b (g a)) Cheers, Stefan