On Fri, Jul 17, 2009 at 1:14 PM, Petr Pudlak <deb@pudlak.name> wrote:

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
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