[Haskell-cafe] Foldable for (,)

[MarLinn]

On 2017-05-03 20:35, Dmitry Olshansky wrote:
> So I don't know an answer... Really, why we have this constraint? The 
> same question is about Functor.

Functor is easy:

instance Traversable f => Functor f where
     fmap = (runIdentity .) . traverse . (pure .)