example of overlappable instance ?

[pcarbonn]

The tutorial says:

These situations can be handled with the type class approach just discussed with overlappable instances.

Could you give an example for those of us not expert in type hackery ? I have some difficulty writing the Overlappable instance. A draft would be something like:

instance {-# OVERLAPPABLE #-} ( OpenAlg r ?? ?? (OpenADT r)) => OverList a a' r ?? where
  fmapList' _ ?? = ??

Thanks !

[woehr]

I don't think what I had in mind when I wrote that sentence can be accomplished with OpenADT. It can with the EADT library, because the types of a row are its indices. However, with row-types, the label is the index, so it's not possible (I think) to write overlappable instances of the form instance {-# OVERLAPPABLE #-} ( OpenAlg r l f (OpenADT r)) => OverList a a' r f because the label isn't known.

However, what I think can be done is writing type classes on the elements of a row. Continuing with the list example, something with a function like (a -> a') -> f x -> g x. Then we can operate on VarF's, which have labels. We should be able to write a function (similar in implementation to VarF's functor instances) that transforms VarF r x -> VarF s x. The key difference compared to before being, the type class instances no longer depend on the label.

I'm working on updating the tutorial with such an example and will let you know what I come up with!