Thoughts on re-exporting Contravariant?

[locallycompact]

Now that contravariant is in base I wonder if it makes sense to re-export it in rio? My instinct says that contramap, Predicate, Equivalence and Comparison have a low risk of collision and are sufficiently general that they should be simply re-exported. However, divide, choose, etc may be better namespaced.

[snoyberg]

What version of base added this typeclass? We want to keep compatibility with older versions of base, and I'd rather avoid either a conditional API or adding extra dependencies for older versions of GHC.

[locallycompact]

It's just gone in in 4.12.0.0. http://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Functor-Contravariant.html

[snoyberg]

In which case, I'd say we should hold off until at least GHC 8.6 is considered the default compiler version to start with, and probably until at least GHC 8.2 is no longer commonly supported. Then we can introduce a conditional dependency.