Looks sensible to me. It looks like the build is failing on older GHCs due to UndecidableInstances not being enabled—can you add that?

Also, would you mind mentioning this in the CHANGELOG.md? Thanks!

Yeah busy day, but I'll get to those two things.

Related to this, I also have use for a FlipC: https://github.com/obsidiansystems/constraints-extras/pull/4/files#diff-5ed8f2541b375e8ecca027c7d55495bbR38 Mind if I add that too? And if so, what module?

Related to this, I also have use for a FlipC: https://github.com/obsidiansystems/constraints-extras/pull/4/files#diff-5ed8f2541b375e8ecca027c7d55495bbR38 Mind if I add that too? And if so, what module?

I'm not technically the maintainer of this library (@ekmett is), so I can't really speak as to whether constraints is an appropriate place for FlipC or not. Adding ComposeC seems like a no-brainer, since we're just exposing functionality that's already there, but FlipC is a different beast entirely. (I'm only acting conservatively here since @ekmett has expressed reservations in the past about adding generic Constraint type combinators, such as in #47 (comment)).

Regardless, it might be best to leave that for a separate PR.

Makes sense; no problem.

Oh but the second think proposed in the thread you linked is Compose exactly; and it also falls under the "avoid higher kinds" dictum.

I discovered recently that sop-core also exposes a Compose class here.