A general library for higher-kinded data

[chris-martin]

Rel8 defines the term "higher-kinded data" as:

Higher-kinded data types are data types of the pattern:

data MyType f =
  MyType { field1 :: Column f T1 OR HK1 f
         , field2 :: Column f T2 OR HK2 f
         , ...
         , fieldN :: Column f Tn OR HKn f
         }

where Tn is any Haskell type, and HKn is any higher-kinded type.

Aside from the specific detail of the Column type family, this notion seems very general! Do y'all think there's any potential for a library that would help others replicate the pattern for other applications? (For example, I have in mind a similar sort of approach to filesystem storage, with database rows and fields corresponding to directories and files.)

[ocharles]

Maybe! Though I will note I think those libraries maybe already exist. In this space, we have:

1. https://hackage.haskell.org/package/barbies - probably the one I'd most recommend
2. https://hackage.haskell.org/package/rank2classes
3. https://hackage.haskell.org/package/hkd

and there may be others. So I think we'd have to be confident that we have something substantially different. There's a good chance that if https://ghc-proposals.readthedocs.io/en/latest/proposals/0242-unsaturated-type-families.html makes it into GHC proper, we can drop a lot of what we've had to invent in Rel8 and just use barbies - but that remains to be proven.