Poly kinded barbie types

[vapourismo]

Barbie types are currently restricted to kind (Type -> Type) -> Type.

As far as I can tell, all of the type class abstractions in this package would still work if one extends this restriction to a more poly-kinded one: (k -> Type) -> Type.

Thoughts?

[jcpetruzza]

Yes, I too think that everything should work exactly the same with barbies of kind (k -> Type) -> Type (as is done, e.g. in parameterized-utils). I couldn't think of interesting examples of this at the time, but it is probably better not to introduce artificial restrictions, though, so I'll try to to lift this.

[vapourismo]

This is a use case I currently have:

type RowKind k = (k -> Type) -> Type

newtype Single :: k -> (k -> Type) -> Type

data Expression :: Type -> Type

class Row (row :: RowKind k)

data Statement :: RowKind Type -> Type where
    TableOnly :: Text -> Statement row

    Select
        :: (TraversableB source, ConstraintsB source, AllB Row source)
        => (source (Single Expression) -> row Expression)
        -> (source (Single Expression) -> Expression Bool)
        -> source Statement
        -> Statement row

The limitation here is that source :: RowKind Type -> Type.

[jcpetruzza]

This is a very interesting example, thanks! So, b :: RowKind Type is a type parameterized by a functor, i.e., a barbie, and source is then a type parameterized by a barbie, which, as you observe, should also be a barbie.

Gave it a quick try: lifting the restriction for FunctorB, TraversableB and ProductB was easy enough. For ConstraintsB , the generic derivation of AllB breaks with an ugly compiler message, but I think this is because in AllB c b, the kind of c is now a function on b, so hopefully fixable...

[vapourismo]

I think you've hit the same nasty error message that I encountered when I tried to refactor it to use PolyKinds. Something about an unnamed type variable <tv> not matching. I have been trying to figure out what it means ever since.

[jcpetruzza]

Yeah, that one :smile:

[jcpetruzza]

@vapourismo Still probably a bit rough around the edges, but if could you give #8 a try and report back, it'd be great.

[vapourismo]

I've tried a few base cases and they seem to work well. Haven't had the time to use it extensively, yet.

[jcpetruzza]

Released as v1.1.0.0