error "Could not match kind # Type with kind Type" when using ( | r ) with RowApply

[srghma]

for example I can write

type The1 r = ( x :: String | r )
type The2 r = ( y :: String | r )

type MyRec = { | The1 + The2 + () }

but when I need The2 empty and write

type The1 r = ( x :: String | r )
type The2 r = ( | r )

type MyRec = { | The1 + The2 + () }

it throws error

  Could not match kind

    # Type

  with kind

    Type


while checking the kind of Record (RowApply The1 (RowApply The2 ()))
in type synonym MyRec

[JordanMartinez]

I'm pretty sure the ( | r) syntax only works when there are "concrete" rows between the ( and |. Otherwise, The2 is better defined as type The2 r = r.

[natefaubion]

@JordanMartinez is correct, due to a quirk in the Type AST there is no distinction between r and ( | r ) so it will default to Type. With PolyKinds, The2 will have a polymorphic signature (type The2 :: forall a. a -> a) so it will work in a future release.