Allow runtime-representation polymorphic data families

This is an offshoot of #12369 (closed) (‘data families shouldn't be required to have return kind *, data instances should’), allowing family declarations something like:

data family Array a :: TYPE rep

mentioned in issue #12369 (closed)

changed weight to 5

added Tfeature request Trac import TypeInType labels

changed the description

Explore how ticket:12369#comment:126120, ticket:12369#comment:126124 fit into this.

TODO How Free fits with Levity-Polymorphic Type Classes, can we define **something** like this

newtype instance Free (k :: TYPE rep -> Constraint) (p :: TYPE rep) :: TYPE rep where
  Free0 :: (forall q. k q => (p -> q) -> q) -> Free k p

TODO or like this

newtype instance Fix (f :: TYPE rep -> TYPE rep) :: TYPE rep where
  In :: f (Fix f) -> Fix f

And would they be useful

TODO Look at Ran and Lan and their hierarchies.

The UnliftedNewtypes patch (!364 (closed)) added support for levity polymorphic data families, among other things. See the typecheck/should_compile/UnliftedNewtypesUnassociatedFamily test case.

changed milestone to %8.10.1

added UnliftedNewtypes label

closed

added Pnormal label