Permit abstract data types in signatures that don't have kind *
Here is a fascinating program that I would like to write, but cannot with Backpack today:
unit p where signature Key where import GHC.Exts (Constraint) data Key k :: Constraint instance Key Bool signature Map where import Key data Map k a empty :: Map k a insert :: Key k => k -> a -> Map k a -> Map k a lookup :: Key k => k -> Map k a -> Maybe a module M where import Map import Key x = insert True "foo" empty
The point is to make the *constraint* for Map parametrizable, so that HashMap can levy a different constraint (Hashable) than Map (Ord).
data Key k :: Constraint is implementable, because we can implement abstract data types using type synonyms in Backpack, and synonyms can have more kinds than just
*. However, GHC chokes on this declaration, because it thinks that data types always have to have kind