Hi.
If I get it right, the trick is to use a *-kinded (instead of a *->*-kinded) argument for the class & instances and have a "type function" that is able to phantom-retag an already applied type-constructor `Foo_ Unres` to a differently applied type-constructor `Foo_ Res`.
Yes.
The only thing that disturbs me is that I have to define explicit boilerplate type family declarations for all twenty or so non-polymorphic instances which'd look always the same, i.e.
type ResFun (t r) a = t a type ResArg (t r) a = t ~ a
...could that be defined as an override-able default somehow?
The only way I can quickly think of that might work is to go via another type family. However, this requires "UndecidableInstances": type family GetF e :: Resolved -> * type instance GetF (f x) = f -- provides operation to transform an unresolved `Foo_ Unres` to a resolved `Foo_ Res` class Resolvable e where type ResFun e (a :: Resolved) :: * type ResFun e a = GetF e a type ResArg e (a :: Resolved) :: Constraint type ResArg e a = GetF e a ~ e ... Then, for types like Foo, you can just write: instance Resolvable (Foo_ r) where resolve _ x = return Foo I.e., you shouldn't need the type family declarations. Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com