DerivingViaPolyKinds.hs

{-# LANGUAGE DerivingVia, StandaloneDeriving, PolyKinds, RankNTypes #-}
module DerivingViaPolyKinds where


class Foo (f :: k -> *)

newtype Bar (f :: k -> *) (a :: k) = Bar (f a)
  -- deriving Foo via f -- Works

-- Standalone deriving instances only work if you instantiate the polykinded variable:
-- deriving via (f :: * -> *) instance Foo f => Foo (Bar f)
-- deriving via (f :: (* -> *) -> *) instance Foo f => Foo (Bar f)

-- None of these work:
{-
deriving via f instance Foo f => Foo (Bar f)

deriving via (f :: k -> *) instance Foo f => Foo (Bar f)

deriving via (f :: k -> *) instance Foo (f :: k -> *) => Foo (Bar f)
deriving via (f :: k -> *) instance Foo f => Foo (Bar (f :: k -> *))
deriving via (f :: k -> *) instance Foo (f :: k -> *) => Foo (Bar f :: k -> *)

deriving via f instance Foo (f :: k -> *) => Foo (Bar f)


deriving via f instance Foo (f :: k -> *) => Foo (Bar f :: k -> *)


forall-ing won't help.
forall k. on the LHS interprets it as an RankNType
forall k. on the RHS interprets it a universally quantified variable free from the LHS.

deriving via (forall k. f :: k -> *) instance Foo (f :: k -> *) => Foo (Bar f :: k -> *)

deriving via (forall k. f :: k -> *) instance Foo f => Foo (Bar f)

deriving via (forall f k. f :: k -> *) instance Foo (f :: k -> *) => Foo (Bar f :: k -> *)

deriving via (forall f k. f :: k -> *) instance Foo f => Foo (Bar f)


-}