{-# LANGUAGE ExistentialQuantification, RankNTypes #-} data List a = Nil | a ::: (List a) data C c = forall c. C (List c) data D d = D (forall d. List d) main = do let s = (3 :: Int) ::: Nil let a = C ((3 :: Int) ::: Nil) let b = D Nil let c = D (undefined ::: Nil) -- error: -- let d = D ((3 :: Int) ::: Nil) -- let e = D (() ::: Nil) return ()