Existentially qualified types in Scala

existentials.scala

/*
 data ShowBox = forall s. Show s => SB s

 heteroList :: [ShowBox]
 heteroList = [SB (), SB 5, SB True]

-----

 instance Show ShowBox where
  show (SB s) = show s        

 f :: [ShowBox] -> IO ()
 f xs = mapM_ print xs

 main = f heteroList
 */

def show(as : List[F[_]] forSome { type F[_] <: Show[_]}): IO[String] =
  as.traverse(a => IO.pure(a.show(_)))