Haskell closed classes using DataKinds

DKCC.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}

-- Needed only for example below
-- {-# LANGUAGE UndecidableInstances #-}

data T1 = T1A | T1B

class Test (a :: T1) b | a -> b, b -> a where tcc :: b -> String
instance Test T1A Int where tcc = show
instance Test T1B [Char] where tcc = id

-- New instances fail with fundep conflicts.  Try, for example,
-- instance Test T1A Float where tcc = show
-- instance Test a Float where tcc = show