test.hs

{-# LANGUAGE DataKinds, GADTs, KindSignatures #-}

data RedBlack :: (Bool -> *) where
    Red :: RedBlack 'False -> RedBlack 'True
    Black :: RedBlack a -> RedBlack b
    Stop :: RedBlack a

test = Red $ Black $ Red $ Stop

--test1 = Red $ Red $ Stop
--
-- causes:
-- src/Lol.hs:10:15:
--    Couldn't match type ‘'True’ with ‘'False’
--    Expected type: RedBlack 'False
--      Actual type: RedBlack 'True
--    In the second argument of ‘($)’, namely ‘Red $ Stop’
--    In the expression: Red $ Red $ Stop
--    In an equation for ‘test1’: test1 = Red $ Red $ Stop