GoodProgram.hs

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

data K = X | Y

data Box (a :: K) where
  Box :: Box a

class Foo (a :: K) where
  name :: Box a -> String

instance Foo X where
  name _ = "X"

instance Foo Y where
  name _ = "Y"

main = do
  putStrLn $ aName (Box :: Box X)

aName :: Box a -> String
-- shouldn't need the Foo since K is closed
-- aName :: Foo a => Box a -> String
aName = name