err.out

/Users/kb/workspace/typelabels/typelabels.hs

The first argument of ‘Main.TypedLabel1’ should have kind ‘*’,
  but ‘Main.Onion’ has kind ‘Main.VegType’
In the type signature for ‘Main.embraceOnion3’:
  Main.embraceOnion3 :: Main.TypedLabel1 Main.Onion Main.Vegetable
                        -> GHC.Types.IO ()

typelabels.hs

{-# LANGUAGE DataKinds      #-}
{-# LANGUAGE KindSignatures #-}

module Main where

main :: IO ()
main = putStrLn "Hi!"

data Vegetable = Vegetable { vegName :: String
                           , vetType :: VegType }
    deriving (Show, Eq)
data VegType = Potato
             | Tomato
             | Cucumber
             | Onion
    deriving (Show, Eq)
newtype TypedVegetable (t::VegType) = TypedVegetable Vegetable

unType :: TypedVegetable (t::VegType) -> Vegetable
unType (TypedVegetable v) = v

-- | bad
embraceOnion :: Vegetable -> IO ()
embraceOnion veg = putStrLn ("Yay for onion! It's name is: " ++ vegName veg)

-- | good, but little ugly because of boilerplate
embraceOnion2 :: TypedVegetable Onion -> IO ()
embraceOnion2 onion = putStrLn ("Yay for onion! It's name is: " ++ vegName (unType onion))

-- | doesn't work, need to specify kind of `t`
newtype TypedLabel1 t a = TypedLabel1 a
unTyped1 :: TypedLabel1 t a -> a
unTyped1 (TypedLabel1 a) = a
embraceOnion3 :: TypedLabel1 Onion Vegetable -> IO ()
embraceOnion3 = undefined