Zero cost dirty subtypes for Haskell

Subtypes.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
module Main where

import Unsafe.Coerce

main :: IO ()
main = do
  let wolf = Wolf "snarl" "branch" "ahoooo!"
  putStrLn $ speak wolf

speak :: a <: Animal => a -> String
speak a = animalSpeak $ upcast a

class a <: b | a -> b

instance {-# OVERLAPPABLE #-} forall a b c. (b <: c, a <: b) => a <: c

upcast :: a <: b => a -> b
upcast = unsafeCoerce

data Animal = Animal { animalSpeak :: String }

data Cat = Cat { catSpeak :: String, catHack :: String }
instance Cat <: Animal

data Dog = Dog { dogSpeak :: String, dogFetch :: String }
instance Dog <: Animal

data Wolf = Wolf { wolfSpeak :: String, wolfFetch :: String, wolfHowl :: String }
instance Wolf <: Dog