Printing out data constructor names using GHC.Generics

cn.hs

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE TypeOperators #-}

import Data.Proxy
import GHC.Generics

data Foo = Bar Int | Baz Float deriving Generic

class CN (f :: * -> *) where
  constructorNames :: proxy f -> [String]

instance CN f => CN (D1 c f) where
  constructorNames _ = constructorNames (Proxy :: Proxy f)

instance (CN x, CN y) => CN (x :+: y) where
  constructorNames _ = constructorNames (Proxy :: Proxy x) ++ constructorNames (Proxy :: Proxy y)

instance Constructor c => CN (C1 c f) where
  constructorNames _ = [conName (undefined :: t c f a)]

main :: IO ()
main = do
  print $ constructorNames (Proxy :: Proxy (Rep Foo))
  return ()