{-# LANGUAGE FunctionalDependencies, TypeFamilies, AllowAmbiguousTypes #-}
main :: IO ()
main = do
  pureId (putStrLn "hello")
  print =<< pureId True



class PureId arg m where
  pureId :: arg -> Res m arg

type family Res m a where
  Res n (m a) = m a
  Res m a = m a
  

instance {-# OVERLAPPABLE #-} (Monad m, Res m a ~ m a) => PureId a m where
  pureId = pure
  
instance (m ~ n) => PureId (m a) n where
  pureId = id