{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}

data Nullable a = X a

type family AllowAssign a b where
  AllowAssign (Nullable a) (Nullable b) = 'True
  AllowAssign (Nullable a) b = 'False
  AllowAssign a b = 'True

assign :: AllowAssign a b ~ 'True => a -> b -> IO ()
assign _ _ = return ()

main :: IO ()
main = do
  assign (X ()) (X ())
  -- assign (X ()) ()  -- uncommenting this yields a type error
  assign () (X ())
  assign () ()
  putStrLn "yay"