data Foo a = Foo Int -- a is a phantom type

main :: IO ()
main = do
  let e1 = Foo 3 :: Foo Int
  let e2 = Foo 3 :: Foo Bool
  -- the next line is illegal, because while they have the same value,
  -- they have different types
  if e1 == e2 then putStrLn "same" else putStrLn "different"