ex11 - https://medium.com/@lu4nm3/monad-transformers-vs-mtl-d3a1167e2649

ex11_mvm.scala

def createMonadWriter[L](implicit M: Monoid[L]): IO[MonadWriter[IO, L]] = {
  for {
    writer <- Ref.of[IO, L](M.empty)
  } yield new MonadWriter[IO, L] {
    val monad: Monad[IO] = Monad[IO]
    def tell(l: L): IO[Unit] = writer.update(_.combine(l))
    def logs: IO[L] = writer.get
  }
}

def createMonadState[S](initial: S): IO[MonadState[IO, S]] = {
  for {
    state <- Ref.of[IO, S](initial)
  } yield new MonadState[IO, S] {
    val monad: Monad[IO] = Monad[IO]
    def get: IO[S] = state.get
    def set(s: S): IO[Unit] = state.set(s)
  }
}