11.Answer

object Pull:
  def unfoldEval[F[_], O, R](init: R)(f: R => F[Either[R, (O, R)]]): Pull[F, O, R] =
    Pull.Eval(f(init)).flatMap:
      case Left(r) => Result(r)
      case Right((o, r2)) => Output(o) >> unfoldEval(r2)(f)

object Stream:
  def unfoldEval[F[_], O, R](init: R)(f: R => F[Option[(O, R)]]): Stream[F, O] =
    Pull.Eval(f(init)).flatMap:
      case None => Stream.empty
      case Some((o, r)) => Pull.Output(o) ++ unfoldEval(r)(f)