Simplified pull free monad without type parameters or overt reference to monads

pull-no-params.scala

enum Pull {
  case Done
  case Output(name: String)
  case UnconsThen(pull: Pull, f: Option[(String, Pull)] => Pull)
  case Then(pull: Pull, next: () => Pull)
}

import Pull._
val done = Done
def single(name: String) = Output(name)
def append(pull: Pull, name: String) = Then(pull, () => single(name))

def repeat(pull: Pull): Pull = Then(pull, () => repeat(pull))

def take(n: Int, pull: Pull): Pull = {
  def onOutput(element: Option[(String, Pull)]): Pull =
    element match {
      case Some((name, rest)) =>
        Then(
          Output(name),
          () => take(n - 1, rest)
        )
      case None => done
    }

  if n <= 0 then done else UnconsThen(pull, onOutput)
}

def step(pull: Pull): Either[(String, Pull), Unit] =
  pull match {
    case Done         => Right(())
    case Output(name) => Left((name, done))
    case UnconsThen(pull, f) =>
      step(pull) match {
        case Left((name, next)) =>
          step(f(Some((name, next))))
        case Right(a) =>
          step(f(None))
      }
    case Then(pull, rest) =>
      step(pull) match {
        case Left((name, next)) =>
          Left((name, Then(next, rest)))
        case Right(()) => step(rest())
      }
  }

def last(pull: Pull, prev: Option[String]): Option[String] = step(
  pull
) match {
  case Left((name, next)) => last(next, Some(name))
  case Right(())          => prev
}
val kittens = append(single("Mao"), "Popcorn")
println(last(take(4, repeat(kittens)), None))