Process.scala

package org.teckhooi.fp.dojo2.fpis.iostream

import org.teckhooi.fp.dojo2.fpis.iostream.Process.{Await, Emit, Halt}

sealed trait Process[I, O] {

  import Process._

  def apply(s: LazyList[I]): LazyList[O] = this match {
    case Halt()           => LazyList()
    case Emit(head, tail) => head #:: tail.apply(s)
    case Await(recv) =>
      s match {
        case h #:: t => recv(Some(h))(t)
        case xs      => recv(None)(xs)
      }
  }

  def repeat: Process[I, O] = {
    def go(p: Process[I, O]): Process[I, O] = p match {
      case Halt()           => go(this)
      case Emit(head, tail) => Emit(head, go(tail))
      case Await(recv) =>
        Await {
          case None => recv(None)
          case x    => go(recv(x))
        }
    }

    go(this)
  }
}

object Process {
  case class Emit[I, O](head: O, tail: Process[I, O] = Halt[I, O]()) extends Process[I, O]
  case class Await[I, O](recv: Option[I] => Process[I, O])           extends Process[I, O]
  case class Halt[I, O]()                                            extends Process[I, O]
}

object RunMe extends App {
  def liftOne[I, O](f: I => O): Process[I, O] =
    Await {
      case Some(x) => Emit(f(x))
      case _       => Halt()
    }

  val i = liftOne((i: Int) => i * 2).repeat(LazyList(1, 2, 3)).toList
  println(i.mkString(","))
}