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Algebraic Effects for Scala 3

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package turbolift.handlers
import turbolift.!!
import turbolift.Extensions._
import turbolift.effects.{ChoiceEffect, ChoiceSignature}


extension [Fx <: ChoiceEffect](fx: Fx)
  def choiceHandler_first: fx.ThisHandler[Identity, Option, Any] =
    new fx.impl.Stateless[Identity, Option, Any] with fx.impl.Parallel with ChoiceSignature:
      override def onReturn(a: Unknown): Option[Unknown] !! Any = !!.pure(Some(a))

      override def onRestart(as: Option[Unknown]): Unknown !! ThisEffect =
        as match
          case Some(a) => !!.pure(a)
          case None => fx.empty

      override def onZip[A, B, C](as: Option[A], bs: Option[B], k: (A, B) => C): Option[C] =
        as.flatMap(a => bs.map(b => k(a, b)))

      override def empty: Nothing !! ThisEffect = Control.abort(None)

      override def choose[A](as: Iterable[A]): A !! ThisEffect =
        Control.capture: k =>
          val it = as.iterator
          def loop(): Option[Unknown] !! Fx =
            if it.hasNext then
              k(it.next()).flatMap {
                case None => loop()
                case x => !!.pure(x)
              }
            else
              !!.none
          loop()

      override def choosePar[A](as: Iterable[A]): A !! ThisEffect = choose(as)

    .toHandler