Future Option 地獄の対処法 - OptionTの使い方 (Scala)

実コードでは引数の依存や仕様で順番は変えられないものを想定
OptionTを使わない場合と使う場合のコード例
for式を二重にする方法は取り出した値がOptionに包まれた状態で、次のgeneratorで利用できないためここでは触れない

import scalaz.OptionT
import scala.concurrent.{Await, ExecutionContext, Future}
import scala.concurrent.duration.Duration

object Hello extends App {
  import scala.concurrent.ExecutionContext.Implicits.global
  import scalaz.Scalaz._
  import Factories._

  // 実コードでは引数の依存や仕様で順番は変えられないものを想定
  private val option: Option[String]                = Option("hoge")
  private val futureOption: Future[Option[String]]  = Future.successful(Some("fuga"))
  private val future: Future[Piyo]                  = Future.successful(Piyo("piyo", "piyo"))
  private val futureOption2: Future[Option[String]] = Future.successful(Some("moge"))

// OptionTを使わない場合
  val futureOptionString: Future[Option[String]] = option
    .map(
      hoge =>
        futureOption.flatMap(
          _.map(
            fuga =>
              future.flatMap {
                case Piyo(piyo, piyopiyo) =>
                  futureOption2.map(_.map(moge => s"$hoge $fuga $piyo $piyopiyo $moge"))
            }
          ).getOrElse(Future.successful(None))
      )
    ).getOrElse(Future.successful(None))
  val res1: Option[String] = Await.result(futureOptionString, Duration.Inf)

// OptionTを使った場合
  val optionT: OptionT[Future, String] = for {
    hoge                 <- option.toOptionT
    fuga                 <- futureOption.toOptionT
    Piyo(piyo, piyopiyo) <- future.toOptionT
    moge                 <- futureOption2.toOptionT
  } yield {
    s"$hoge $fuga $piyo $piyopiyo $moge"
  }
  val res2: Option[String] = Await.result(optionT.run, Duration.Inf)

  println(res1 == res2)
}

object Factories {
  import scalaz._
  implicit class RichOption[A](val opt: Option[A]) {
    def toOptionT(implicit ec: ExecutionContext): OptionT[Future, A] = OptionT(Future.successful(opt))
  }
  implicit class RichFutureOption[A](val f: Future[Option[A]]) {
    def toOptionT(implicit ec: ExecutionContext): OptionT[Future, A]               = OptionT(f)
    def toOptionTOption(implicit ec: ExecutionContext): OptionT[Future, Option[A]] = OptionT(f.map(Option(_)))
  }
  implicit class RichFuture[A](val f: Future[A]) {
    def toOptionT(implicit ec: ExecutionContext): OptionT[Future, A] = OptionT(f.map(Option(_)))
  }
}
case class Piyo(piyo: String, piyopiyo: String)