在 scalaz 中使用 Either 分层状态

Question

在Integrating State with Either（幻灯片88）中，鉴于State 的模式在Either 下分层，是否有推荐的方法来添加另一种类型的状态，例如，通过Writer 之类的方式进行日志记录？似乎新状态必须存在于现有 State 和 Either 之间，以便利用 flatMap 中 Either 的快速失败行为。

以下是演示文稿中代码的可运行示例，经过调整后可在 Scalaz 7.2.8 上运行 2.11.8。有没有一种方法可以在现有行为之上干净地添加新的 monad 转换器，从而简化重构？ Stacking StateT in Scalaz 适用于堆叠，但不处理由 Either 的快速失败 flatMap 行为造成的排序问题。

// Based on slide 88+ in https://speakerdeck.com/mpilquist/scalaz-state-monad
// Adjusted for Scala 2.11 (invariant A), Scalaz 7.2 (Pointed->Applicative) and Throwable on lhs of Either
object IntegratingStateAndEither {
  import scalaz._
  import scalaz.Scalaz._
  import EitherT._
  import scalaz.StateT.stateMonad

  type QueryStateS[A] = State[QueryState, A]
  type ET[F[_], A] = EitherT[F, Throwable, A]
  type QueryStateES[A] = ET[QueryStateS, A]

  object QueryStateES {
    def apply[A](s: QueryStateS[Throwable \/ A]): QueryStateES[A] = EitherT(s)
    def liftE[A](e: Throwable \/ A): QueryStateES[A] = apply(Applicative[QueryStateS].point(e))
    def liftS[A](s: QueryStateS[A]): QueryStateES[A] = MonadTrans[ET].liftM(s)
  }

  def runQuery(s: String, m: Model): QueryStateES[QueryResult] = for {
    query <- parseQuery(s)
    res <- performQuery(query, m)
  } yield res

  def parseQuery(s: String): QueryStateES[StatsQuery] =
    QueryStateES.liftE(new Exception("TODO parse").left)

  def performQuery(q: StatsQuery, m: Model): QueryStateES[QueryResult] =
    QueryStateES.liftE(new Exception("TODO perform").left)

  // Just examples that do nothing
  case class Model()
  case class StatsQuery()
  case class QueryResult()
  case class QueryState()

  def test = runQuery("a + b", Model()).run.run(QueryState())
}

Answer 1

要回答有关日志记录的具体示例，您可以执行以下操作：

object LayeringReaderWriterStateWithEither {
  // Based on slide 88+ in https://speakerdeck.com/mpilquist/scalaz-state-monad
  // Adjusted for Scala 2.11 (invariant A), Scalaz 7.2 (Pointed->Applicative) and Throwable on lhs of Either
  object IntegratingStateAndEither {
    import scalaz._
    import scalaz.Scalaz._
    import EitherT._

    type QueryStateS[A] = ReaderWriterState[List[String], String, QueryState, A]
    type ET[F[_], A] = EitherT[F, Throwable, A]
    type QueryStateES[A] = ET[QueryStateS, A]

    object QueryStateES {
      def apply[A](s: QueryStateS[Throwable \/ A]): QueryStateES[A] = EitherT(s)
      def liftE[A](e: Throwable \/ A): QueryStateES[A] = apply(Applicative[QueryStateS].point(e))
      def liftS[A](s: QueryStateS[A]): QueryStateES[A] = MonadTrans[ET].liftM(s)
      def log(msg: String): QueryStateES[Unit] = liftS {
        ReaderWriterState[List[String], String, QueryState, Unit] {
          case (r, s) => (msg.format(r, s), (), s).point[Id]
        }
      }
    }

    def runQuery(s: String, m: Model): QueryStateES[QueryResult] = for {
      _ ← log("Starting")
      query <- parseQuery(s)
      _ ← log(s"Got a query: $query")
      res <- performQuery(query, m)
    } yield res

    def log(msg: String): QueryStateES[Unit] =
      QueryStateES.log(msg)

    def parseQuery(s: String): QueryStateES[StatsQuery] =
      QueryStateES.liftE(new Exception(s"TODO parse $s").left)

    def performQuery(q: StatsQuery, m: Model): QueryStateES[QueryResult] =
      QueryStateES.liftE(new Exception(s"TODO perform $q in $m").left)

    // Just examples that do nothing
    case class Model()
    case class StatsQuery()
    case class QueryResult()
    case class QueryState()

    def test = runQuery("a + b", Model()).run.run(Nil, QueryState())
  }
}