使用 State 和 IO 的堆叠单子时停止理解中间流

Question

在这个 Scala 示例中，我需要在结果为 StopNow 时停止，我需要在调用 decisionStep 后执行此操作。

我该怎么做？

  case class BusinessState()

  trait BusinessResult
  case object KeepGoing extends BusinessResult
  case object StopNow extends BusinessResult

  type IOState[S, A] = StateT[IO, S, A]
  type BusinessIOState[A] = IOState[BusinessState, A]

  trait SomeSteps {
    def step1:BusinessIOState[Unit]
    def step2:BusinessIOState[BusinessState]
    def decisionStep:BusinessIOState[BusinessResult]
    def step3:BusinessIOState[BusinessResult]
    def step4:BusinessIOState[BusinessResult]

    def program = for {
      _ <- step1
      businessState <- step2

      businessResult <- decisionStep

      businessResult1 <- step3
      businessResult2 <- step4
    } yield()
  }

Answer 1

如果你想保持状态，那么你可以将另一个 monad 转换器放入组合中，即OptionT，用于短路。然后将包含所有可能返回StopNow 的步骤的整个块提升到OptionT，最后使用getOrElse 带回BusinessIOState：

import cats._
import cats.data._
import cats.syntax._
import cats.effect._

object StopIO extends IOApp {
  case class BusinessState()

  trait BusinessResult
  case object KeepGoing extends BusinessResult
  case object StopNow extends BusinessResult

  type IOState[S, A] = StateT[IO, S, A]
  type BusinessIOState[A] = IOState[BusinessState, A]

  trait SomeSteps {
    def step1: BusinessIOState[Unit]
    def step2: BusinessIOState[BusinessState]
    def decisionStep: BusinessIOState[BusinessResult]
    def step3: BusinessIOState[BusinessResult]
    def step4: BusinessIOState[BusinessResult]

    def toOpt(a: BusinessIOState[BusinessResult])
    : OptionT[BusinessIOState, BusinessResult] = {
      OptionT.liftF(a).filter(_ == KeepGoing)
    }

    def program: BusinessIOState[Unit] = (for {
      _ <- step1
      businessState <- step2
      _ <- (for {
        _ <- toOpt(decisionStep)
        _ <- toOpt(step3)
        _ <- toOpt(step4)
      } yield ()).getOrElse(())
    } yield ())
  }

  object Impl extends SomeSteps {
    def step1 = Monad[BusinessIOState].unit
    def step2 = Monad[BusinessIOState].pure(BusinessState())
    def decisionStep = StateT.liftF(IO { println("dS"); KeepGoing })
    def step3 = StateT.liftF(IO { println("3"); StopNow })
    def step4 = StateT.liftF(IO { println("4"); KeepGoing })
  }

  def run(args: List[String]) = for {
    _ <- Impl.program.runA(BusinessState())
  } yield ExitCode.Success
}

输出是：

dS
3

注意4没有出现，程序提前停止，因为step3返回一个StopNow。

---

您可能想知道为什么不使用MonadError[IO, Throwable] 的功能进行短路，因为IO 已经可以处理因抛出异常而停止的计算。下面是它的样子：

import cats._
import cats.data._
import cats.syntax._
import cats.effect._

object StopIO extends IOApp {
  case class BusinessState()

  trait BusinessResult
  case object KeepGoing extends BusinessResult
  case object StopNow extends BusinessResult

  type IOState[S, A] = StateT[IO, S, A]
  type BusinessIOState[A] = IOState[BusinessState, A]

  trait SomeSteps {
    def step1: BusinessIOState[Unit]
    def step2: BusinessIOState[BusinessState]
    def decisionStep: BusinessIOState[BusinessResult]
    def step3: BusinessIOState[BusinessResult]
    def step4: BusinessIOState[BusinessResult]

    def raiseStop(a: BusinessIOState[BusinessResult])
    : BusinessIOState[Unit] = {
      a.flatMap {
        case KeepGoing => StateT.liftF(IO.unit)
        case StopNow => StateT.liftF(
          MonadError[IO, Throwable].raiseError(new Exception("stop now"))
        )
      }
    }

    def program = (for {
      _ <- step1
      businessState <- step2
      _ <- raiseStop(decisionStep)
      _ <- raiseStop(step3)
      _ <- raiseStop(step4)
    } yield ())
  }

  object Impl extends SomeSteps {
    def step1 = Monad[BusinessIOState].unit
    def step2 = Monad[BusinessIOState].pure(BusinessState())
    def decisionStep = StateT.liftF(IO { println("dS"); KeepGoing })
    def step3 = StateT.liftF(IO { println("3"); StopNow })
    def step4 = StateT.liftF(IO { println("4"); KeepGoing })
  }

  def run(args: List[String]) = for {
    _ <- Impl.program.runA(BusinessState()).handleErrorWith(_ => IO.unit)
  } yield ExitCode.Success
}

同样，输出是：

dS
3

我认为它与OptionT 版本相比既不短也不清晰，而且它还有一个缺点，即在StopNow 结果的情况下，状态没有正确传递，而是所有内容都被删除了，并且() 在“世界尽头”返回。这有点类似于对控制流使用异常，还有一个缺点是它所能做的就是完全退出整个程序。所以，我可能会尝试使用OptionT。