左flatMap on EitherT

Question

假设我有返回Future[Either[_, _]的函数，我希望在发生故障时应用其中一些函数，这意味着只将它们应用到左侧。简化的例子是：

def operation1: Future[Either[String, Int]] = Future.successful(Right(5))
def operation2: Future[Either[String, Int]] = Future.successful(Left("error"))
def operation2FallBackWork = Future.successful{
  println("Doing some revert stuff")
  Left("Error happened, but reverting was successful")
}

val res = for {
  res1 <- EitherT.fromEither(operation1)
  res2 <- EitherT.fromEither(operation2)//.leftFlatMap(operation2FallBackWork) -????
} yield res1 + res2

Await.result(res.toEither, 5 seconds)

如何做到这一点？

Answer 1

与leftFlatMap最接近的是MonadError的handleError，它具有您希望从leftFlatMap中获得的签名(但请注意，您需要将回退操作更改为EitherT，并提供一个常量函数，而不是按原样传递)。

import scala.concurrent.{ Await, Future }
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._
import scalaz._, Scalaz._

def operation1: Future[Either[String, Int]] = Future.successful(Right(5))
def operation2: Future[Either[String, Int]] = Future.successful(Left("error"))

def operation2FallBack: EitherT[Future, String, Int] = EitherT(
  Future.successful {
    println("Doing some revert stuff")
    "Error happened, but reverting was successful".left
  }
)

val E: MonadError[({ type L[x] = EitherT[Future, String, x] })#L, String] =
  implicitly

val res = for {
  a <- EitherT.fromEither(operation1)
  b <- E.handleError(EitherT.fromEither(operation2))(_ => operation2FallBack)
} yield a + b

Await.result(res.toEither, 5.seconds)

您还可以使用MonadError提供的语法，使其看起来像是EitherT有一个handleError方法，尽管要让Scala编译器认识到您的操作具有正确的形状需要更多的步骤：

import scala.concurrent.{ Await, Future }
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._
import scalaz._, Scalaz._

type FE[x] = EitherT[Future, String, x]

def operation1: FE[Int] = EitherT(Future.successful(5.right))
def operation2: FE[Int] = EitherT(Future.successful("error".left))

def operation2FallBack: FE[Int] = EitherT(
  Future.successful {
    println("Doing some revert stuff")
    "Error happened, but reverting was successful".left
  }
)

val res = for {
  a <- operation1
  b <- operation2.handleError(_ => operation2FallBack)
} yield a + b

Await.result(res.toEither, 5.seconds)

我更喜欢这第二个版本，但这是一个风格和品味的问题。