EitherT 上的左平面图

Question

假设我有 return Future[Either[_, _] 的函数，我想在出现故障时应用其中的一些函数，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)

如何实现？

Answer 1

最接近 leftFlatMap 的是 MonadError 的 handleError，它具有您期望的 leftFlatMap 签名（尽管请注意您需要将回退操作更改为 EitherT 并提供常量函数而不是传递它 as-is)。您可以像这样直接使用 EitherT 实例：

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 提供的语法使它看起来像 EitherT 有一个 handleError 方法，尽管需要更多的仪式才能让 Scala 编译器认识到您的操作具有正确的形状：

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)

我更喜欢第二个版本，但这是风格和品味的问题。

EitherT 上的左平面图

Left flatMap on EitherT

scala

scalaz

monad-transformers