假设我有返回Future[Either[_, _]
的函数,我想在失败的情况下应用其中一些函数,这意味着只将它们应用于左侧。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?
答案 0 :(得分:11)
与leftFlatMap
最接近的是MonadError
&#39; s handleError
,它具有您对leftFlatMap
内容所期望的签名(虽然请注意,您需要将回退操作更改为EitherT
并提供常量函数,而不是按原样传递它。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
提供的语法使其看起来像EitherT
具有handleError
方法,尽管需要更多的仪式才能让Scala编译器识别出来您的操作形式正确:
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,但这只是风格和品味的问题。