将潜在的未来恢复到 Cats'EitherT's Left 中? [英] Recovering underlying Future into Cats' EitherT's Left?

问题描述

如果我有一个 Future[Either[String, Int]] 表示可能的错误消息 (String) 或成功的计算 (Int)，将 Future 的潜在故障作为错误消息移到左侧很简单:

If I have a Future[Either[String, Int]] that represents either a possible error message (String) or a successful computation (Int), it is simple to move the Future's potential failure into the left side as an error message:

def handleFailure(fe: Future[Either[String,Int]]) =
  f.recover({ case e: Exception => Left(s"failed because ${e.getMessage}"))

我希望 EitherT 存在类似的东西，但也许我只是找不到它的名字.它相对简单，但涉及拆箱和重新装箱，感觉很笨拙:

I would expect something similar to exist for EitherT, but maybe I just can't find out what it is called. It is relatively simple, but involves unboxing and re-boxing the EitherT which feels kludgey:

def handleFailureT(fe: EitherT[Future, String, Int]) =
  EitherT(handleFailure(et.value)) // See above for handleFailure definition

Cats 确实在 不久前添加了一个 MonadError 实例，但它专门用于直接恢复到Either 的right，而不是替换Either 本身.

Cats did add a MonadError instance a while ago, but it's specifically for recovering straight into the Either's right, not for replacing the Either itself.

handleFailureT 是在 Cats 中实现的吗?如果是，它叫什么?

Is handleFailureT implemented it Cats, and if so what is it called?

推荐答案

这并不明显，但我认为这就是 attempt 和 attemptT 的用途.例如:

It is not at all obvious, but I think this is what attempt and attemptT are for. For example:

val myTry: Try[Int] = Try(2)
val myFuture: Future[String] = Future.failed(new Exception())
val myTryET: EitherT[Try, Throwable, Int] = myTry.attemptT
val myFutureET: EitherT[Future, Throwable, String] = myFuture.attemptT

// alternatively
val myFutureET: EitherT[Future, Throwable, String] = EitherT(myFuture.attempt)

有一个 PR 将此添加到文档中:https://github.com/typelevel/cats/pull/3178 -- 但它目前没有出现在文档中.但是，您可以在这里看到它:https://github.com/typelevel/cats/blob/master/docs/src/main/tut/datatypes/eithert.md#from-applicativeerrorf-e-to-要么tf-ea

There was a PR to add this to the documentation: https://github.com/typelevel/cats/pull/3178 -- but it doesn't appear in the documentation currently. However, you can see it here: https://github.com/typelevel/cats/blob/master/docs/src/main/tut/datatypes/eithert.md#from-applicativeerrorf-e-to-eithertf-e-a

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