Scala:将整个列表中的每个元素与每个元素中的每个元素组合 [英] Scala: Combine Either per the whole List with Either per elements

问题描述

我有一个Either列表，表示错误:

I have a list of Either, which represents error:

type ErrorType = List[String]
type FailFast[A] = Either[ErrorType, A]

import cats.syntax.either._
val l = List(1.asRight[ErrorType], 5.asRight[ErrorType])

如果它们都正确，我想获得[A]的列表，在这种情况下-List[Int]

If all of them are right, I want to get a list of [A], in this case - List[Int]

如果剩下任何Either，我想合并所有两个的所有错误并将其返回.

If any Either is left, I want to combine all errors of all either and return it.

我在[如何将Seq [Either [A，B]]简化为Either [A，Seq [B]]

但是很久以前.例如，答案之一提供了使用partitionMap的能力，目前我找不到.可能有一个更好，更优雅的解决方案.斯卡拉猫的例子很好.

But it was quite long ago. For instance, one of the answers offers to use partitionMap, which I cannot find at this moment. Probably there is a better, more elegant solution. Example with scala-cats would be great.

我想如何使用它:

for {
  listWithEihers <- someFunction
  //if this list contains one or more errors, return Left[List[String]]
  //if everything is fine, convert it to:
  correctItems <- //returns list of List[Int] as right
} yield correctItems

此理解的返回类型必须为:

Return type of this for-comprehension must be:

Either[List[String], List[Int]]

推荐答案

正如注释中已经提到的，Either对于快速失败行为很有用.为了积累多个错误，您可能需要类似Validated的东西.而且:

As already mentioned in the comments, Either is good for fail-fast behavior. For accumulating multiple errors, you probably want something like Validated. Moreover:

• 列表是可遍历的(具有Traverse的实例)
• 已验证是适用的
• Validated.fromEither将Either[List[String], X]映射到Validated[List[String], X]，这正是traverse中的功能所需要的.

• List is traversable (has instance of Traverse)
• Validated is applicative
• Validated.fromEither maps Either[List[String], X] to Validated[List[String], X], that's exactly what you need as function in traverse.

因此，您可以尝试:

• l.traverse(Validated.fromEither)(如果您对Validated
没问题)
• l.traverse(Validated.fromEither).toEither，如果您真的想要最后一个Either.

• l.traverse(Validated.fromEither) if you are OK with a Validated
• l.traverse(Validated.fromEither).toEither if you really want an Either in the end.

包含所有导入的完整示例:

Full example with all imports:

import cats.data.Validated
import cats.syntax.validated._
import cats.syntax.either._
import cats.syntax.traverse._
import cats.instances.list._
import cats.Traverse
import scala.util.Either

type ErrorType = List[String]
type FailFast[A] = Either[ErrorType, A]
val l: List[Either[ErrorType, Int]] = List(1.asRight[ErrorType], 5.asRight[ErrorType])

// solution if you want to keep a `Validated`
val validatedList: Validated[ErrorType, List[Int]] =
  l.traverse(Validated.fromEither)

// solution if you want to transform it back to `Either`
val eitherList: Either[ErrorType, List[Int]] =    
  l.traverse(Validated.fromEither).toEither

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