如何将Seq [[A，B]]减少到[A，Seq [B]] [英] How to reduce a Seq[Either[A,B]] to a Either[A,Seq[B]]

问题描述

给出一个 Seq [或者[String，A]] 和 Left 作为错误消息的序列。我想获得任何[String，Seq [A]] ，我得到 Right （这将是一个 Seq [A] ），如果序列的所有元素都是 Right 。如果至少有一个 Left （错误消息），我想要获得第一个错误消息或所有错误消息的串联。

Given a sequence of eithers Seq[Either[String,A]] with Left being an error message. I want to obtain an Either[String,Seq[A]] where I get a Right (which will be a Seq[A]), if all elements of the sequence are Right. If there is at least one Left (an error message), I'd like to obtain the first error message or a concatenation of all error messages.

当然，您可以发布scalaz代码，但我也对代码不感兴趣。

Of course you can post scalaz code but I'm also interested in code not using it.

我改变了标题，最初要求 [Seq [A]，Seq [B]] 来反映主体。

I've changed the title, which originally asked for an Either[Seq[A],Seq[B]] to reflect the body of the message.

推荐答案

编辑：我错过了要求标题 [Seq [A]，Seq [B]] ，但我确实读过我想获得第一条错误消息或所有错误消息的串联，这会给你前者： / b>

I missed that the title of your question asked for Either[Seq[A],Seq[B]], but I did read "I'd like to obtain the first error message or a concatenation of all error messages", and this would give you the former:

def sequence[A, B](s: Seq[Either[A, B]]): Either[A, Seq[B]] =
  s.foldRight(Right(Nil): Either[A, List[B]]) {
    (e, acc) => for (xs <- acc.right; x <- e.right) yield x :: xs
  }

scala> sequence(List(Right(1), Right(2), Right(3)))
res2: Either[Nothing,Seq[Int]] = Right(List(1, 2, 3))

scala> sequence(List(Right(1), Left("error"), Right(3)))
res3: Either[java.lang.String,Seq[Int]] = Left(error)

---

使用Scalaz：

---

Using Scalaz:

val xs: List[Either[String, Int]] = List(Right(1), Right(2), Right(3))

scala> xs.sequenceU
res0:  scala.util.Either[String,List[Int]] = Right(List(1, 2, 3))

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