如何将 Seq[Either[A,B]] 减少为 [A,Seq[B]]? [英] How to reduce Seq[Either[A,B]] to Either[A,Seq[B]]?
问题描述
给定一个 Seq[Either[String,A]]
的序列,其中 Left
是一条错误消息.我想获得一个 Either[String,Seq[A]]
,在那里我得到一个 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),我想获取第一条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 cat 或 scalaz 代码,但我也对不使用它的代码感兴趣.
Of course you can post cats or scalaz code but I'm also interested in code not using it.
我更改了标题,最初要求使用 Either[Seq[A],Seq[B]]
来反映消息正文.
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
我错过了您问题的标题要求 Either[Seq[A],Seq[B]]
,但是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:
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)
<小时>
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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