How to reduce Seq[Either[A,B]] to Either[A,Seq[B]]?
Edit: 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)
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))
Given a starting sequence xs
, here's my take:
xs collectFirst { case x@Left(_) => x } getOrElse
Right(xs collect {case Right(x) => x})
This being in answer to the body of the question, obtaining only the first error as an Either[String,Seq[A]]
. It's obviously not a valid answer to the question in the title
To return all errors:
val lefts = xs collect {case Left(x) => x }
def rights = xs collect {case Right(x) => x}
if(lefts.isEmpty) Right(rights) else Left(lefts)
Note that rights
is defined as a method, so it'll only be evaluated on demand, if necessary
Here is the scalaz code:
_.sequence