Getting Value of Either

问题

Besides using match, is there an Option-like way to getOrElse the actual content of the Right or Left value?

scala> val x: Either[String,Int] = Right(5)
scala> val a: String = x match { 
                                case Right(x) => x.toString
                                case Left(x) => "left" 
                       }
a: String = 5

回答1:

I don't particularly like Either and as a result I'm not terribly familiar with it, but I believe you're looking for projections: either.left.getOrElse or either.right.getOrElse.

Note that projections can be used in for-comprehensions as well. This is an example straight from the documentation:

def interactWithDB(x: Query): Either[Exception, Result] =
  try {
    Right(getResultFromDatabase(x))
  } catch {
    case ex => Left(ex)
  }

// this will only be executed if interactWithDB returns a Right
val report =
  for (r <- interactWithDB(someQuery).right) yield generateReport(r)
if (report.isRight)
  send(report)
else
  log("report not generated, reason was " + report.left.get)

回答2:

Nicolas Rinaudo's answer regarding calling getOrElse on either the left or right projection is probably the closest to Option.getOrElse.

Alternatively, you can fold the either:

scala> val x: Either[String,Int] = Right(5)
x: Either[String,Int] = Right(5)

scala> val a: String = x.fold(l => "left", r => r.toString)
a: String = 5

As l is not used in the above fold, you could also write x.fold(_ => "left", r => r.toString)

Edit: Actually, you can literally have Option.getOrElse by calling toOption on the left or right projection of the either, eg,

scala> val o: Option[Int] = x.right.toOption
o: Option[Int] = Some(5)

scala> val a: String = o.map(_.toString).getOrElse("left")
a: String = 5

回答3:

Given type A on both sides, that is, Either[A, A], we can use Either.merge

...to extract values from Either instances regardless of whether they are Left or Right.

Note if left and right types differ then result becomes Any:

val e: Either[Int, String] = Right("hello")
e.merge // hello: Any

回答4:

In Scala 2.12 there is a getOrElse method for getting the "right" value but you cannot use it for the "left" value directly. However, you can do it like this: e.swap.getOrElse(42).

来源：https://stackoverflow.com/questions/19503647/getting-value-of-either

标签

scala

either