Scala Either map Right or return Left

scala either
Michał Jurczuk picture Michał Jurczuk · Dec 31, 2015 · Viewed 17.7k times · Source

Is it possible to handle Either in similar way to Option? In Option, I have a getOrElse function, in Either I want to return Left or process Right. I'm looking for the fastest way of doing this without any boilerplate like:

val myEither:Either[String, Object] = Right(new Object())
myEither match {
    case Left(leftValue) => value
    case Right(righValue) => 
        "Success"
}

Answer

Alexey Romanov picture Alexey Romanov · Nov 22, 2016

In Scala 2.12,

Either is right-biased, which means that Right is assumed to be the default case to operate on. If it is Left, operations like map, flatMap, ... return the Left value unchanged

so you can do

myEither.map(_ => "Success").merge

if you find it more readable than fold.

Related questions

Getting Value of Either

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 …

Read More
Using Either to process failures in Scala code

Option monad is a great expressive way to deal with something-or-nothing things in Scala. But what if one needs to log a message when "nothing" occurs? According to the Scala API documentation, The Either type is often used as an …

Read More
Convert Option to Either in Scala

Suppose I need to convert Option[Int] to Either[String, Int] in Scala. I'd like to do it like this: def foo(ox: Option[Int]): Either[String, Int] = ox.fold(Left("No number")) {x => Right(x)} Unfortunately the code …

Read More