Programming on the Edge

Learning Scala: scala.Either

Published by Matt Hicks under , on Thursday, October 21, 2010
I would consider myself pretty competent with Scala at this point, but there is still so much I have yet to learn. Something I always used to do in Java was to work my way one class at a time through the JavaDocs of the language I am attempting to do with Scala now as well.

I've just run across the Either class in the ScalaDocs and really like the idea.

The idea is that you may have two different return types that may result from one method. Classically that is solved with all sorts of hackery or returning the common superclass to the two and then doing an instanceof check, but in Scala the Either class provides a much more elegant solution:

object Test {
 def main(args: Array[String]): Unit = {
  numOrString("5") match {
   case Left(s) => println("String: " + s)
   case Right(i) => println("Integer: " + i)
 def numOrString(s: String) = {
  try {
  } catch {
   case exc: NumberFormatException => Left(s)

I'm not really sure how practical this example I wrote is, but my method "numOrString" takes in a String and attempts to convert it to an Int. If it can return an Int it does so or it returns a String. By use of the Left and Right subclasses of Either you can define which type of result you are sending back. Finally, with Scala's awesome matching you can easily determine which type of result you got.