Either to Try and vice versa in Scala

Question

Are there any conversions from Either to Try and vice versa in the Scala standard library ? Maybe I am missing something but I did not find them.

Answer 1

(NOTICE: this answer applies to Scala 2.11. for more recent versions of Scala, see other answers)

To the best of my knowledge this does not exist in the standard library. Although an Either is typically used with the Left being a failure and the Right being a success, it was really designed to support the concept of two possible return types with one not necessarily being a failure case. I'm guessing these conversions that one would expect to exist do not exist because Either was not really designed to be a Success/Fail monad like Try is. Having said that it would be pretty easy to enrich Either yourself and add these conversions. That could look something like this:

object MyExtensions {
  implicit class RichEither[L <: Throwable,R](e:Either[L,R]){
    def toTry:Try[R] = e.fold(Failure(_), Success(_))
  }
  
  implicit class RichTry[T](t:Try[T]){
    def toEither:Either[Throwable,T] = t.transform(s => Success(Right(s)), f => Success(Left(f))).get
  }  
}

object ExtensionsExample extends App{
  import MyExtensions._
  
  val t:Try[String] = Success("foo")
  println(t.toEither)
  val t2:Try[String] = Failure(new RuntimeException("bar"))
  println(t2.toEither)
  
  val e:Either[Throwable,String] = Right("foo")
  println(e.toTry)
  val e2:Either[Throwable,String] = Left(new RuntimeException("bar"))
  println(e2.toTry)
}