Elegant Handling of Scala Future[Either]]

Question

I have a type whose shape is like this:

val myType: Future[Either[MyError, TypeA]] = // some value

I know that I could pattern match on this and get to the Right or Left type, but the problem is that I would have to nest my pattern matching logic. I'm looking for much more elegant way of handling this? Any suggestions?

Answer 1

If you encode your MyError as an exception, you don't need the Either anymore and can simply patternMatch against the completion, or use a recoverWith to map it to another type:

myType.onComplete {
  case Success(t) =>
  case Failure(e) =>
}

To map your existing Either types you could do something like this:

case class MyException(e: MyError) extends Exception

def eitherToException[A](f: Future[Either[MyError,A]]): Future[A] = {
  f.flatMap {
    case Left(e) => Future.failed(MyException(e))
    case Right(x) => Future.successful(x)
  }
}

val myType2 = eitherToException(myType)

Alternatively, if MyError and TypeA are under your control, you could create a common super type and pattern match against that:

sealed trait MyResult
final case class MyError() extends MyResult
final case class TypeA() extends MyResult

myType.map {
  case MyError() => ...
  case TypeA() => ...
}

Answer 2

You can create custom extractor objects:

object FullSuccess {
  def unapply[T](x: Try[Either[MyError, T]]) = x match {
    case Success(Right(x)) => Some(x)
    case _ => None
  }
}

object PartSuccess {
  def unapply[T](x: Try[Either[MyError, T]]) = x match {
    case Success(Left(err)) => Some(err)
    case _ => None
  }
}

And

val myType: Future[Either[MyError, TypeA]] = // some value

myType.onComplete {
  case FullSuccess(x) => ... // equivalent to case Success(Right(x))
  case PartSuccess(x) => ... // equivalent to case Success(Left(x))
  case Failure(e) => ...
}