Await a future, receive an either

Question

I'd like to await a scala future that may have failed. If I use Await.result the exception will be thrown. Instead, if I have f: Future[String] I would like a method Await.resultOpt(f): Option[String] or Await.resultEither(f): Either[String].

I could get this by using scala.util.control.Exception.catching or I could f map (Right(_)) recover { case t: Throwable => Left(t) }, but there must be a more straightforward way.

Solution 1

You could use Await.ready which simply blocks until the Future has either succeeded or failed, then returns a reference back to that Future.

From there, you would probably want to get the Future's value, which is an Option[Try[T]]. Due to the Await.ready call, it should be safe to assume that the value is a Some. Then it's just a matter of mapping between a Try[T] and an Either[Throwable, T].

The short version:

val f: Future[T] = ...

val result: Try[T] = Await.ready(f, Duration.Inf).value.get

val resultEither = result match {
  case Success(t) => Right(t)
  case Failure(e) => Left(e)
}

Solution 2

The shorter version, just to promote the API:

scala> val f = Future(7)
f: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@13965637

scala> f.value.get
res0: scala.util.Try[Int] = Success(7)

scala> import scala.util._
import scala.util._

scala> Either.cond(res0.isSuccess, res0.get, res0.failed.get)
res2: scala.util.Either[Throwable,Int] = Right(7)

scala> val f = Future[Int](???)
f: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@64c4c1

scala> val v = f.value.get
v: scala.util.Try[Int] = Failure(java.util.concurrent.ExecutionException: Boxed Error)

scala> Either.cond(v.isSuccess, v.get, v.failed.get)
res4: scala.util.Either[Throwable,Int] = Left(java.util.concurrent.ExecutionException: Boxed Error)

It has a slight advantage in being a one-liner.

But of course, after adding a .toEither extension method, you don't care how many lines it took.

Solution 3

You could start to make your own type utils and do something like so

  trait RichTypes {

    import scala.util.{Try, Success, Failure}
    import scala.concurrent.{Await, Future}
    import scala.concurrent.duration.Duration

    implicit class RichFuture[T](f: Future[T]) {
      def awaitResult(d: Duration): Either[Throwable, T] = {
        Try(Await.result(f, d)).toEither
      }
    }

    implicit class RichTry[T](tri: Try[T]) {
      def toEither(): Either[Throwable, T] = {
        tri.fold[Either[Throwable, T]](Left(_), Right(_))
      }
    }
  }

  object Example
    extends App
      with RichTypes {

    import scala.concurrent.Future
    import scala.concurrent.duration._

    val succ = Future.successful("hi").awaitResult(5.seconds)
    val fail = Future.failed(new Exception("x")).awaitResult(5.seconds)

    println(succ) // Right(hi)
    println(fail) // Left(Exception(x))
  }

I separated it out for a Try to also have a .fold :).