Scala, ZIO - convert Future[Either[...]] into ZIO

Question

I have a simple method which returns Future[Either[Throwable, MyValue]] . Now I would like to convert it to ZIO. I created:

def zioMethod(): ZIO[Any, Throwable, MyValue] =
    ZIO.fromFuture {implicit ec =>
       futureEitherMethod().flatMap { result => 
            Future(result)
       }
    }

But it didn't work without trace: No implicit found for parameter trace: Trace .

So, after I added Trace as implicit it still doesn't work correctly. Is any other way to convert Future[Either[,]] into ZIO ? Docs are not clear in this topic.

Answer 1

I do not use ZIO at all, but: the signature (in the code or in the scaladoc), seems to be rather clear on this topic:

ZIO.fromFuture[A] converts a Future[A] into a ZIO[Any, Throwable, A] .

You pass in a Future[Either[Throwable, MyValue]] , so it would return a ZIO[Any, Throwable, Either[Throwable, MyValue]] - not a ZIO[Any, Throwable, MyValue] as your code expects.

To transfer the Left case of your Either onto the "error channel" of the ZIO , you have several options, eg:

Convert Future[Either[Throwable, MyValue]] to Future[Throwable, MyValue] and then to ZIO:

def zioMethod(): ZIO[Any, Throwable, MyValue] =
  ZIO.fromFuture { implicit ec =>
    futureEitherMethod().flatMap { 
      case Right(myValue)  => Future.successful(myValue)
      case Left(throwable) => Future.failed(throwable)
    }
  }

Convert to ZIO[Any, Throwable, Either[Throwable, MyValue]] and then to ZIO[Any, Throwable, MyValue] :

def zioMethod(): ZIO[Any, Throwable, MyValue] = {
  val fromFuture: ZIO[Any, Throwable, Either[Throwable, MyValue]] =
    ZIO.fromFuture { implicit ec =>
      futureEitherMethod()
    }
  fromFuture.flatMap(ZIO.fromEither)
}

BTW: for me, this compiles without bringing any additional implicits in code: https://scastie.scala-lang.org/dRefNpH0SEO5aIjegV8RKw