How to use transformWith() to ignore failed futures? - scala

Question

I need to process sequence of transformations into one transformation, in order to if I have some failed Future it should be just ignored(I am trying to do it without recover or recoverWith, but smth going wrong, code fails when meet any failure)
type Transformation[T] = T => Future[T]
//in - Seq(trans1, trans2, trans3)
in.reduce[Transformation[T]](
(acc, el) =>
acc.andThen[Future[T]](
ft =>
ft.flatMap(el)
.transformWith[T](
t =>
t match {
case Failure(exception) => ft //this line is the most suspicious for me
case Success(value) => Future.successful(value)
}
)
)
)

Answer 1

The transformWith doesn't seem to provide you the possibility to recover with the original input value from the previous step, because it has to work with a Try. What is it supposed to do if the Try turns out to be a failure? By that time, it does not have the original input to fall back to, it only has a Throwable, not a T. So, transformWith seems insufficient.
If the pipeline is just a few transformations long, you can try foldLeft with fallbackTo:
import scala.concurrent._
import scala.concurrent.duration._
import scala.concurrent.ExecutionContext.Implicits.global
type Transformation[T] = T => Future[T]
def chainTrafos[T](
trafos: List[Transformation[T]],
value: T
): Future[T] = {
trafos.foldLeft(Future { value })(
(f, t) => f.flatMap(x => t(x).fallbackTo(Future { x }))
)
}
val ta: Transformation[Int] = x => Future { x * x }
val tb: Transformation[Int] = x => Future.failed(new Error("oops"))
val tc: Transformation[Int] = x => Future { x - 58 }
println(Await.result(chainTrafos(List(ta, tb, tc), 10), 10.seconds))
prints
42