Simple way to filter a list based on future results of items in that list?

[Ryan LeCompte]

Basically, if you have a method that returns a future, and you want to filter a list based on those futures (and the futures are built using values from the list), is there something simple that I can use out of the box? 

I came up with the following (example code, not actual code):

scala> val nums = 0 to 10
res17: scala.collection.immutable.Range.Inclusive = Range(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
 
scala> def lookup(id: Int): Future[Option[Int]] = future { if (rnd.nextBoolean) Some(5) else None }
lookup: (id: Int)scala.concurrent.Future[Option[Int]]
 
scala> val collapsed = Future.sequence(nums.map { n => lookup(n).map { (n, _) } })
reduced: scala.concurrent.Future[scala.collection.immutable.IndexedSeq[(Int, Option[Int])]] = scala.concurrent.impl.Promise$DefaultPromise@5f0900d2
 
scala> val filtered = collapsed.map { results => results.collect { case (i, opt) if opt.isDefined => i } }
filtered: scala.concurrent.Future[scala.collection.immutable.IndexedSeq[Int]] = scala.concurrent.impl.Promise$DefaultPromise@7f5451cc
 

[Ryan LeCompte]

(I used a Future[Option[Int]] as the result from lookup(), but it's really some other Future[MyClass] that has an isEnabled method (I used isDefined on option to be similar in the example).

[Som Snytt]

Future.collect will effectively filter before you sequence; then, just flatten the bad results.

In your example, you filter with collect on the aggregate results.

The difference is that you could lookup(i).collect { case p => ... }.onSuccess(...)

object Test extends App {
  import util.{ Random => rnd, _ }
  import concurrent._
  import concurrent.duration._
  import ExecutionContext.Implicits.global

  val nums = 0 to 10

  def value(i: Int) = if (rnd.nextBoolean) Some(5) else None
  def lookup(id: Int) = future(value(id))
  def f(i: Int) = lookup(i) map ((i, _)) collect { case (i, Some(v)) => v }
  val all = nums map f
  val done = Future sequence all
  Await ready (done, Duration.Inf)
  val vs = all map (_.value.get.toOption)
  Console println vs.flatten
}

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[Ryan LeCompte]

Nice alternative, I had forgotten about Future.collect. Thanks!