How to flatten a List of Futures in Scala

Question

I want to take this val:

val f = List(Future(1), Future(2), Future(3))

Perform some operation on it (I was thinking flatten)

f.flatten

And get this result

scala> f.flatten = List(1,2,3)

If the flatten method isn't appropriate here, that's fine. As long as I get to the result.

Thanks!

Solution

Future.sequence takes a List[Future[T]] and returns a Future[List[T]].

You can do

Future.sequence(f) 

and then use map or onComplete on it to access the list of values.

Comments

[mljohns89]

This is the output I'm getting: scala.concurrent.Future[List[Int]] = scala.concurrent.impl.Promise$DefaultPromise@3227f076 Is this expected? It is transforming a List[Future[T]] into a Future[List[T]]. But I was expecting something along the lines of: scala.concurrent.Future[List[Int]] = Future(List(1,2,3)) Thanks!

[Gabriele Petronella]

Yes, it's a Future. Now you can handle it (either map, use onComplete or anything you want). The result inside the Future will be your list in case of success

[lmm]

Future doesn't normally print what's inside it (that would mean blocking). If you do Future(1), you'll get something that looks like scala.concurrent.impl.Promise$DefaultPromise@78393fdb. For testing you can use something like Await.result(f, 2.seconds).

[Gabriele Petronella]

@mikejones1477 it's almost always preferable not to block. Consider handling the result asynchronously

[mljohns89]

Yes. I wasn't planning on blocking for my final result. But in the interest of learning, I'd like to ask what you mean about asynchronously?

[Gabriele Petronella]

I mean in a non-blocking way. Mapping over a future is already asynchronous, since the code will run eventually in the future (if the computation has success)