Combining Scala Futures and collections in for comprehensions

Question

I'm trying to use a for expression to iterate over a list, then do a transformation on each element using a utility that returns a Future. Long story short, it doesn't compile, and I'd like to understand why. I read this question, which is similar, and was a great help, but what I'm trying to do is even simpler, which is all the more confusing as to why it doesn't work. I'm trying to do something like:

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

val numberList = List(1, 2, 3)
def squareInTheFuture(number: Int): Future[Int] = Future { number * number}
val allTheSquares = for {
                          number <- numberList
                          square <- squareInTheFuture(number)
                        } yield { square }

And what I get is:

error: type mismatch; found : scala.concurrent.Future[Int] required: scala.collection.GenTraversableOnce[?] square <- squareInTheFuture(number) ^

Can someone help me understand why this doesn't work and what the best alternative is?

Answer 1

The Future companion object has a traverse method that does exactly what you want:

val allTheSquares: Future[List[Int]] =
  Future.traverse(numberList)(squareInTheFuture)

This will asynchronously start all the computations and return a future that will be completed once all of those futures are completed.

Answer 2

flatMap requires that the type constructors of numberList and squareInTheFuture(number) are the same (modulo whatever implicit conversions the collection library does). That isn't the case here. Instead, this is a traversal:

val allSquaresInTheFuture: Future[List[Int]] =
    Future.traverse(numberList)(squareInTheFuture)