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This is the second of several posts describing the evolution of scala.concurrent.Future in Scala 2.12.x. For the previous post, click here.

Missing canonical combinators: zipWith

Being able to join two Futures together to produce a new Future which contains a Tuple of the results of both has been available for a while with the zip operation, and this looks something like this:

val future1: Future[String] = 
val future2: Future[Int] = 
val zippedFuture: Future[(String, Int)] = future1 zip future2

But what if we don’t want a Tuple? What if we want to combine the results of the two Futures to create something else?

This typically means having to combine zip and map:

/* The reason why we use a «PartialFunction literal» (yes, I made that up)
 * here is because otherwise the code has to use the arguably uglier _._1 and
 * _._2 syntax for `Tuple` value extraction.
 */
val combinedFuture: Future[String] =
    future1 zip future2 map { case (string, int) => s"$string & $int" }

Now, if you’re like me and you dislike allocations, tuples and lack of genericity, you’ll see that zip is a specialization of x.zipWith(y)(Tuple2.apply)! Well, perhaps it helps if we share the signature of said zipWith:

def zipWith[U, R](that: Future[U])(f: (T, U) => R)(implicit executor: ExecutionContext): Future[R]

This allows us to express the example above as:

val combinedFuture: Future[String] =
    future1.zipWith(future2)((string, int) => s"$string & $int")

Benefits:

  1. Less to type
  2. Less to read
  3. Fewer allocations
  4. Fewer asynchronous steps
  5. More general than zip

Click here for the next part in this blog series.

Cheers,