Randomize Randomize - 1 year ago 102
Scala Question

How can I combine two Future[JsArray]?

I have two

, and I need to wait that both are completed with a result.

If they both succeed, the final result should be a
holding the sum of their contents. If at least one of then fails, the final result should be a

How can I do that?

Answer Source

One solution

  1. Define a map2 function for Futures:

    def map2[A, B, C](fa: Future[A], fb: Future[B])(g: (A, B) => C)
                     (implicit executor: ExecutionContext): Future[C] =
      for {
        a <- fa
        b <- fb
      } yield g(a, b)
  2. Define a merge function for merging the two JsArrays (i.e. extracting and concatenating the elements, and use the result to create a new JsArray):

    def merge(array1: JsArray, array2: JsArray): JsArray =
      JsArray(array1.elements ++ array2.elements: _*)

    Note: you need the type ascription in order to pass the resulting Vector to JsArray's apply method.

  3. Call

    map2(f1, f2)(merge)

Trial run in the REPL

scala> import spray.json._
import spray.json._

scala> import scala.concurrent.Future
import scala.concurrent.Future

scala> import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.ExecutionContext.Implicits.global

scala> val f1 = Future(JsArray(JsNumber(1), JsNumber(2))) 
f1: scala.concurrent.Future[spray.json.JsArray] = List()

scala> val f2 = Future(JsArray(JsNumber(3), JsNumber(4), JsNumber(5)))
f2: scala.concurrent.Future[spray.json.JsArray] = List()

scala> map2(f1, f2)(merge).onComplete(println)
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