Creating parallel executing List combinators

Question

I have function that splits List[B] in half and returns tuple. I need to release combinators for executing them in parallel.

If I do something like fl.map(x => x.map(...)) it is not parallel.

object Parallel {

  implicit class ParList[A](fl: Future[List[A]]) {

    private def split[B](l: List[B]): (List[B], List[B]) = l.splitAt(l.length / 2)

    def paralFilter(f: A => Boolean): Future[List[A]] = ???

    def paralFold(init: A)(f: (A, A) => A): Future[A] = ???
  }

I understand that I must split list elementwise via split[B] and execute combinators with elements in parallel. How can I do it?

Answer 1

To achieve this use method par of scala.collection.Parallelizable :

object Parallel {

  implicit class ParList[A](fl: Future[List[A]]) {

    private def split[B](l: List[B]): (List[B], List[B]) = l.splitAt(l.length / 2)

    def paralFilter(f: A => Boolean)(implicit ec:ExecutionContext): Future[List[A]] = 
      fl.map(_.par.filter(f).toList)

    def paralFold(init: A)(f: (A, A) => A)(implicit ec:ExecutionContext): Future[A] =
      fl.map(_.par.fold(init)(f))
  }
}