+str #19020 reduce combinator

akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala

@@ -706,9 +706,28 @@ trait FlowOps[+Out, +Mat] {
    * '''Completes when''' upstream completes
    *
    * '''Cancels when''' downstream cancels
+   *
+   * See also [[FlowOps.scan]]
    */
   def fold[T](zero: T)(f: (T, Out) ⇒ T): Repr[T] = andThen(Fold(zero, f))
 
+  /**
+   * Similar to `fold` but uses first element as zero element.
+   * Applies the given function towards its current and next value,
+   * yielding the next current value.
+   *
+   * '''Emits when''' upstream completes
+   *
+   * '''Backpressures when''' downstream backpressures
+   *
+   * '''Completes when''' upstream completes
+   *
+   * '''Cancels when''' downstream cancels
+   *
+   * See also [[FlowOps.fold]]
+   */
+  def reduce[T >: Out](f: (T, T) ⇒ T): Repr[T] = via(new Reduce[T](f))
+
   /**
    * Intersperses stream with provided element, similar to how [[scala.collection.immutable.List.mkString]]
    * injects a separator between a List's elements.