diff --git a/akka-docs/rst/java/code/docs/stream/javadsl/cookbook/RecipeReduceByKeyTest.java b/akka-docs/rst/java/code/docs/stream/javadsl/cookbook/RecipeReduceByKeyTest.java
index 34acdb8b45..3770d4804b 100644
--- a/akka-docs/rst/java/code/docs/stream/javadsl/cookbook/RecipeReduceByKeyTest.java
+++ b/akka-docs/rst/java/code/docs/stream/javadsl/cookbook/RecipeReduceByKeyTest.java
@@ -55,8 +55,10 @@ public class RecipeReduceByKeyTest extends RecipeTest {
         final Source<Pair<String, Integer>, NotUsed> counts = words
             // split the words into separate streams first
           .groupBy(MAXIMUM_DISTINCT_WORDS, i -> i)
+          //transform each element to pair with number of words in it
+          .map(i -> new Pair<>(i, 1))
           // add counting logic to the streams
-          .fold(new Pair<>("", 0), (pair, elem) -> new Pair<>(elem, pair.second() + 1))
+          .reduce((left, right) -> new Pair<>(left.first(), left.second() + right.second()))
           // get a stream of word counts
           .mergeSubstreams();
         //#word-count
@@ -77,17 +79,13 @@ public class RecipeReduceByKeyTest extends RecipeTest {
   static public <In, K, Out> Flow<In, Pair<K, Out>, NotUsed> reduceByKey(
       int maximumGroupSize,
       Function<In, K> groupKey,
-      Function<K, Out> foldZero,
-      Function2<Out, In, Out> fold,
-      Materializer mat) {
+      Function<In, Out> map,
+      Function2<Out, Out, Out> reduce) {
 
     return Flow.<In> create()
-      .groupBy(maximumGroupSize, i -> i)
-      .fold((Pair<K, Out>) null, (pair, elem) -> {
-        final K key = groupKey.apply(elem);
-        if (pair == null) return new Pair<>(key, fold.apply(foldZero.apply(key), elem));
-        else return new Pair<>(key, fold.apply(pair.second(), elem));
-      })
+      .groupBy(maximumGroupSize, groupKey)
+      .map(i -> new Pair<>(groupKey.apply(i), map.apply(i)))
+      .reduce((left, right) -> new Pair<>(left.first(), reduce.apply(left.second(), right.second())))
       .mergeSubstreams();
   }
   //#reduce-by-key-general
@@ -104,9 +102,8 @@ public class RecipeReduceByKeyTest extends RecipeTest {
         Source<Pair<String, Integer>, NotUsed> counts = words.via(reduceByKey(
           MAXIMUM_DISTINCT_WORDS,
           word -> word,
-          key -> 0,
-          (count, elem) -> count + 1,
-          mat));
+          word -> 1,
+          (left, right) -> left + right));
 
         //#reduce-by-key-general2
         final Future<List<Pair<String, Integer>>> f = counts.grouped(10).runWith(Sink.head(), mat);
diff --git a/akka-docs/rst/java/stream/stream-cookbook.rst b/akka-docs/rst/java/stream/stream-cookbook.rst
index de0e26a603..18095f5575 100644
--- a/akka-docs/rst/java/stream/stream-cookbook.rst
+++ b/akka-docs/rst/java/stream/stream-cookbook.rst
@@ -113,7 +113,7 @@ we have a stream of streams, where every substream will serve identical words.
 To count the words, we need to process the stream of streams (the actual groups
 containing identical words). ``groupBy`` returns a :class:`SubSource`, which
 means that we transform the resulting substreams directly. In this case we use
-the ``fold`` combinator to aggregate the word itself and the number of its
+the ``reduce`` combinator to aggregate the word itself and the number of its
 occurrences within a :class:`Pair<String, Integer>`. Each substream will then
 emit one final value—precisely such a pair—when the overall input completes. As
 a last step we merge back these values from the substreams into one single
@@ -133,8 +133,8 @@ stream cannot continue without violating its resource bound, in this case
 By extracting the parts specific to *wordcount* into
 
 * a ``groupKey`` function that defines the groups
-* a ``foldZero`` that defines the zero element used by the fold on the substream given the group key
-* a ``fold`` function that does the actual reduction
+* a ``map`` map each element to value that is used by the reduce on the substream
+* a ``reduce`` function that does the actual reduction
 
 we get a generalized version below:
 
diff --git a/akka-docs/rst/scala/code/docs/stream/cookbook/RecipeReduceByKey.scala b/akka-docs/rst/scala/code/docs/stream/cookbook/RecipeReduceByKey.scala
index 5e80a6124a..494303fb32 100644
--- a/akka-docs/rst/scala/code/docs/stream/cookbook/RecipeReduceByKey.scala
+++ b/akka-docs/rst/scala/code/docs/stream/cookbook/RecipeReduceByKey.scala
@@ -21,10 +21,10 @@ class RecipeReduceByKey extends RecipeSpec {
       val counts: Source[(String, Int), NotUsed] = words
         // split the words into separate streams first
         .groupBy(MaximumDistinctWords, identity)
+        //transform each element to pair with number of words in it
+        .map(_ -> 1)
         // add counting logic to the streams
-        .fold(("", 0)) {
-          case ((_, count), word) => (word, count + 1)
-        }
+        .reduce((l, r) => (l._1, l._2 + r._2))
         // get a stream of word counts
         .mergeSubstreams
       //#word-count
@@ -46,26 +46,19 @@ class RecipeReduceByKey extends RecipeSpec {
       def reduceByKey[In, K, Out](
         maximumGroupSize: Int,
         groupKey: (In) => K,
-        foldZero: (K) => Out)(fold: (Out, In) => Out): Flow[In, (K, Out), NotUsed] = {
+        map: (In) => Out)(reduce: (Out, Out) => Out): Flow[In, (K, Out), NotUsed] = {
 
         Flow[In]
-          .groupBy(maximumGroupSize, groupKey)
-          .fold(Option.empty[(K, Out)]) {
-            case (None, elem) =>
-              val key = groupKey(elem)
-              Some((key, fold(foldZero(key), elem)))
-            case (Some((key, out)), elem) =>
-              Some((key, fold(out, elem)))
-          }
-          .map(_.get)
+          .groupBy[K](maximumGroupSize, groupKey)
+          .map(e => groupKey(e) -> map(e))
+          .reduce((l, r) => l._1 -> reduce(l._2, r._2))
           .mergeSubstreams
       }
 
-      val wordCounts = words.via(reduceByKey(
-        MaximumDistinctWords,
-        groupKey = (word: String) => word,
-        foldZero = (key: String) => 0)(fold = (count: Int, elem: String) => count + 1))
-
+      val wordCounts = words.via(
+        reduceByKey(MaximumDistinctWords,
+          groupKey = (word: String) => word,
+          map = (word: String) => 1)((left: Int, right: Int) => left + right))
       //#reduce-by-key-general
 
       Await.result(wordCounts.grouped(10).runWith(Sink.head), 3.seconds).toSet should be(Set(
diff --git a/akka-docs/rst/scala/stream/stream-cookbook.rst b/akka-docs/rst/scala/stream/stream-cookbook.rst
index 4a4516eaaa..823f98f5ab 100644
--- a/akka-docs/rst/scala/stream/stream-cookbook.rst
+++ b/akka-docs/rst/scala/stream/stream-cookbook.rst
@@ -111,7 +111,7 @@ we have a stream of streams, where every substream will serve identical words.
 To count the words, we need to process the stream of streams (the actual groups
 containing identical words). ``groupBy`` returns a :class:`SubFlow`, which
 means that we transform the resulting substreams directly. In this case we use
-the ``fold`` combinator to aggregate the word itself and the number of its
+the ``reduce`` combinator to aggregate the word itself and the number of its
 occurrences within a tuple :class:`(String, Integer)`. Each substream will then
 emit one final value—precisely such a pair—when the overall input completes. As
 a last step we merge back these values from the substreams into one single
@@ -131,8 +131,8 @@ this case ``groupBy`` will terminate with a failure.
 By extracting the parts specific to *wordcount* into
 
 * a ``groupKey`` function that defines the groups
-* a ``foldZero`` that defines the zero element used by the fold on the substream given the group key
-* a ``fold`` function that does the actual reduction
+* a ``map`` map each element to value that is used by the reduce on the substream
+* a ``reduce`` function that does the actual reduction
 
 we get a generalized version below:
 
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowReduceSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowReduceSpec.scala
new file mode 100644
index 0000000000..7624f645a2
--- /dev/null
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowReduceSpec.scala
@@ -0,0 +1,59 @@
+/**
+ * Copyright (C) 2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.stream.scaladsl
+
+import scala.concurrent.Await
+import scala.util.control.NoStackTrace
+
+import akka.stream.ActorMaterializer
+import akka.stream.testkit.AkkaSpec
+import akka.stream.testkit.Utils._
+import scala.concurrent.duration._
+
+class FlowReduceSpec extends AkkaSpec {
+  implicit val materializer = ActorMaterializer()
+
+  "A Reduce" must {
+    val input = 1 to 100
+    val expected = input.sum
+    val inputSource = Source(input).filter(_ ⇒ true).map(identity)
+    val reduceSource = inputSource.reduce[Int](_ + _).filter(_ ⇒ true).map(identity)
+    val reduceFlow = Flow[Int].filter(_ ⇒ true).map(identity).reduce(_ + _).filter(_ ⇒ true).map(identity)
+    val reduceSink = Sink.reduce[Int](_ + _)
+
+    "work when using Source.runReduce" in assertAllStagesStopped {
+      Await.result(inputSource.runReduce(_ + _), 3.seconds) should be(expected)
+    }
+
+    "work when using Source.reduce" in assertAllStagesStopped {
+      Await.result(reduceSource runWith Sink.head, 3.seconds) should be(expected)
+    }
+
+    "work when using Sink.reduce" in assertAllStagesStopped {
+      Await.result(inputSource runWith reduceSink, 3.seconds) should be(expected)
+    }
+
+    "work when using Flow.reduce" in assertAllStagesStopped {
+      Await.result(inputSource via reduceFlow runWith Sink.head, 3.seconds) should be(expected)
+    }
+
+    "work when using Source.reduce + Flow.reduce + Sink.reduce" in assertAllStagesStopped {
+      Await.result(reduceSource via reduceFlow runWith reduceSink, 3.seconds) should be(expected)
+    }
+
+    "propagate an error" in assertAllStagesStopped {
+      val error = new Exception with NoStackTrace
+      val future = inputSource.map(x ⇒ if (x > 50) throw error else x).runReduce(Keep.none)
+      the[Exception] thrownBy Await.result(future, 3.seconds) should be(error)
+    }
+
+    "complete future with failure when reducing function throws" in assertAllStagesStopped {
+      val error = new Exception with NoStackTrace
+      val future = inputSource.runReduce[Int]((x, y) ⇒ if (x > 50) throw error else x + y)
+      the[Exception] thrownBy Await.result(future, 3.seconds) should be(error)
+    }
+
+  }
+
+}
diff --git a/akka-stream/src/main/scala/akka/stream/impl/Stages.scala b/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
index e33ee8481c..34469b4722 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
@@ -45,6 +45,7 @@ private[stream] object Stages {
     val dropWhile = name("dropWhile")
     val scan = name("scan")
     val fold = name("fold")
+    val reduce = name("reduce")
     val intersperse = name("intersperse")
     val buffer = name("buffer")
     val conflate = name("conflate")
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
index c1074b6c45..e12d14c672 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
@@ -1071,3 +1071,38 @@ private[stream] final class DropWithin[T](timeout: FiniteDuration) extends Simpl
 
   override def toString = "DropWithin"
 }
+
+/**
+ * INTERNAL API
+ */
+private[stream] final class Reduce[T](f: (T, T) ⇒ T) extends SimpleLinearGraphStage[T] {
+  override def initialAttributes: Attributes = DefaultAttributes.reduce
+
+  override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new GraphStageLogic(shape) {
+    override def toString = s"Reduce.Logic(aggregator=$aggregator)"
+    var aggregator: T = _
+
+    setHandler(in, new InHandler {
+      override def onPush(): Unit = {
+        aggregator = grab(in)
+        pull(in)
+        setHandler(in, rest)
+      }
+    })
+    def rest = new InHandler {
+      override def onPush(): Unit = {
+        aggregator = f(aggregator, grab(in))
+        pull(in)
+      }
+      override def onUpstreamFinish(): Unit = {
+        push(out, aggregator)
+        completeStage()
+      }
+    }
+
+    setHandler(out, new OutHandler {
+      override def onPull(): Unit = pull(in)
+    })
+  }
+  override def toString = "Reduce"
+}
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
index 136511f26b..739094c194 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
@@ -555,6 +555,22 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
   def fold[T](zero: T)(f: function.Function2[T, Out, T]): javadsl.Flow[In, T, Mat] =
     new Flow(delegate.fold(zero)(f.apply))
 
+  /**
+   * 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
+   */
+  def reduce(f: function.Function2[Out, Out, Out @uncheckedVariance]): javadsl.Flow[In, Out, Mat] =
+    new Flow(delegate.reduce(f.apply))
+
   /**
    * Intersperses stream with provided element, similar to how [[scala.collection.immutable.List.mkString]]
    * injects a separator between a List's elements.
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala
index fb1bc07461..b7854e0253 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala
@@ -29,6 +29,16 @@ object Sink {
   def fold[U, In](zero: U, f: function.Function2[U, In, U]): javadsl.Sink[In, Future[U]] =
     new Sink(scaladsl.Sink.fold[U, In](zero)(f.apply))
 
+  /**
+   * A `Sink` that will invoke the given function for every received element, giving it its previous
+   * output (from the second element) and the element as input.
+   * The returned [[scala.concurrent.Future]] will be completed with value of the final
+   * function evaluation when the input stream ends, or completed with `Failure`
+   * if there is a failure signaled in the stream.
+   */
+  def reduce[In](f: function.Function2[In, In, In]): Sink[In, Future[In]] =
+    new Sink(scaladsl.Sink.reduce[In](f.apply))
+
   /**
    * Helper to create [[Sink]] from `Subscriber`.
    */
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
index 2aa378591e..580cfd59eb 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
@@ -457,6 +457,17 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
   def runFold[U](zero: U, f: function.Function2[U, Out, U], materializer: Materializer): Future[U] =
     runWith(Sink.fold(zero, f), materializer)
 
+  /**
+   * Shortcut for running this `Source` with a reduce function.
+   * The given function is invoked for every received element, giving it its previous
+   * output (from the second ones) an the element as input.
+   * The returned [[scala.concurrent.Future]] will be completed with value of the final
+   * function evaluation when the input stream ends, or completed with `Failure`
+   * if there is a failure is signaled in the stream.
+   */
+  def runReduce[U >: Out](f: function.Function2[U, U, U], materializer: Materializer): Future[U] =
+    runWith(Sink.reduce(f), materializer)
+
   /**
    * Concatenate this [[Source]] with the given one, meaning that once current
    * is exhausted and all result elements have been generated,
@@ -1006,6 +1017,22 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
   def fold[T](zero: T)(f: function.Function2[T, Out, T]): javadsl.Source[T, Mat] =
     new Source(delegate.fold(zero)(f.apply))
 
+  /**
+   * 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
+   */
+  def reduce(f: function.Function2[Out, Out, Out @uncheckedVariance]): javadsl.Source[Out, Mat] =
+    new Source(delegate.reduce(f.apply))
+
   /**
    * Intersperses stream with provided element, similar to how [[scala.collection.immutable.List.mkString]]
    * injects a separator between a List's elements.
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/SubFlow.scala b/akka-stream/src/main/scala/akka/stream/javadsl/SubFlow.scala
index 7198d01dc0..ca5d32d0ab 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/SubFlow.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/SubFlow.scala
@@ -400,6 +400,22 @@ class SubFlow[-In, +Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Flo
   def fold[T](zero: T)(f: function.Function2[T, Out, T]): SubFlow[In, T, Mat] =
     new SubFlow(delegate.fold(zero)(f.apply))
 
+  /**
+   * 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
+   */
+  def reduce(f: function.Function2[Out, Out, Out @uncheckedVariance]): SubFlow[In, Out, Mat] =
+    new SubFlow(delegate.reduce(f.apply))
+
   /**
    * Intersperses stream with provided element, similar to how [[scala.collection.immutable.List.mkString]]
    * injects a separator between a List's elements.
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/SubSource.scala b/akka-stream/src/main/scala/akka/stream/javadsl/SubSource.scala
index 08bec7646a..137904da40 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/SubSource.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/SubSource.scala
@@ -396,6 +396,22 @@ class SubSource[+Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source
   def fold[T](zero: T)(f: function.Function2[T, Out, T]): SubSource[T, Mat] =
     new SubSource(delegate.fold(zero)(f.apply))
 
+  /**
+   * 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
+   */
+  def reduce(f: function.Function2[Out, Out, Out @uncheckedVariance]): SubSource[Out, Mat] =
+    new SubSource(delegate.reduce(f.apply))
+
   /**
    * Intersperses stream with provided element, similar to how [[scala.collection.immutable.List.mkString]]
    * injects a separator between a List's elements.
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
index 339e9b94ba..ebd8326b2c 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
+++ b/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.
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
index 2674e7d292..d9463aa84d 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
@@ -230,6 +230,16 @@ object Sink {
   def fold[U, T](zero: U)(f: (U, T) ⇒ U): Sink[T, Future[U]] =
     Flow[T].fold(zero)(f).toMat(Sink.head)(Keep.right).named("foldSink")
 
+  /**
+   * A `Sink` that will invoke the given function for every received element, giving it its previous
+   * output (from the second element) and the element as input.
+   * The returned [[scala.concurrent.Future]] will be completed with value of the final
+   * function evaluation when the input stream ends, or completed with `Failure`
+   * if there is a failure signaled in the stream.
+   */
+  def reduce[T](f: (T, T) ⇒ T): Sink[T, Future[T]] =
+    Flow[T].reduce(f).toMat(Sink.head)(Keep.right).named("reduceSink")
+
   /**
    * A `Sink` that when the flow is completed, either through a failure or normal
    * completion, apply the provided function with [[scala.util.Success]]
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
index 88e32bd8f6..19170ab203 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
@@ -101,6 +101,17 @@ final class Source[+Out, +Mat](private[stream] override val module: Module)
   def runFold[U](zero: U)(f: (U, Out) ⇒ U)(implicit materializer: Materializer): Future[U] =
     runWith(Sink.fold(zero)(f))
 
+  /**
+   * Shortcut for running this `Source` with a reduce function.
+   * The given function is invoked for every received element, giving it its previous
+   * output (from the second element) and the element as input.
+   * The returned [[scala.concurrent.Future]] will be completed with value of the final
+   * function evaluation when the input stream ends, or completed with `Failure`
+   * if there is a failure signaled in the stream.
+   */
+  def runReduce[U >: Out](f: (U, U) ⇒ U)(implicit materializer: Materializer): Future[U] =
+    runWith(Sink.reduce(f))
+
   /**
    * Shortcut for running this `Source` with a foreach procedure. The given procedure is invoked
    * for each received element.