Refining signatures on fold and reduce

akka-actor/src/main/scala/akka/dispatch/Future.scala

@@ -75,7 +75,7 @@ object Futures {
    * the result will be the first failure of any of the futures, or any failure in the actual fold,
    * or the result of the fold.
    */
-  def fold[R,T](zero: R, timeout: Long = Actor.TIMEOUT)(futures: Iterable[Future[T]])(foldFun: (R, T) => R): Future[R] = {
+  def fold[T,R](zero: R, timeout: Long = Actor.TIMEOUT)(futures: Iterable[Future[T]])(foldFun: (R, T) => R): Future[R] = {
     val result = new DefaultCompletableFuture[R](timeout)
     val results = new ConcurrentLinkedQueue[T]()
     val waitingFor = new AtomicInteger(futures.size)
@@ -104,13 +104,16 @@ object Futures {
   /**
    * Initiates a fold over the supplied futures where the fold-zero is the result value of the Future that's completed first
    */
-  def reduce[T](futures: Iterable[Future[T]], timeout: Long = Actor.TIMEOUT)(op: (T,T) => T): Future[T] = {
-    val result = new DefaultCompletableFuture[T](timeout)
+  def reduce[T, R >: T](futures: Iterable[Future[T]], timeout: Long = Actor.TIMEOUT)(op: (R,T) => T): Future[R] = {
+    if (futures.isEmpty)
+      throw new UnsupportedOperationException("empty reduce left")
+
+    val result = new DefaultCompletableFuture[R](timeout)
     val seedFound = new AtomicBoolean(false)
     val seedFold = (f: Future[T]) => {
       if (seedFound.compareAndSet(false, true)){ //Only the first completed should trigger the fold
         if (f.exception.isDefined) result completeWithException f.exception.get //If the seed is a failure, we're done here
-        else (fold[T,T](f.result.get, timeout)(futures.filterNot(_ eq f))(op)).onComplete(result.completeWith(_)) //Fold using the seed
+        else (fold[T,R](f.result.get, timeout)(futures.filterNot(_ eq f))(op)).onComplete(result.completeWith(_)) //Fold using the seed
       }
       () //Returns Unit
     }