Simplify reduce/scan generics

Updates based on discussion at #369 (comment)

Author: benjchristensen

rxjava-core/src/main/java/rx/Observable.java

@@ -3193,7 +3193,7 @@ public Observable<T> aggregate(Func2<T, T, T> accumulator) {
      * @see <a href="http://msdn.microsoft.com/en-us/library/hh229154(v%3Dvs.103).aspx">MSDN: Observable.Aggregate</a>
      * @see <a href="http://en.wikipedia.org/wiki/Fold_(higher-order_function)">Wikipedia: Fold (higher-order function)</a>
      */
-    public <R> Observable<R> reduce(R initialValue, Func2<? super R, ? super T, ? extends R> accumulator) {
+    public <R> Observable<R> reduce(R initialValue, Func2<R, ? super T, R> accumulator) {
         return create(OperationScan.scan(this, initialValue, accumulator)).takeLast(1);
     }
 
@@ -3204,7 +3204,7 @@ public <R> Observable<R> reduce(R initialValue, Func2<? super R, ? super T, ? ex
      * 
      * @see #reduce(Object, Func2)
      */
-    public <R> Observable<R> aggregate(R initialValue, Func2<? super R, ? super T, ? extends R> accumulator) {
+    public <R> Observable<R> aggregate(R initialValue, Func2<R, ? super T, R> accumulator) {
         return reduce(initialValue, accumulator);
     }
 
@@ -3288,7 +3288,7 @@ public Observable<T> sample(long period, TimeUnit unit, Scheduler scheduler) {
      * @return an Observable that emits the results of each call to the accumulator function
      * @see <a href="http://msdn.microsoft.com/en-us/library/hh211665(v%3Dvs.103).aspx">MSDN: Observable.Scan</a>
      */
-    public <R> Observable<R> scan(R initialValue, Func2<? super R, ? super T, ? extends R> accumulator) {
+    public <R> Observable<R> scan(R initialValue, Func2<R, ? super T, R> accumulator) {
         return create(OperationScan.scan(this, initialValue, accumulator));
     }
 

rxjava-core/src/main/java/rx/operators/OperationScan.java

@@ -55,7 +55,7 @@ public final class OperationScan {
      * @return An observable sequence whose elements are the result of accumulating the output from the list of Observables.
      * @see <a href="http://msdn.microsoft.com/en-us/library/hh212007%28v=vs.103%29.aspx">Observable.Scan(TSource, TAccumulate) Method (IObservable(TSource), TAccumulate, Func(TAccumulate, TSource, TAccumulate))</a>
      */
-    public static <T, R> OnSubscribeFunc<R> scan(Observable<? extends T> sequence, R initialValue, Func2<? super R, ? super T, ? extends R> accumulator) {
+    public static <T, R> OnSubscribeFunc<R> scan(Observable<? extends T> sequence, R initialValue, Func2<R, ? super T, R> accumulator) {
         return new Accumulator<T, R>(sequence, initialValue, accumulator);
     }
 
@@ -76,11 +76,11 @@ public static <T> OnSubscribeFunc<T> scan(Observable<? extends T> sequence, Func
 
     private static class AccuWithoutInitialValue<T> implements OnSubscribeFunc<T> {
         private final Observable<? extends T> sequence;
-        private final Func2<? super T, ? super T, ? extends T> accumulatorFunction;
+        private final Func2<T, T, T> accumulatorFunction;
 
         private AccumulatingObserver<T, T> accumulatingObserver;
 
-        private AccuWithoutInitialValue(Observable<? extends T> sequence, Func2<? super T, ? super T, ? extends T> accumulator) {
+        private AccuWithoutInitialValue(Observable<? extends T> sequence, Func2<T, T, T> accumulator) {
             this.sequence = sequence;
             this.accumulatorFunction = accumulator;
         }
@@ -116,9 +116,9 @@ public void onCompleted() {
     private static class Accumulator<T, R> implements OnSubscribeFunc<R> {
         private final Observable<? extends T> sequence;
         private final R initialValue;
-        private final Func2<? super R, ? super T, ? extends R> accumulatorFunction;
+        private final Func2<R, ? super T, R> accumulatorFunction;
 
-        private Accumulator(Observable<? extends T> sequence, R initialValue, Func2<? super R, ? super T, ? extends R> accumulator) {
+        private Accumulator(Observable<? extends T> sequence, R initialValue, Func2<R, ? super T, R> accumulator) {
             this.sequence = sequence;
             this.initialValue = initialValue;
             this.accumulatorFunction = accumulator;
@@ -133,11 +133,11 @@ public Subscription onSubscribe(final Observer<? super R> observer) {
 
     private static class AccumulatingObserver<T, R> implements Observer<T> {
         private final Observer<? super R> observer;
-        private final Func2<? super R, ? super T, ? extends R> accumulatorFunction;
+        private final Func2<R, ? super T, R> accumulatorFunction;
 
         private R acc;
 
-        private AccumulatingObserver(Observer<? super R> observer, R initialValue, Func2<? super R, ? super T, ? extends R> accumulator) {
+        private AccumulatingObserver(Observer<? super R> observer, R initialValue, Func2<R, ? super T, R> accumulator) {
             this.observer = observer;
             this.accumulatorFunction = accumulator;