fix(merge): support union types

Allows xs.merge(streamOfNumbers, streamOfStrings) to return a Stream of numbers or strings, as the return type in TypeScript.

Modifis static `merge` method to implement a new interface:
MergeSignature. This interface is constructed similarly to the
CombineSignature interface. Export the new interface.

This change allows consumers invoking the static method to declare types
and leverage TypeScript to verify type safety when merging streams. The
change brings `merge` in line with other static methods which already
include this feature.

PR #82.

Should resolve #80.

src/core.ts

@@ -132,6 +132,37 @@ export class MergeProducer<T> implements Aggregator<T, T>, InternalListener<T> {
   }
 }
 
+export interface MergeSignature {
+  (): Stream<any>;
+  <T1>(s1: Stream<T1>): Stream<T1>;
+  <T1, T2>(
+    s1: Stream<T1>,
+    s2: Stream<T2>): Stream<T1 | T2>;
+  <T1, T2, T3>(
+    s1: Stream<T1>,
+    s2: Stream<T2>,
+    s3: Stream<T3>): Stream<T1 | T2 | T3>;
+  <T1, T2, T3, T4>(
+    s1: Stream<T1>,
+    s2: Stream<T2>,
+    s3: Stream<T3>,
+    s4: Stream<T4>): Stream<T1 | T2 | T3 | T4>;
+  <T1, T2, T3, T4, T5>(
+    s1: Stream<T1>,
+    s2: Stream<T2>,
+    s3: Stream<T3>,
+    s4: Stream<T4>,
+    s5: Stream<T5>): Stream<T1 | T2 | T3 | T4 | T5>;
+  <T1, T2, T3, T4, T5, T6>(
+    s1: Stream<T1>,
+    s2: Stream<T2>,
+    s3: Stream<T3>,
+    s4: Stream<T4>,
+    s5: Stream<T5>,
+    s6: Stream<T6>): Stream<T1 | T2 | T3 | T4 | T5 | T6>;
+  <T>(...stream: Array<Stream<T>>): Stream<T>;
+}
+
 export interface CombineSignature {
   (): Stream<Array<any>>;
   <T1>(s1: Stream<T1>): Stream<[T1]>;
@@ -1377,9 +1408,10 @@ export class Stream<T> implements InternalListener<T> {
    * or more streams may be given as arguments.
    * @return {Stream}
    */
-  static merge<T>(...streams: Array<Stream<T>>): Stream<T> {
-    return new Stream<T>(new MergeProducer(streams));
-  }
+  static merge: MergeSignature = <MergeSignature>
+    function merge(...streams: Array<Stream<any>>): Stream<any> {
+      return new Stream<any>(new MergeProducer(streams));
+    };
 
   /**
    * Combines multiple input streams together to return a stream whose events

tests/factory/merge.ts

@@ -75,4 +75,25 @@ describe('xs.merge', () => {
     assert.strictEqual(stream instanceof Stream, true);
     done();
   });
+
+  it('should have a correct TypeScript signature', () => {
+    const bools = xs.create<boolean>({
+      start: listener => {},
+      stop: () => {}
+    });
+
+    const numbers = xs.create<number>({
+      start: listener => {},
+      stop: () => {}
+    });
+
+    const strings = xs.create<string>({
+      start: listener => {},
+      stop: () => {}
+    });
+
+    const emptyIsh: Stream<boolean> = xs.merge<boolean>();
+    const doubled: Stream<boolean | string> = xs.merge(bools, strings);
+    const tripled: Stream<boolean | number | string> = xs.merge(bools, strings, numbers);
+  });
 });