feat(pipe): add pipe method ot Observable

Also adds type overloads for pipe and for compose

NOTE: For some reason TypeScript would not let me call compose(...operations) in the pipe method

spec/Observable-spec.ts

@@ -3,6 +3,7 @@ import * as sinon from 'sinon';
 import * as Rx from '../dist/cjs/Rx';
 import {TeardownLogic} from '../dist/cjs/Subscription';
 import marbleTestingSignature = require('./helpers/marble-testing'); // tslint:disable-line:no-require-imports
+import { map } from '../dist/cjs/operators';
 
 declare const { asDiagram, rxTestScheduler };
 declare const cold: typeof marbleTestingSignature.cold;
@@ -621,6 +622,34 @@ describe('Observable', () => {
       });
     });
   });
+
+  describe('pipe', () => {
+    it('should exist', () => {
+      const source = Observable.of('test');
+      expect(source.pipe).to.be.a('function');
+    });
+
+    it('should pipe multiple operations', (done) => {
+      Observable.of('test')
+        .pipe(
+          map((x: string) => x + x),
+          map((x: string) => x + '!!!')
+        )
+        .subscribe(
+          x => {
+            expect(x).to.equal('testtest!!!');
+          },
+          null,
+          done
+        );
+    });
+
+    it('should return the same observable if there are no arguments', () => {
+      const source = Observable.of('test');
+      const result = source.pipe();
+      expect(result).to.equal(source);
+    });
+  });
 });
 
 /** @test {Observable} */

src/Observable.ts

@@ -7,6 +7,8 @@ import { toSubscriber } from './util/toSubscriber';
 import { IfObservable } from './observable/IfObservable';
 import { ErrorObservable } from './observable/ErrorObservable';
 import { observable as Symbol_observable } from './symbol/observable';
+import { OperatorFunction } from './interfaces';
+import { compose } from './util/compose';
 
 export interface Subscribable<T> {
   subscribe(observerOrNext?: PartialObserver<T> | ((value: T) => void),
@@ -286,4 +288,43 @@ export class Observable<T> implements Subscribable<T> {
   [Symbol_observable]() {
     return this;
   }
+
+  /* tslint:disable:max-line-length */
+  pipe(): Observable<T>
+  pipe<A>(op1: OperatorFunction<T, A>): Observable<A>
+  pipe<A, B>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>): Observable<B>
+  pipe<A, B, C>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>): Observable<C>
+  pipe<A, B, C, D>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>): Observable<D>
+  pipe<A, B, C, D, E>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>): Observable<E>
+  pipe<A, B, C, D, E, F>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>): Observable<F>
+  pipe<A, B, C, D, E, F, G>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>): Observable<G>
+  pipe<A, B, C, D, E, F, G, H>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>, op8: OperatorFunction<G, H>): Observable<H>
+  pipe<A, B, C, D, E, F, G, H, I>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>, op8: OperatorFunction<G, H>, op9: OperatorFunction<H, I>): Observable<I>
+  /* tslint:enable:max-line-length */
+
+  /**
+   * Used to stitch together functional operators into a chain.
+   * @method pipe
+   * @return {Observable} the Observable result of all of the operators having
+   * been called in the order they were passed in.
+   *
+   * @example
+   *
+   * import { map, filter, scan } from 'rxjs/operators';
+   *
+   * Rx.Observable.interval(1000)
+   *   .pipe(
+   *     filter(x => x % 2 === 0),
+   *     map(x => x + x),
+   *     scan((acc, x) => acc + x)
+   *   )
+   *   .subscribe(x => console.log(x))
+   */
+  pipe<R>(...operations: OperatorFunction<T, R>[]): Observable<R> {
+    if (operations.length === 0) {
+      return this as any;
+    }
+
+    return compose.apply(this, operations)(this);
+  }
 }

src/util/compose.ts

@@ -1,6 +1,19 @@
 import { noop } from './noop';
 import { UnaryFunction } from '../interfaces';
 
+/* tslint:disable:max-line-length */
+export function compose<T>(): UnaryFunction<T, T>;
+export function compose<T, A>(op1: UnaryFunction<T, A>): UnaryFunction<T, A>;
+export function compose<T, A, B>(op1: UnaryFunction<T, A>, op2: UnaryFunction<A, B>): UnaryFunction<T, B>;
+export function compose<T, A, B, C>(op1: UnaryFunction<T, A>, op2: UnaryFunction<A, B>, op3: UnaryFunction<B, C>): UnaryFunction<T, C>;
+export function compose<T, A, B, C, D>(op1: UnaryFunction<T, A>, op2: UnaryFunction<A, B>, op3: UnaryFunction<B, C>, op4: UnaryFunction<C, D>): UnaryFunction<T, D>;
+export function compose<T, A, B, C, D, E>(op1: UnaryFunction<T, A>, op2: UnaryFunction<A, B>, op3: UnaryFunction<B, C>, op4: UnaryFunction<C, D>, op5: UnaryFunction<D, E>): UnaryFunction<T, E>;
+export function compose<T, A, B, C, D, E, F>(op1: UnaryFunction<T, A>, op2: UnaryFunction<A, B>, op3: UnaryFunction<B, C>, op4: UnaryFunction<C, D>, op5: UnaryFunction<D, E>, op6: UnaryFunction<E, F>): UnaryFunction<T, F>;
+export function compose<T, A, B, C, D, E, F, G>(op1: UnaryFunction<T, A>, op2: UnaryFunction<A, B>, op3: UnaryFunction<B, C>, op4: UnaryFunction<C, D>, op5: UnaryFunction<D, E>, op6: UnaryFunction<E, F>, op7: UnaryFunction<F, G>): UnaryFunction<T, G>;
+export function compose<T, A, B, C, D, E, F, G, H>(op1: UnaryFunction<T, A>, op2: UnaryFunction<A, B>, op3: UnaryFunction<B, C>, op4: UnaryFunction<C, D>, op5: UnaryFunction<D, E>, op6: UnaryFunction<E, F>, op7: UnaryFunction<F, G>, op8: UnaryFunction<G, H>): UnaryFunction<T, H>;
+export function compose<T, A, B, C, D, E, F, G, H, I>(op1: UnaryFunction<T, A>, op2: UnaryFunction<A, B>, op3: UnaryFunction<B, C>, op4: UnaryFunction<C, D>, op5: UnaryFunction<D, E>, op6: UnaryFunction<E, F>, op7: UnaryFunction<F, G>, op8: UnaryFunction<G, H>, op9: UnaryFunction<H, I>): UnaryFunction<T, I>;
+/* tslint:enable:max-line-length */
+
 export function compose<T, R>(...fns: Array<UnaryFunction<T, R>>): UnaryFunction<T, R> {
   if (!fns) {
     return noop as UnaryFunction<any, any>;