feat(scan): add higher-order lettable version of scan

note arguments length checking in prototype version

src/operator/scan.ts

@@ -1,6 +1,6 @@
-import { Operator } from '../Operator';
+
 import { Observable } from '../Observable';
-import { Subscriber } from '../Subscriber';
+import { scan as higherOrderScan } from '../operators';
 
 /* tslint:disable:max-line-length */
 export function scan<T>(this: Observable<T>, accumulator: (acc: T, value: T, index: number) => T, seed?: T): Observable<T>;
@@ -46,67 +46,8 @@ export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T,
  * @owner Observable
  */
 export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed?: T | R): Observable<R> {
- let hasSeed = false;
- // providing a seed of `undefined` *should* be valid and trigger
- // hasSeed! so don't use `seed !== undefined` checks!
- // For this reason, we have to check it here at the original call site
- // otherwise inside Operator/Subscriber we won't know if `undefined`
- // means they didn't provide anything or if they literally provided `undefined`
  if (arguments.length >= 2) {
- hasSeed = true;
- }
-
- return this.lift(new ScanOperator(accumulator, seed, hasSeed));
-}
-
-class ScanOperator<T, R> implements Operator<T, R> {
- constructor(private accumulator: (acc: R, value: T, index: number) => R, private seed?: T | R, private hasSeed: boolean = false) {}
-
- call(subscriber: Subscriber<R>, source: any): any {
- return source.subscribe(new ScanSubscriber(subscriber, this.accumulator, this.seed, this.hasSeed));
- }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-class ScanSubscriber<T, R> extends Subscriber<T> {
- private index: number = 0;
-
- get seed(): T | R {
- return this._seed;
- }
-
- set seed(value: T | R) {
- this.hasSeed = true;
- this._seed = value;
- }
-
- constructor(destination: Subscriber<R>, private accumulator: (acc: R, value: T, index: number) => R, private _seed: T | R,
- private hasSeed: boolean) {
- super(destination);
- }
-
- protected _next(value: T): void {
- if (!this.hasSeed) {
- this.seed = value;
- this.destination.next(value);
- } else {
- return this._tryNext(value);
- }
- }
-
- private _tryNext(value: T): void {
- const index = this.index++;
- let result: any;
- try {
- result = this.accumulator(<R>this.seed, value, index);
- } catch (err) {
- this.destination.error(err);
- }
- this.seed = result;
- this.destination.next(result);
+ return higherOrderScan(accumulator, seed)(this);
  }
+ return higherOrderScan(accumulator)(this);
 }

src/operators/index.ts

@@ -2,4 +2,5 @@ export { concatMap } from './concatMap';
 export { filter } from './filter';
 export { map } from './map';
 export { mergeMap } from './mergeMap';
+export { scan } from './scan';
 export { switchMap } from './switchMap';

src/operators/scan.ts

@@ -0,0 +1,115 @@
+import { Operator } from '../Operator';
+import { Observable } from '../Observable';
+import { Subscriber } from '../Subscriber';
+import { OperatorFunction } from '../interfaces';
+
+/* tslint:disable:max-line-length */
+export function scan<T>(accumulator: (acc: T, value: T, index: number) => T, seed?: T): OperatorFunction<T, T>;
+export function scan<T>(accumulator: (acc: T[], value: T, index: number) => T[], seed?: T[]): OperatorFunction<T, T[]>;
+export function scan<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed?: R): OperatorFunction<T, R>;
+/* tslint:enable:max-line-length */
+
+/**
+ * Applies an accumulator function over the source Observable, and returns each
+ * intermediate result, with an optional seed value.
+ *
+ * <span class="informal">It's like {@link reduce}, but emits the current
+ * accumulation whenever the source emits a value.</span>
+ *
+ * <img src="./img/scan.png" width="100%">
+ *
+ * Combines together all values emitted on the source, using an accumulator
+ * function that knows how to join a new source value into the accumulation from
+ * the past. Is similar to {@link reduce}, but emits the intermediate
+ * accumulations.
+ *
+ * Returns an Observable that applies a specified `accumulator` function to each
+ * item emitted by the source Observable. If a `seed` value is specified, then
+ * that value will be used as the initial value for the accumulator. If no seed
+ * value is specified, the first item of the source is used as the seed.
+ *
+ * @example <caption>Count the number of click events</caption>
+ * var clicks = Rx.Observable.fromEvent(document, 'click');
+ * var ones = clicks.mapTo(1);
+ * var seed = 0;
+ * var count = ones.scan((acc, one) => acc + one, seed);
+ * count.subscribe(x => console.log(x));
+ *
+ * @see {@link expand}
+ * @see {@link mergeScan}
+ * @see {@link reduce}
+ *
+ * @param {function(acc: R, value: T, index: number): R} accumulator
+ * The accumulator function called on each source value.
+ * @param {T|R} [seed] The initial accumulation value.
+ * @return {Observable<R>} An observable of the accumulated values.
+ * @method scan
+ * @owner Observable
+ */
+export function scan<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed?: T | R): OperatorFunction<T, R> {
+ let hasSeed = false;
+ // providing a seed of `undefined` *should* be valid and trigger
+ // hasSeed! so don't use `seed !== undefined` checks!
+ // For this reason, we have to check it here at the original call site
+ // otherwise inside Operator/Subscriber we won't know if `undefined`
+ // means they didn't provide anything or if they literally provided `undefined`
+ if (arguments.length >= 2) {
+ hasSeed = true;
+ }
+
+ return function scanOperatorFunction(source: Observable<T>): Observable<R> {
+ return source.lift(new ScanOperator(accumulator, seed, hasSeed));
+ };
+}
+
+class ScanOperator<T, R> implements Operator<T, R> {
+ constructor(private accumulator: (acc: R, value: T, index: number) => R, private seed?: T | R, private hasSeed: boolean = false) {}
+
+ call(subscriber: Subscriber<R>, source: any): any {
+ return source.subscribe(new ScanSubscriber(subscriber, this.accumulator, this.seed, this.hasSeed));
+ }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class ScanSubscriber<T, R> extends Subscriber<T> {
+ private index: number = 0;
+
+ get seed(): T | R {
+ return this._seed;
+ }
+
+ set seed(value: T | R) {
+ this.hasSeed = true;
+ this._seed = value;
+ }
+
+ constructor(destination: Subscriber<R>, private accumulator: (acc: R, value: T, index: number) => R, private _seed: T | R,
+ private hasSeed: boolean) {
+ super(destination);
+ }
+
+ protected _next(value: T): void {
+ if (!this.hasSeed) {
+ this.seed = value;
+ this.destination.next(value);
+ } else {
+ return this._tryNext(value);
+ }
+ }
+
+ private _tryNext(value: T): void {
+ const index = this.index++;
+ let result: any;
+ try {
+ result = this.accumulator(<R>this.seed, value, index);
+ } catch (err) {
+ this.destination.error(err);
+ }
+ this.seed = result;
+ this.destination.next(result);
+ }
+}