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

note arguments length checking in prototype version

Author: benlesh

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);
+  }
+}