Pass update function to store setup callbacks

This non-breaking change allows more complex store logic to be
implemented, such as a derived store that accumulates a history of
its parent store's values.

src/runtime/store/index.ts

@@ -13,7 +13,7 @@ export type Updater<T> = (value: T) => T;
 type Invalidator<T> = (value?: T) => void;
 
 /** Start and stop notification callbacks. */
-export type StartStopNotifier<T> = (set: Subscriber<T>) => Unsubscriber | void;
+export type StartStopNotifier<T> = (set: Subscriber<T>, update?: (fn: Updater<T>) => void) => Unsubscriber | void;
 
 /** Readable interface for subscribing. */
 export interface Readable<T> {
@@ -92,7 +92,7 @@ export function writable<T>(value?: T, start: StartStopNotifier<T> = noop): Writ
 		const subscriber: SubscribeInvalidateTuple<T> = [run, invalidate];
 		subscribers.add(subscriber);
 		if (subscribers.size === 1) {
-			stop = start(set) || noop;
+			stop = start(set, update) || noop;
 		}
 		run(value);
 
@@ -125,7 +125,7 @@ type StoresValues<T> = T extends Readable<infer U> ? U :
  */
 export function derived<S extends Stores, T>(
 	stores: S,
-	fn: (values: StoresValues<S>, set: (value: T) => void) => Unsubscriber | void,
+	fn: (values: StoresValues<S>, set: Subscriber<T>, update?: (fn: Updater<T>) => void) => Unsubscriber | void,
 	initial_value?: T
 ): Readable<T>;
 
@@ -163,7 +163,7 @@ export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Rea
 
 	const auto = fn.length < 2;
 
-	return readable(initial_value, (set) => {
+	return readable(initial_value, (set, update) => {
 		let inited = false;
 		const values = [];
 
@@ -175,7 +175,7 @@ export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Rea
 				return;
 			}
 			cleanup();
-			const result = fn(single ? values[0] : values, set);
+			const result = fn(single ? values[0] : values, set, update);
 			if (auto) {
 				set(result as T);
 			} else {

test/store/index.ts

@@ -128,6 +128,50 @@ describe('store', () => {
 			assert.deepEqual(values, [0, 1, 2]);
 		});
 
+		it('passes an optional update function', () => {
+			let running;
+			let tick;
+			let add;
+
+			const store = readable(undefined, (set, update) => {
+				tick = set;
+				running = true;
+				add = n => update(value => value + n);
+
+				set(0);
+
+				return () => {
+					tick = () => { };
+					add = _ => { };
+					running = false;
+				};
+			});
+
+			assert.ok(!running);
+
+			const values = [];
+
+			const unsubscribe = store.subscribe(value => {
+				values.push(value);
+			});
+
+			assert.ok(running);
+			tick(1);
+			tick(2);
+			add(3);
+			add(4);
+			tick(5);
+			add(6);
+
+			unsubscribe();
+
+			assert.ok(!running);
+			tick(7);
+			add(8);
+
+			assert.deepEqual(values, [0, 1, 2, 5, 9, 5, 11]);
+		});
+
 		it('creates an undefined readable store', () => {
 			const store = readable();
 			const values = [];
@@ -231,6 +275,39 @@ describe('store', () => {
 			assert.deepEqual(values, [0, 2, 4]);
 		});
 
+		it('passes optional set and update functions', () => {
+			const number = writable(1);
+			const evensAndSquaresOf4 = derived(number, (n, set, update) => {
+				if (n % 2 === 0) set(n);
+				if (n % 4 === 0) update(n => n * n);
+			}, 0);
+
+			const values = [];
+
+			const unsubscribe = evensAndSquaresOf4.subscribe(value => {
+				values.push(value);
+			});
+
+			number.set(2);
+			number.set(3);
+			number.set(4);
+			number.set(5);
+			number.set(6);
+			assert.deepEqual(values, [0, 2, 4, 16, 6]);
+
+			number.set(7);
+			number.set(8);
+			number.set(9);
+			number.set(10);
+			assert.deepEqual(values, [0, 2, 4, 16, 6, 8, 64, 10]);
+
+			unsubscribe();
+
+			number.set(11);
+			number.set(12);
+			assert.deepEqual(values, [0, 2, 4, 16, 6, 8, 64, 10]);
+		});
+
 		it('prevents glitches', () => {
 			const lastname = writable('Jekyll');
 			const firstname = derived(lastname, n => n === 'Jekyll' ? 'Henry' : 'Edward');