Implement cut operator

README.md

@@ -52,6 +52,48 @@ Patronum had 3 breaking changes: 1) from `0.14` to `0.100`, 2) from `0.100` to `
 
 We have [migration guide](https://patronum.effector.dev/docs/migration-guide).
 
+## Cut
+
+[Method documentation & API](/src/cut)
+
+```ts
+import { createEvent } from 'effector';
+import { cut } from 'patronum/cut';
+
+type WSInitEvent =  { type: 'init'; key: string };
+type WSIncrementEvent =  { type: 'increment'; count: number; name: string };
+type WSResetEvent =   { type: 'reset'; name: string };
+type WSEvent =
+  | WSInitEvent
+  | WSIncrementEvent
+  | WSResetEvent
+
+export const websocketEventReceived = createEvent<WSEvent[]>();
+
+const { init, increment, reset, __ } = cut({
+  source: websocketEventReceived,
+  cases: {
+    init: (events) => events.filter((wsEvent: WSEvent): wsEvent is WSInitEvent => wsEvent.type === 'init'),
+    increment: (events) => events.filter((wsEvent: WSEvent): wsEvent is WSIncrementEvent => wsEvent.type === 'increment'),
+    reset: (events) => events.filter((wsEvent: WSEvent): wsEvent is WSResetEvent => wsEvent.type === 'reset'),
+  },
+});
+
+init.watch(initEvents => {
+  console.info(`inited for ${initEvents.length}`);
+});
+
+increment.watch(incrementEvents => {
+  console.info('should be incremented', incrementEvents.map(wsEvent => wsEvent.count).reduce((a, b) => a + b));
+});
+
+websocketEventReceived([{ type: 'increment', name: 'demo', count: 5 }, { type: 'increment', name: 'demo', count: 15 }]);
+// => inited for 0
+// => should be incremented 20
+```
+
+[Try it]()
+
 # Development
 
 You can review [CONTRIBUTING.md](./CONTRIBUTING.md)

src/cut/cut.fork.test.ts

@@ -0,0 +1,113 @@
+import 'regenerator-runtime/runtime';
+import { createDomain, fork, serialize, allSettled } from 'effector';
+
+import { cut } from './index';
+
+test('works in forked scope', async () => {
+  const app = createDomain();
+  const source = app.createEvent<{ first?: number; another?: boolean }>();
+  const out = cut({
+    source,
+    cases: {
+      first: (payload) => payload.first,
+    },
+  });
+
+  const $data = app.createStore(0);
+
+  $data
+    .on(out.first, (state, payload) => state + payload)
+    .on(out.__, (state, payload) => (payload ? -state : 0));
+
+  const scope = fork(app);
+
+  await allSettled(source, {
+    scope,
+    params: { first: 15 },
+  });
+  expect(serialize(scope)).toMatchInlineSnapshot(`
+    Object {
+      "-4r0u7g": 15,
+    }
+  `);
+
+  await allSettled(source, {
+    scope,
+    params: { another: true },
+  });
+  expect(serialize(scope)).toMatchInlineSnapshot(`
+    Object {
+      "-4r0u7g": -15,
+    }
+  `);
+});
+
+test('do not affect another fork', async () => {
+  const app = createDomain();
+  const source = app.createEvent<{ first?: number; another?: boolean }>();
+  const out = cut({
+    source,
+    cases: {
+      first: (payload) => payload.first,
+    },
+  });
+
+  const $data = app.createStore(0);
+
+  $data
+    .on(out.first, (state, payload) => state + payload)
+    .on(out.__, (state, payload) => (payload ? -state : 0));
+
+  const scopeA = fork(app);
+  const scopeB = fork(app);
+
+  await allSettled(source, {
+    scope: scopeA,
+    params: { first: 200 },
+  });
+  expect(serialize(scopeA)).toMatchInlineSnapshot(`
+    Object {
+      "-hjldon": 200,
+    }
+  `);
+
+  await allSettled(source, {
+    scope: scopeB,
+    params: { first: -5 },
+  });
+  expect(serialize(scopeB)).toMatchInlineSnapshot(`
+    Object {
+      "-hjldon": -5,
+    }
+  `);
+});
+
+test('do not affect original store value', async () => {
+  const app = createDomain();
+  const source = app.createEvent<{ first?: number; another?: boolean }>();
+  const out = cut({
+    source,
+    cases: {
+      first: (payload) => payload.first,
+    },
+  });
+
+  const $data = app.createStore(0);
+
+  $data
+    .on(out.first, (state, payload) => state + payload)
+    .on(out.__, (state, payload) => (payload ? -state : 0));
+
+  const scope = fork(app);
+
+  await allSettled(source, {
+    scope,
+    params: { first: 15 },
+  });
+  expect(serialize(scope)).toMatchInlineSnapshot(`
+    Object {
+      "-tv4arn": 15,
+    }
+  `);
+  expect($data.getState()).toBe($data.defaultState);
+});