adding a redux todo list to playground

playground/src/reducers/createReducer.ts

@@ -0,0 +1,15 @@
+import { Reducer } from 'redux';
+import { ActionTypes, TodoAction } from '../actions';
+
+export function createReducer<T>(
+  initialState: T,
+  handlers: { [actionType in ActionTypes]?: (state: T, action: TodoAction) => T }
+): Reducer<T> {
+  return function reducer(state = initialState, action: TodoAction): T {
+    if (handlers.hasOwnProperty(action.type)) {
+      return handlers[action.type](state, action);
+    } else {
+      return state;
+    }
+  };
+}

playground/src/reducers/index.ts

@@ -0,0 +1,22 @@
+import { createReducer } from './createReducer';
+import { Store, FilterTypes } from '../store';
+import { combineReducers } from 'redux';
+
+let counter = 0;
+
+export const reducer = combineReducers<Store>({
+  todos: createReducer(
+    {},
+    {
+      add(state, action) {
+        const id = String(counter++);
+        return { ...state, [id]: { label: action.label, completed: false } };
+      }
+    }
+  ),
+  filter: createReducer<FilterTypes>('all', {
+    filter(state, action) {
+      return action.filter;
+    }
+  })
+});