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;
+    }
+  };
+}