placed the redux-utils goop in a different directory

playground/src/redux-utils/reducer.ts

@@ -0,0 +1,33 @@
+import { Reducer } from 'redux';
+import { Draft, produce } from 'immer';
+
+export type ImmerReducer<T, A> = (state: Draft<T>, action?: A) => T;
+export type HandlerMap<T, AT extends string, Lookup extends { [t in AT]: any }> = {
+  [actionType in AT]?: ImmerReducer<T, Lookup[actionType]>
+};
+
+function isHandlerFunction<T, A, AT extends string, Lookup extends { [t in AT]: any }>(
+  handlerOrMap: HandlerMap<T, AT, Lookup> | ImmerReducer<T, A>
+): handlerOrMap is ImmerReducer<T, A> {
+  if (typeof handlerOrMap === 'function') {
+    return true;
+  }
+
+  return false;
+}
+
+export function createGenericReducer<T, AT extends string, Lookup extends { [t in AT]: any }>(
+  initialState: T,
+  handlerOrMap: HandlerMap<T, AT, Lookup> | ImmerReducer<T, Lookup[AT]>
+): Reducer<T> {
+  return function reducer(state = initialState, action: Lookup[AT]): T {
+    if (isHandlerFunction(handlerOrMap)) {
+      return produce(state, draft => handlerOrMap(draft, action as Lookup[AT]));
+    } else if (handlerOrMap.hasOwnProperty(action.type)) {
+      const handler = (handlerOrMap as any)[action.type] as ImmerReducer<T, Lookup[AT]>;
+      return produce(state, draft => handler(draft, action));
+    } else {
+      return state;
+    }
+  };
+}