import { unionize, ofType } from 'unionize'

// Define a record mapping tag literals to value types
const Action = unionize({
  ADD_TODO:                ofType<{ id: string; text: string }>(),
  SET_VISIBILITY_FILTER:   ofType<'SHOW_ALL' | 'SHOW_ACTIVE' | 'SHOW_COMPLETED'>(),
  TOGGLE_TODO:             ofType<{ id: string }>(),
}, 'type', 'payload'); // optionally override tag and value property names

type Action = typeof Action._Union;

type Action =
  | { type: ADD_TODO; payload: { id: string; text: string } }
  | { type: SET_VISIBILITY_FILTER; payload: 'SHOW_ALL' | 'SHOW_ACTIVE' | 'SHOW_COMPLETED' }
  | { type: TOGGLE_TODO; payload: { id: string } }

store.dispatch(Action.ADD_TODO({ id: 'c819bbc1', text: 'Take out the trash' }));
store.dispatch(Action.SET_VISIBILITY_FILTER('SHOW_COMPLETED'));

const todosReducer = (state: Todo[] = [], action: Action) => Action.match({
  // handle cases as pure functions instead of switch statements
  ADD_TODO: ({ id, text }) => [
    ...state,
    { id, text, completed: false }
  ],
  TOGGLE_TODO: ({ id }) => state.map(todo =>
    todo.id === id
      ? {...todo, completed: !todo.completed}
      : todo
  )
}, () => state /* default; if not provided, cases must be exhaustive */)(action);

(action$: Observable<Action>) => action$
  .filter(Action.is.TOGGLE_TODO)
  // The type of the resulting observable is refined appropriately...
  .mergeMap(({ text }) => /*...*/)

const { id, text } = Action.as.ADD_TODO(someAction) // throws if someAction is not an ADD_TODO