public class AppStateReducer implements Reducer<AppState> {

    private Reducer<List<TodoItem>> itemsReducer = TodoReducer.create();
    private Reducer<TodoFilter> filterReducer = FilterReducer.create();

    @Override
    public AppState reduce(AppState state, Action action) {
        List<TodoItem> items = itemsReducer.reduce(state.todoItems, action);
        TodoFilter filter = filterReducer.reduce(state.filter, action);
        return new AppState(items, filter);
    }
}