export function formReducer(state: IFormState = getDefaultFormState(), action: Action): IFormState {
    switch(action.type) {
        ...
        case COUNTRIES_LOADED: {
            const typedAction = <ICountriesLoadedAction>action;
            return { ...state, countries: typedAction.payload.countries };
        }
        case FORM_COUNTRY_CHANGED: {
            const typedAction = <IFormCountryChangedAction>action;
            return { ...state, country: typedAction.payload.country, isDirty: true };
        }
        case CITIES_FOR_COUNTRY_LOADED: {
            const typedAction = <ICitiesForCountryLoadedAction>action;
            return { ...state, cities: typedAction.payload.cities };
        }
        case FORM_CITY_CHANGED: {
            const typedAction = <IFormCityChangedAction>action;
            return { ...state, city: typedAction.payload.city, isDirty: true };
        }
        default: {
            return state;
        }
    }
}