callback implementation to fetch user orders

user_orders_callback.kt

fun showUserOrders(username: String, password: String) {
    login(username, password) {
        user -> fetchUserOrders(user.userId) {
            orders -> showUserOrders(orders)
        }
    }
}

fun login(username: String, password: String, callback: (User) -> Unit)

fun fetchUserOrders(userId: Long, callback: (List<Orders>) -> Unit)