factory-function-final-approach-main.kt

fun main() {
    val contact = Contact(PhoneNumber("12345"))
    show(contact.phoneNumber) // "Phone number: 12345"

    val contactWithInvalidPhoneNumber = Contact(PhoneNumber(""))
    show(contactWithInvalidPhoneNumber.phoneNumber) // prints nothing
}

private fun show(phoneNumber: PhoneNumber) {
    if (phoneNumber is ValidPhoneNumber) {
        val phoneNumberScreen = PhoneNumberScreen(phoneNumber)
        phoneNumberScreen.render()
    }
}