Imagine, if you can, that you need to capture data about your Customers

data class Customer(
    val id: String,
    val firstName: String,
    val lastName: String,
    val email: String,
    val addressLine1: String?,
    val addressLine2: String?,
    val addressLine3: String?,
    val postcode: String?,
    val iso2CountryCode: String,
    val phone: String?
)

Due to the power of Kotlin null safety, we can see that we are required to capture a firstName, lastName, email and iso2CountryCode, but that everything else is optional.

We dutifully write an HTML form with validation rules to ensure that we have what we need, post it off to some HTTP handler, and set to to write the code to persist our data class to some datastore -

interface Customers {
    fun save(customer: Customer)
    fun findById(id: String): Customer?
    fun gdpr() = TODO("2018-05-25")
}

If you’re ahead of me here, or just a run of the mill pedant, you’ll be saying, “But Duncan, what about that id?”

Ah yes. Identifiers are usually dished out by the datastore, so we don’t have one at the point that we create a Customer to save. In Java null would be a valid value for the id (as well as every other reference), but if we specify the id as nullable in Kotlin; every time we reference a Customer id we will have to explicitly allow for the null - it’s going to get tedious pretty quickly.

An alternative is to have a separate class to model the unsaved customer.

data class UnsavedCustomer(
    val firstName: String,
    val lastName: String,
    val email: String,
    val addressLine1: String?,
    val addressLine2: String?,
    val addressLine3: String?,
    val postcode: String?,
    val iso2CountryCode: String,
    val phone: String?
)

interface Customers {
    fun save(customer: UnsavedCustomer): Customer
    fun findById(id: String): Customer
}

That’s quite nice, because it shows that act of saving a customer is to turn an UnsavedCustomer into a (Saved)Customer. But the duplication between Customer and UnsavedCustomer is going to be a source of eternal shame.

Nothing? can save us

Let’s make the type of the id generic - constrained to String?

data class CustomerData<out T : String?>(
    val id: T,
    val firstName: String,
    val lastName: String,
    val email: String,
    val addressLine1: String? = null,
    val addressLine2: String? = null,
    val addressLine3: String? = null,
    val postcode: String? = null,
    val iso2CountryCode: String,
    val phone: String? = null
)

So our id must be compatible with nullable String.

A saved Customer has an id, so we can tighten String? to String in this case.

typealias Customer = CustomerData<String>

val existingCustomer = Customer("123", "Fred", "Flintstone", "fred@bedrock.org", iso2CountryCode = "US")

Now a customer’s id must be non-null, which makes our life simple

val thisIsFine: String = existingCustomer.id

val doesntCompile = Customer(null, "Fred", "Flintstone", "fred@bedrock.org", iso2CountryCode = "US")

We want our unsaved customers have a null id. The type of null is Nothing?, which is helpfully substitutable for String?

typealias UnsavedCustomer = CustomerData<Nothing?>

val newCustomer = UnsavedCustomer(null, "Fred", "Flintstone", "fred@bedrock.org", iso2CountryCode = "US")

You can’t use a String where you expect a Nothing? though, so null is the only acceptable value of an UnsavedCustomer id

val typeMismatch = UnsavedCustomer("123", "Fred", "Flintstone", "fred@bedrock.org", iso2CountryCode = "US")
    // Required: Nothing? Found: String

val typeMismatch2: String = newCustomer.id
    // Required: String Found: Nothing?

Because of this you can’t use an UnsavedCustomer where you expected a saved one, or vice versa

val typeMismatch3: Customer = newCustomer
val typeMismatch4: UnsavedCustomer = existingCustomer

although if you want to treat them alike you can define AnyCustomer

typealias AnyCustomer = CustomerData<*>

val ok: AnyCustomer = newCustomer
val okToo: AnyCustomer = existingCustomer

val AnyCustomer.isSaved get() = id != null

To convert an Unsaved to a saved customer, supply the missing id

@Suppress("UNCHECKED_CAST")
fun UnsavedCustomer.saved(id: String) = (this as Customer).copy(id)

Full-disclosure - I thought this trick up a while ago, but we’ve only just put it into some production code, so I don’t know how it will turn out in practice. I’ll report back if there turn out to be any wrinkles that I haven’t spotted.