Inline Tiny Types With Validation

It’s been a long time since my last post. This is largely because my writing efforts have been directed towards a book! Nat Pryce (co-author of the excellent GOOS) and I are hard at work on Java to Kotlin, A Refactoring Guidebook, due to be published in 2021 by O’Reilly. You can read our work in progress on O’Reilly Online Learning.

Nat and I are both fans of tiny-types. The basic idea here is to have a separate type for individual domain concepts rather than just using (usually) Strings. So we might define GivenName and FamilyName types so that a Customer can be

data class Customer(
    val id: CustomerId,
    val givenName: GivenName,
    val familyName: FamilyName

Compared to raw strings, we can’t mix up the order of our given and family names when we create a customer, nor can we accidentally pass an OrderId where we need a CustomerId.

Here’s a typical tiny type in Kotlin:

data class GivenName(val value: String)

As you can imagine, tiny types are a bit of a pain in Java, where we have to define equals, hashCode and toString by hand, but Kotlin helps them make economic sense. For bonus points, we can even have our types behave like a CharSequence (the closest interface to String.

data class FamilyName(val value: String): CharSequence by value

This lets us write, for example


because isNotBlank is helpfully defined on CharSequence not String.

On the subject of non-blank-ness, it would be really helpful if we were prevented from creating a blank CustomerId. That would go a long way to avoiding the errors we get when functions are partial - only defined on some of their inputs (see Failure is not an Option, Part 7 - Avoiding Failure).

We can prevent the creation of an invalid CustomerId by checking the value in an init block.

data class CustomerId(val value: String): CharSequence by value {
    init {

Now we may get an error when we try to create a CustomerId, but if we have an instance, we can rely on the fact that it isn’t blank.

Given how easy they are to write in Kotlin, about the only drawback with tiny types is we actually end up with 2 objects to represent one value. That’s the initial String, and the data class wrapper around the string. The extra object probably consumes an extra 16 bytes these days, which is quite an overhead if we have a lot of wrapped strings. Potentially worse is the issue of cache coherency, because the initial string was probably created in the bowels of some JDBC code and may end up a long way in memory from its wrapper. So calling a method on a tiny type reference may involve fetching it into cache, and then the value that it is wrapping.

Kotlin Inline Classes, whilst still experimental, are aimed at removing this overhead.

inline class GivenName(val value: String)

In most circumstances the compiler arranges for instances of inline classes to be passed around as just the single contained value, so that we don’t have the overhead of creating an extra wrapper instance. This should be perfect for tiny types.

There’s one major problem though, we can’t easily validate inline classes.

inline class CustomerId(val value: String) {
    init { // Error: Inline class cannot have an initializer block

init blocks are disallowed because Java code is permitted to call Kotlin methods that take inline classes, passing the base type (String above). In this case the init block won’t have been called, so users of inline classes can’t know that validation will have been performed. So init blocks are disallowed.

For the same reason the primary constructor must be public. In fact, every route to validating an inline class is deliberately blocked because it might not have been taken when called from Java.

If you have a pure-Kotlin codebase, or just only ever call down into Java rather than back into Kotlin, this is a shame. It would be really nice to use inline classes to implement validated tiny types. Can we find a way to make this work?

What we need to do is either make sure that calling CustomerId("") throws an exception, or prevent callers from calling the CustomerId(val value: String) at all, instead making them go through some factory which can perform pre-validation.

We can’t throw an exception from the constructor because we can’t have an init block, but there is a sneaky way to achieve the second, at least for pure Kotlin code - make the value another inline class.

inline class CustomerId(private val __value: __DontMakeMe) {
    val value get(): String = __value.value

inline class __DontMakeMe(val value: String)

Now, in Kotlin, we can’t call the constructor with a String:

val id = CustomerId("42") // Type mismatch: inferred type is String but __DontMakeMe was expected

We can still call it, but there’s enough warning that we are subverting things:

val id = CustomerId(__DontMakeMe("42"))

Now we can create a factory function to validate a String and create a CustomerId with it.

fun customerIdOf(s: String): CustomerId? =
    when {
        s.isNotBlank() -> CustomerId(__DontMakeMe(s))
        else -> null

Looking at the bytecode, we end up calling a static ‘constructor’ function for both CustomerId and __DontMakeMe here, but both just check that their parameter is not null.

I’ve chosen to return a nullable CustomerId? from customerIdOf so that callers know the call might fail, rather than having to read the code or documentation to see that it might throw an exception.

Inline classes can implement interfaces, although not through delegation at present.

inline class FamilyName(private val __value: __DontMakeMe): CharSequence {
    val value get(): String = __value.value

    override val length get() = value.length

    override fun get(index: Int) = value.get(index)

    override fun subSequence(startIndex: Int, endIndex: Int) =
        value.subSequence(startIndex, endIndex)

fun familyNameOf(s: String): FamilyName? =
    when {
        s.isNotBlank() -> FamilyName(__DontMakeMe(s))
        else -> null

We can also define toString in order to mask sensitive information

inline class CardNumber(private val __value: __DontMakeMe) {
    val value get(): String = __value.value

    override fun toString() = "************" + value.substring(12)

fun cardNumberOf(s: String): CardNumber? =
    when {
        s.length == 16 && s.all(Char::isDigit) -> CardNumber(__DontMakeMe(s))
        else -> null

As I’ve said, this is a subversion of the inline class initialisation rules and won’t be safe if you accept calls in from Java. I’m also not sure how marshalling technologies like Jackson or kotlinx.serialisation will interact with the technique. For pure Kotlin codebases though I think it’s worth a try, as tiny types can significantly improve the readability and safety of our code.

Come back soon, because I’ll present a tiny tiny types framework using phantom types to reduce the amount of boilerplate code required.

Postscript - there is some discussion of other ways to achieve validation on the reddit discussion.

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