Quick "getting started" type question - is this a valid definition of a data class? `data class Name...

# getting-started

[v79]

Quick "getting started" type question - is this a valid definition of a data class? data class Name(val title: String, val lowerTitle = title.lowercase()) ? I.e. a default value dependent on the value of another field? It seems to compile...

[Jacob]

nitpicking on the example itself, A class like this would let me create a Name("My Title", "TOTALLY NOT MY TITLE Or LOWER")

[Jacob]

moving lowerTitle into a val outside the constructor is probably what you really want. The only downside is that it won't be a part of the toString method

[v79]

In real code, it's not just lowercase. And it needs to be in the constructor because it's a Serialized class. It's working but I'm not sure it's a good idea, as you say, could be open to abuse.

[Jacob]

also keep in mind calling .copy will break the relationship between these properties

[v79]

They are allowed to be unrelated in my usecase, it's more about providing a calculated default value if the "lowerTitle" isn't supplied. Though ideally I'd want some sort of contract in place to ensure the value is valid... The true class is:

@Serializable
data class PostMetadata(val title: String, val template: String = "post", val slug: String = title.toSlug(), val date: LocalDate)
// and String extension function 'toSlug()'...
fun String.toSlug() : String {
    val reserved = "[;/?:@&=+\$, ]"
    return this.replace(Regex(pattern = reserved),"-").lowercase()
}

I do feel I'm abusing power here.

[Rob Elliot]

I think it's fine - but I'd define a Slug class (as a value class if that works OK for you) and enforce the invariant in its constructor.

Untitled.kt

[v79]

Thanks @Rob Elliot - I have yet to learn value classes (though I'm suddenly, pointlessly, exploring context receivers when I should be focusing on more important things!)

[Rob Elliot]

Doesn't have to be a value class, could be a plain old data class with one value. The important bit is that by having a checked type that enforces the invariant you ensure that you can't end up with an invalid slug.

[Rob Elliot]

A value class is just a way to get that without actually boxing the string in another object in memory. Basically value class Slug only exists at compile time, at runtime it will just be a String and the constructor will just be a static method that calls require(slug.matches(legitimate)).

[Rob Elliot]

(That's not quite the whole story - there are times when the compiler has to box the value in a real Slug instance at runtime - but I can't remember when that happens...)