Pop quiz: Anyone know why this fails to compile (a...

# getting-started

[Daniel Pitts]

Pop quiz: Anyone know why this fails to compile (at least on JVM):

@JvmInline
value class Namespace(val uri: String) {
}
open class WithNamespace(val namespace: Namespace) {
    constructor(url: String) : this(Namespace(url))
}

Spoiler: What type is Namespace on the JVM?

[Youssef Shoaib [MOD]]

Conflicting signature for the constructor!

[Daniel Pitts]

Exactly! I was only using the String overload, so I just removed the other constructor, but I'm wondering if there is a way to exclude the secondary from the JVM.

[Youssef Shoaib [MOD]]

I mean, a factory function might be good enough:

fun WithNamespace(url: String) = WithNamespace(Namespace(url))

[Daniel Pitts]

The expected usage is:

data object CalDav : WithNamespace("urn:ietf:params:xml:ns:caldav") {...}

[Daniel Pitts]

I did get this to work, but its slightly more wordy:
``````

[Youssef Shoaib [MOD]]

Oh yeah you could also do: constructor(url: String, unit: Unit = Unit) : this(Namespace(url))

[Daniel Pitts]

interface Namespaced {
    val namespace: Namespace
}

@JvmInline
value class Namespace(val uri: String) : Namespaced {
    override val namespace: Namespace get() = this
    fun name(name: String) = Name(name, this)
}

data object CalDav : Namespaced by Namespace("urn:ietf:params:xml:ns:caldav") {

[Daniel Pitts]

Yeah, thought about the unit: Unit . I think this is better though.

[Youssef Shoaib [MOD]]

The interface solution is definitely way better tho.
For the original, you could also do this for maximum compatibility:

open class WithNamespace(val namespace: Namespace, val unit: Unit = Unit) {
    constructor(url: String) : this(Namespace(url))
}

Both constructors are accessible in Kotlin. The main constructor is (probably) marked synthetic or something, and the secondary constructor is accessible from Java.

[ephemient]

https://youtrack.jetbrains.com/issue/KT-56424

[Klitos Kyriacou]

The whole point of having a value class is to give you stronger typing, so you don't accidentally pass a plain String when you should be passing a Namespace. Why do you want to circumvent this by introducing a wrapper function/constructor?

[Daniel Pitts]

This is a convenience for declaring the namespace values in the first place. In my example above, the CalDav object is the “source of truth” for this particular Namespace.