in Rust, I can define a

trait

(which roughly translated to a Kotlin

interface

), and then implement that

trait

for types which were defined elsewhere. can I do something similar in Kotlin? I know I can kind of simulate this with extension functions.

that wouldn't be compatible with the Java object model

Context receivers are aboslutely the way to go here. Here's an example of how they can be used:

interface Summable<T> { // similar to a trait Summable that is implemented for some type T
    operator fun T.plus(other: T)
    val zero: T
}

// You can even define methods that need T to be summable
context(Summable<T>)
fun <T> List<T>.sumAll(): T = fold(zero) { acc, t -> t + acc }

// implementing Summable for a type is trivial
object IntSummable: Summable<Int> {
    override val zero = 0
    override fun Int.plus(other: Int) = this + other
}

//usage is slightly ugly in that you have to explicitly bring the implementation of Summable<Int> into scope

with(IntSummable) {
    listOf(1, 2, 3, 4, 5).sumAll()
}

that… looks quite good. thanks a lot. I’ll give it a try and see how it works for my use case.