Sure, it may work fine in some cases. And then in others it doesn't, eg.

data class Foo(var x: Int)

fun main(args: Array<String>) {
    val f = Foo(3)
    val s = mutableSetOf(f)
    f.x = 7
    println(s == setOf(Foo(7)))
}

on try.kotlinlang.org.

it works with

Set

on JVM but not on JavaScript 🙃

Aren't they both

HashSet

behind the scenes?

No

Well doesn't really matter even, the point is that it sometimes randomly fails.

👍

I'll spend some time finding a hashset - hashset equals that fails 😒imple_smile:

I once submitted a bug for java.util.HashMap because if you mutate the keys, you can't remove it with the iterator. I argued that if you iterate all the elements and call remove for each one, the map should be empty (assuming no concurrent stuff going on) no matter what. I guess it's similar to this - you shouldn't mutate elements, but to what extent should a collection function if it does happen? Bug was fixed - there was another reason - but still.

because the implementation is

public fun <T> setOf(element: T): Set<T> = java.util.Collections.singleton(element)

ah, makes sense

with this implementation it doesn’t have much choice

singleton

method name always annoys me