Kotlin's standard collection are "immutable collections" only in the sense that they are read-only. Adding an element to a Kotlin immutable set copies the whole

Kotlin's standard collection are "immutable collections" only in the sense that they are read-only. Adding an element to a Kotlin immutable set copies the whole thing, https://github.com/JetBrains/kotlin/blob/924c28507067cbfbf78a6509ea89eabe496e34ca/libraries/stdlib/common/src/generated/_Sets.kt#L84 https://github.com/JetBrains/kotlin/blob/924c28507067cbfbf78... This is not that different from Java unmodifiable. In contrast the Scala immutable Set supports constant time addition, https://docs.scala-lang.org/overviews/collections-2.13/performance-characteristics.html https://docs.scala-lang.org/overviews/collections-2.13/perfo... This isn't just an ivory tower concern either. Because Kotlin immutables aren't actually immutable (they just don't expose mutation interfaces) this means that if I get a ref to an immutable Set, I can't modify it but I can't rely on it not changing because it's still back by a mutable one.