<https://kotlinlang.slack.com/archives/C0B8Q383C/p1493038673085550?thread_ts=1492976413.401231&amp;c...

# stdlib

[ilya.gorbunov]

https://kotlinlang.slack.com/archives/C0B8Q383C/p1493038673085550?thread_ts=1492976413.401231&amp;cid=C0B8Q383C
By the way, have you tried this one? https://github.com/Kotlin/kotlinx.collections.immutable
It might be not as good in terms of performance, since right now we're more interested in its API convenience feedback.

[mg6maciej]

Yes, I tried to replace my List usage with this, but the API is not convinient. I even told you a while ago what issues I had 🙂

[mg6maciej]

I don't think reusing kotlin.List will work. It needs to be a separate List interface like this one: https://github.com/vavr-io/vavr/blob/master/vavr/src/main/java/io/vavr/collection/List.java

[ilya.gorbunov]

I've interested to hear why it won't work. Do you have specific considerations against?

[mg6maciej]

Well, maybe it "might" work, but you need to provide implementations for all functions that are there in the stdlib for given supertype, including extension functions.

[mg6maciej]

I was trying to replace list with ImmutableList and then was hitting things like take or drop not defined on this interface.

[mg6maciej]

But the code there was compiling, just returning ordinary List.

[mg6maciej]

So I'd rather not put List as superinterface for ImmutableList and then users would clearly see what's possible and what's not.

[ilya.gorbunov]

That's a valid concern, I've stumbled upon it too, see the issue https://github.com/Kotlin/kotlinx.collections.immutable/issues/10
Though currently it doesn't look entirely impassable, I believe we can find some solution if stdlib operators could get know something about immutable collections.

[mg6maciej]

For me take on ImmutableList simply needs to return ImmutableList. There is no other option to connect it all with data class copy and friends.

[mg6maciej]

I don't want casts or toImmutableList() after every such call.

[mg6maciej]

Gotta go. Always happy to give me more feedback on immutable stuff.

[mg6maciej]

I have a lot of code that does this.copy(someList = this.someList + elem) or val (six, rest) = this.someList.splitAt(6); this.copy(someList = six), so all functions on ImmutableList should return the same type.