[darksaints]

No, it's not hard. In Scala, you are completely free to use those java collections and you can do it exactly how kotlin programmers do it. You want a java list, without any need to convert back and forth between Scala and Java? Import `java.util.List`. All of the same methods and iterators and expressions and constructs are still there. You get all of the lack of capabilities and grace that the java collections provide. Literally no different from using the same collections in Kotlin.

The problem is that those java collections suck in comparison to the Scala collections. So Scala programmers prefer to use Scala collections. Scala programmers would never willfully use java collections if they don't have to, and if they absolutely have to, they have minimal overhead conversions back and forth. The minimal conversion overhead is the price they're willing to pay to use better collections while maintaining java interoperability.

[joshlemer]

Except Kotlin enjoys the compatibility with Java, plus it outfits those java collections with extension methods and some compiler tricks to achieve all the same functionality as the Scala collections (in fact, I would say even more self-consistent and useably than in Scala). Just as in Scala, in Kotlin you can use functional transforations that Scala users are so accustomed to:

Kotlin:

    listOf(1,2,3,4)
        .map{i -> i + 1}
        .filter { it % 2 == 0 }
        .flatMap { listOf(it, it * 2, it * 3) }

Kotlin even has a similar take to Scala's views, which they call sequences:

    listOf(1,2,3,4)
        .asSequence()
        .map{i -> i + 1}
        .filter { it % 2 == 0 }
        .flatMap { listOf(it, it * 2, it * 3) }
        .toList()

Is this really so "lacking in capability and grace" compared to Scala? The only think missing is persistent immutable collections, which are implemented in kotlinx.

[valenterry]

> The only think missing is persistent immutable collections, which are implemented in kotlinx.

You said:

> You have to litter your whole code with `.asScala` and `.asJava`, the java collections don't work with for comprehensions, etc.

So how does kotlinx achieve zero-cost compatibility with Java collections without having something like `.asKotlin` and `.asJava`? Because otherwise there is no difference between Scala and Kotlin in this regards.

[joshlemer]

because the persistentList/Collection/etc in kotlinx implement the appropriate java collection interfaces:

    import java.util.List;

    public class Foo {
        public static void blah(List<Integer> list) {
        }
    }

Kotlin:

    import kotlinx.collections.immutable.persistentListOf

    fun main(): Unit {
        Foo.blah(persistentListOf(1,2,3))
    }

This compiles fine