[esarbe]

It's only a problem if you can't decide which part of your project to write with which language.

If I have to use Java, I put it into a separate sub-project and make sure that I have a nice API to interface between the subprojects. `.asJava` and `.asScala` then only appear at very specific places where the interop happens.

[joshlemer]

What you're describing is in and of itself a severe cost and barrier to the very thing we're describing: interoperability between Java and Scala collections. On the one hand you have Scala where you either have to constantly `.asScala` and `.asJava`, or go your route of ensuring that in any given module only deals with either Scala or Java collections. This may involve additional abstractions or classes to achieve. On the other hand you have Kotlin which just directly, frictionlessly deals with Kotlin/Java collections the same way (in fact, they literally are the same). We're comparing "some small-to-medium cost" against "zero-cost".

Or an other way of saying it is that your approach of walling off modules as either java-collection or scala-collection is kind of like saying "Java and Scala collections work well together, so long as you don't have to use them together too much. If you minimize how much they need to interoperate, the problem is not so bad."

[esarbe]

> Or an other way of saying it is that your approach of walling off modules as either java-collection or scala-collection is kind of like saying "Java and Scala collections work well together, so long as you don't have to use them together too much. If you minimize how much they need to interoperate, the problem is not so bad."

Yes, that's actually a very good way to rephrase it. That being said; I myself wouldn't limit that to just the collections; Java and Scala on the language level have excellent interop. For Scala-the-ecosystem the story is a bit different.

Java is not the primary target for most of the Scala libraries. Not even a secondary one, I dare say. (I'm not even sure how you would model a Java API for a library - say; Cats - that uses implicits for the heavy lifting.)

That of course stems for the fact that Scala makes use of concepts that have no correspondence in neither Java or Kotlin, so there will necessarily be something lost in translation.