Rich Errors look promising to me, but what about interop with Java? What will the return-type of a function be on the Java side be, if the return type on Kotlin side is: Int | ParseError | SomeOtherError?

Background: unions aren't restricted to one normal type and one error type, but to one normal type and any number of error types, so this can't be modelled as syntactic sugar on top of an implicit Either/Result type, can it??

The Java interop compromise is probably the biggest weakness of the proposal - it works beautifully within Kotlin but degrades at boundaries. This is similar to how Kotlin's nullable types (String?) become platform types in Java.

I think it's good nonetheless to add stuff to Kotlin that won't translate 1:1 to Java, both because Java is evolving but also because Kotlin is used in "Native" (non-JVM) contexts as well (not extensively, but hopefully that'll change).

These remind me of checked exceptions in Java. Ironically, Kotlin removed checked exceptions because they tend to be annoying more than useful: there's no clear guideline to whether an exception is checked or unchecked, some functions like IO and reflection have them while others don't, they're verbose especially when closures are involved, and lots of functions simply catch and rethrow checked exceptions in unchecked exceptions.

Which leads me to: why is Kotlin implementing this in a non-JVM compatible way, instead of introducing checked exceptions with better language support? All the problems stated above can be avoided while keeping the core idea of checked exceptions, which seems to be the same as this proposal.

From the GitHub discussion, I see this comment (https://github.com/Kotlin/KEEP/discussions/447#discussioncom...):

> The difference between checked exceptions from java and error unions in this proposal is how they are treated. Checked exceptions are exceptions and always interrupt the flow of execution. On the other hand, errors in this proposal are values and can be passed around as values or intentionally ignored or even aggregated enabling the ability to use them in async and awaitAll etc.

But is this a real difference or something that can be emulated mostly syntactically and no deeper than Kotlin's other features (e.g. nullables, getters and setters)? Checked exceptions are also values, and errors can be caught (then ignored or aggregated) but usually interrupt the flow of execution and get propagated like exceptions.

Slated for release in 2.4 the coming summer. Track in https://youtrack.jetbrains.com/issue/KT-68296