https://youtrack.jetbrains.com/issue/KT-68296/Union-Types-fo...
https://harelang.org/tutorials/introduction#defining-new-err...
However, having an anonymous sum type isn’t a good solution here because there’s nowhere for you to add context to. It’s not useful for your final error to say “permission denied” without the context of a file name, or knowing that you were trying to open configuration file, etc.
Also, depending on your implementation, you either cannot allow multiple errors of the same underlying cause (e.g. two IO errors, one for writing and one for opening) because they are the same type or you have positional error tuples (error index 0 is write, index 1 is open). Neither of those is super ergonomic.
In all seriousness, anonymous sum types would be quite nice. It would make it easier to be precise about return values in situations where `Optional` and `Result` are not sufficient.
The problem with checked exceptions (well, one of the problems) is that the full set of error conditions a function might encounter is often _too_ detailed. Encoding them all into the type signature of every function can be a lot of work, and quite brittle to minor internal changes. So most code bases will still need a catch-call error type that gets bubbled up to a generic handler somewhere.