> No, that's not it. The same problem is true for other error-handling types, such as the "either" or "result" types. Languages without union types or a similar mechanism are unable to define the error types adhoc and force the developer to define them in advance, which is very unergonomic

We are saying the same thing, aren't we?

But it is not only about exceptions. Other effects have the same problem and you need a mechanism to define effect polymorphic functions.