There are so many situations where an error payload is either mandatory for properly handling the error, or required to get somewhat decent error output... Sid

There are so many situations where an error payload is either mandatory for properly handling the error, or required to get somewhat decent error output... Side channels are not really a feasible implementation option. So you have to resort to maintaining the logical information on intermediate levels, returning a result sum type, or stick to good old C paradigms and use a output param. Which negates all value that error sets offer. I understand that it's not trivial to implement, but for me it's a sort of must to avoid ending up with messy APIs.