[sevensor]

Making illegal states unrepresentable sounds like a great idea, and it is, but I see it getting applied without nuance. “Has multiple errors” can be a valid type. Instead of bailing immediately, you can collect all of the errors so that they can be reported all together rather than forcing the user to fix one error at a time.

[mh2266]

Is this not `Result<Whatever, List<Error>>`? There's nothing enforcing that the error side needs to be the value-based equivalent of a single instance of an Exception class.

The important part is not to expose a "String -> Whatever" function publicly.