I mean, you may end up just wanting something like,

    type UsernameError struct {
      name   string
      reason string
    }
    func (e *UsernameError) Error() string { 
      return fmt.Errorf("invalid username %q: %s", e.name, e.reason)
    }

And reason can be "username cannot be empty" or "username may not contain '<'" or something like that.

This is fine for lots of different cases, because it’s likely that your code wants to know how to handle “username is invalid”, but only humans care about why.

I have personally never seen a Go codebase where you parse error strings. I know that people keep complaining about it so it must be happening out there—but every codebase I’ve worked with either has error constants (an exported var set to some errors.New() value) or some kind of custom error type you can check. Or if it doesn’t have those things, I had no interest in parsing the errors.

I write mostly frontends. Sometimes the APIs I talk to give back beautiful English error messages - that I can't just show to the user, because they are using a different language most of the time. And I don't want to write logic that depends on that sentence, far too brittle.

Right—I think the “error code” here is going to be the error type, i.e., UsernameError, or some qualified version of that.

It’s not perfect, but software evolves through many imperfect stages as it gets better, and this is one such imperfect stage that your software may evolve through.

Including a human-readable version of the error is useful because the developers / operators will want to read through the logs for it. Sometimes that is where you stop, because not all errors from all backends will need to be localized.