I'm not sure what you're asking. I was pointing out that if you need to enforce a return result that will have either a value or an error, you can simple write a struct/interface that does that.

Like, you can create an interface that's read only for the return result like:

interface ReadableResult { HasResult() bool Result() whatever Error() error }

Then to create the result you have a writable interface for setting values, and enforce the 'only one of the two' semantics from the setters:

interface WritableResult { SetResult(whatever) SetError(error) }

Struct MyResult{ err error result whatever }

.... Implementation here.

It's extra code, but it should be a rare necessity, at least in my experience. Res, err := getResult() always worked fine for me.