Form validation callables combined with "required"

I had not expected the absence of "required" to end validation and call it successful. After all, in addition to mwhitney's scenario, there might be a field that is not required, but if supplied, must be numeric.

Validation succeeds only if the field is empty, it doesn't blindly trust fields without the "required" rule.

A "required_if" rule might be useful for mwhitney's scenario. I could use one right now as I'm having trouble associating a rule name with a callable rule.

If a rule name is supplied (per the third example in "Use anything as a rule") and the field is blank the validation does not run. If I don't supply the rule name it runs but the error message is not found and it returns "Unable to access an error message corresponding to blah, blah, blah..."