Guards and Records: Testing for Type

The BIF is_record/2 lets you check if something is a record of a particular type:

(defun process-person (p)
  (if (is_record p 'person)
    (do-person-things p)
    (io:format "That's not a person!~n")))

Or in a guard:

(defun process-person (p) (when (is_record p 'person))
  (do-person-things p))

This checks three things:

1. Is p a tuple?
2. Is the first element the atom person?
3. Is the tuple the right size?

If all three conditions hold, congratulations: you probably have a person record. Or an extremely coincidental tuple. The universe tends not to make such distinctions.