Question

I wrote a test function to test my understanding of “return-from” in Lisp

(defun testp (lst)
  (mapc #'(lambda (x y)
            (if (null lst)
                (return-from testp t)))
        lst
        (cdr lst)))

I think the test (testp ‘nil) should return T but it returns NIL.
Could you please help my understanding of why it returns NIL?

Many thanks.

Answer 1

You call MAPC over two empty lists.

How should the LAMBDA function ever be used if the lists don’t have any elements to map over?

Btw., you can write ‘list’ instead of ‘lst’.

(defun testp (list)
  (mapc #'(lambda (x y)
            (if (null list)
                (return-from testp t)))
        list
        (cdr list)))