Mapcan

The Lisp function mapcan works much like mapcar, but with a different way of gathering results:

• The function called by mapcan returns a list of results (perhaps an empty list).
• mapcan concatenates the results; empty lists vanish.

(defun filter (lst predicate)
  (mapcan #'(lambda (item)
              (if (funcall predicate item)
                  (list item)
                  '()))
          lst) )

>(filter '(a 2 or 3 and 7) 'numberp)
          ()(2)()(3)() (7)
(2 3 7)

>(filter '(a 2  or 3  and 7) 'symbolp)
          (a)()(or)()(and)()
(A OR AND)