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)

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

(A OR AND)

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