How to remove nested parentheses in LISP

Question

How can I remove nested parentheses recursively in Common LISP Such as

  (unnest \'(a b c (d e) ((f) g))) => (a b c d e f g)
  (unnest \'(a b))                    


        

Answer 1

Most of the answers have already mentioned a recursive solution to the Flatten problem. Using Common Lisp Object System's multiple dispatching you could solve the problem recursively by defining 3 methods for 3 possible scenarios:

(defmethod flatten ((tree null))
  "Tree is empty list."
  ())
(defmethod flatten ((tree list))
  "Tree is a list."
  (append (flatten (car tree))
          (flatten (cdr tree))))
(defmethod flatten (tree)
  "Tree is something else (atom?)."
  (list tree))

(flatten '(2 ((8) 2 (9 (d (s (((((a))))))))))) ; => (2 8 2 9 D S A)