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

This popular question only has recursive solutions (not counting Rainer's answer).

Let's have a loop version:

(defun flatten (tree &aux todo flat)
  (check-type tree list)
  (loop
     (shiftf todo tree nil)
     (unless todo (return flat))
     (dolist (elt todo)
       (if (listp elt)
           (dolist (e elt)
             (push e tree))
           (push elt flat))))))