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 is a accumulator based approach. The local function %flatten keeps an accumulator of the tail (the right part of the list that's already been flattened). When the part remaining to be flattened (the left part of the list) is empty, it returns the tail. When the part to be flattened is a non-list, it returns that part prefixed onto the tail. When the part to be flattened is a list, it flattens the rest of the list (with the current tail), then uses that result as the tail for flattening the first part of the list.

(defun flatten (list)
  (labels ((%flatten (list tail)
             (cond
               ((null list) tail)
               ((atom list) (list* list tail))
               (t (%flatten (first list)
                            (%flatten (rest list)
                                      tail))))))
    (%flatten list '())))

CL-USER> (flatten '((1 2) (3 4) ((5) 6) 7))
(1 2 3 4 5 6 7)