Racket List elements within a list

Question

I have something that returns a list of elements, but sometimes the list of elements are also lists of elements.

Example of this would be:

(1 2 3 (4 5) (6 7 (8))) 

I can't seem to write a function that just converts it into list with just the elements.

(1 2 3 4 5 6 7 8)

Answer 1

Here's the answer that the original poster came up with, for the benefit of anyone happening upon this question:

(define (test expresssion)
   (cond ((empty? expresssion) null)
         ((not (list? expresssion)) (list expresssion))
         (else (append (test (first expresssion))
                       (test (rest expresssion))))))

(test '(A (B (C) D) A)) ; => '(A B C D A)