Scheme Programming (R5RS) or Dialect of LISP // Functional Programming

can be done in DrRacket.

;;; Function flatten: nested list (arbitrary nesting level) -> flat list

;;; Return a flat list that contains all the elements in the original list

For example: >>> (flatten '(3 4) (5 6 (7) "hi")) -> (3 4 5 6 7 "hi")

What I got so far is wrong:

(define (flatten lst) (if (lst? car) (flatten (car lst) (flatten (cdr lst) (flatten lst)))))