#lang racket

(define foo (lambda (cond ls)

              (if (null? ls)

                  (list)

                  (if (list? (first ls))

                      (append (foo cond (first ls)) (foo cond (rest ls)))

                      ;else:

                      (if (cond (first ls))

                          (append (list (first ls))  (foo cond (rest ls)))

                          ;else:

                          (foo cond (rest ls))

                          )
                      )
              )
        )
)

(foo number? '(1 b (3 c 4) 3 f (3 j (4 4 (5 l))) 5 4 h 8))