flattener.clj

(defn flattener
  [input flattened?]
  (((fn [f] (f f))
    (fn [f]
      (fn [x xs]
        (if (flattened? x)
          (cons x ((f f) (first xs) (rest xs)))
          (if (empty? x)
            xs
            ((f f) (first x) ((f f) (rest x) xs)))))))
   (first input) (rest input)))

 (def tree [1 
            [2 3 
               [4 
                [[5 6] 8]]] 
            [9 4]
            [10 20]
            [[5]]])

(flattener tree integer?)
;;=> (1 2 3 4 5 6 8 9 4 10 20 5)

;;y-combinator