(import std.List :flatten :map)

(let data [[1] 2 [[3 4] 5] [[[]]] [[[6]]] 7 8 []])

(let process (fun (lst)
  (flatten
    (map
      lst
      (fun (sub)
        (if (= "List" (type sub))
          (process sub)
          sub))))))
(assert (= (process data) [1 2 3 4 5 6 7 8]) "list is flat")