defmodule Foo do
  def flatten([]) do
    []
  end

  def flatten([x|xs]) when is_list(x) do
    flatten(x) ++ flatten(xs)
  end

  def flatten([x|xs]) do
    [x|flatten(xs)]
  end
end

IO.inspect Foo.flatten([[[0]], [1, [[2, 3, [[4]]]], [[[[[5]]], 6]]], [7], [[[[[[8]]]]]]])