flatten.exs

defmodule ListFlatten do
  def flatten(list), do: flatten(list, []) |> Enum.reverse
  def flatten([h | t], acc) when h == [], do: flatten(t, acc)
  def flatten([h | t], acc) when is_list(h), do: flatten(t, flatten(h, acc))
  def flatten([h | t], acc), do: flatten(t, [h | acc])
  def flatten([], acc), do: acc
end

list_1 = [[1,2,[3]],4]
list_2 = [[1,5,[3]],4,[[[1,2,[3]],4]]]

IO.puts("Running #{inspect list_1}...")
IO.inspect(ListFlatten.flatten(list_1))

IO.puts("Running #{inspect list_2}...")
IO.inspect(ListFlatten.flatten(list_2))