After a long time without working with Elixir, I was studying Clojure and did a implementation of flatten, so I did the same with Elixir.

flatten.ex

defmodule Flatten do
  def flatten(list, holding_list \\ [], flattened \\ [])

  def flatten([[h | t] | rest], holding_list, flattened) do
    flatten(h, [t, rest | holding_list], flattened)
  end

  def flatten([h | t], holding_list, flattened) do
    flatten(t, holding_list, [h | flattened])
  end

  def flatten([], [h | holding_list], flattened) do
    my_flatten(h, holding_list, flattened)
  end

  def flatten(x, [h | holding_list], flattened) do
    flatten(h, holding_list, [x | flattened])
  end

  def my_flatten([], [], flattened) do
    flattened
    |> Enum.reverse()
    |> IO.inspect(label: :flattened)
  end
end

ExUnit.start()

defmodule FlattenTest do
  use ExUnit.Case

  test "must flatten the list" do
    # require IEx
    # IEx.pry()
    assert Flatten.flatten([[1, 2], [3, [4, [5, 6]]]]) == [1, 2, 3, 4, 5, 6]
  end
end