Flatten a List in Elixir (Custom Implementation)

flatten.exs

defmodule Flatten do

  @doc """
  Flattens a List in Elixir

  ## Examples

  ```
  Flatten.flatten([1,2,3,4])
  # => [1,2,3,4]

  Flatten.flatten([1,[2,3],4])
  # => [1,2,3,4]

  Flatten.flatten([[1, []], [2, [3,4], [[5]]], [[]], 6, [[], 7]])
  # => [1,2,3,4,5,6,7]
  ```
  """
  @spec flatten(List.t) :: List.t
  def flatten(list) do
    list
    |> Enum.reduce([], &do_flatten/2)
    |> Enum.reverse
  end



  # Private Helper
  # Recursively flatten all items in reverse
  # (Reverse the entire list at the end in the calling method)

  defp do_flatten(nested, acc) when is_list(nested) do
    Enum.reduce(nested, acc, &do_flatten/2)
  end

  defp do_flatten(item, acc) do
    [ item | acc ]
  end

end

flatten_test.exs

ExUnit.start

defmodule FlattenTest do
  use ExUnit.Case, async: true

  @moduledoc """
  To run tests directly:

      $ elixir -r flatten.exs flatten_test.exs
  """

  describe "#flatten" do
    test "it works for empty lists" do
      assert Flatten.flatten([]) == []
    end

    test "it works for lists without nesting" do
      assert Flatten.flatten([1,2,3,4,5]) == [1,2,3,4,5]
    end

    test "works for lists with one-level nesting" do
      assert  Flatten.flatten([1,[2,3],4]) == [1,2,3,4]
    end

    test "it works for lists with multi-level nesting" do
      assert [1,2,3,4,5,6,7] ==
        Flatten.flatten([[1, []], [2, [3,4], [[5]]], [[]], 6, [[], 7]])
    end
  end

end