flatten.ex

defmodule Flatten do
  @moduledoc """
  Documentation for Flatten.
  """

  @doc """
  Flatten an array. Ignore nil and empty list values

  ## Examples

      iex> Flatten.flatten([1, 2, [3], [4, 5, [6, 7, 8, [9, 10]]]])
      [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

  """

  @spec flatten(list(integer)) :: list(integer)
  def flatten(list), do: flatten(list, [])

  defp flatten([head | tail], acc) when is_nil(head) or head == [], do: flatten(tail, acc)
  defp flatten([head | tail], acc) when is_list(head), do: flatten(head, flatten(tail, acc))
  defp flatten([head | tail], acc), do: [head | flatten(tail, acc)]
  defp flatten([], acc), do: acc
end

flatten_test.exs

defmodule FlattenTest do
  use ExUnit.Case
  doctest Flatten

  test "flattens array" do
    assert Flatten.flatten([[1, 2, [3]], 4]) == Enum.to_list(1..4)
  end

  test "empty list returns empty list" do
    assert Flatten.flatten([]) == []
  end

  test "ignores empty list and nil values" do
    assert Flatten.flatten([nil, [], [], nil, 1]) == [1]
  end
end