Example to flat an nested arrays in Elixir

flat_anything.exs

defmodule FlatAnything do
  def flatten([]), do: []
  def flatten(this_collection) when is_list(this_collection) do
    [head | tail] = this_collection
    flatten(head) ++ flatten(tail)
  end
  def flatten(element), do: [element]
end

flat_anything_test.exs

defmodule FlatAnythingTest do
  use ExUnit.Case
  doctest FlatAnything

  test "that x became [x]" do
    assert FlatAnything.flatten(1) == [1]
  end

  test "that x as string became [x] with string" do
    assert FlatAnything.flatten("a") == ["a"]
  end

  test "that [x] is flattened to [x]" do
    assert FlatAnything.flatten([1]) == [1]
  end

  test "that [x, y, z] is flattened to [x, y, z]" do
    assert FlatAnything.flatten([1, 2, 3]) == [1, 2, 3]
  end

  test "that [[x]] is flattened to [x]" do
    assert FlatAnything.flatten([[1]]) == [1]
  end

  test "that [[x], y] is flattened to [x, y]" do
    assert FlatAnything.flatten([[1], 2]) == [1, 2]
  end

  test "that [[x], [y]] is flattened to [x, y]" do
    assert FlatAnything.flatten([[1], [2]]) == [1, 2]
  end

  test "that [[[x]], [y]] is flattened to [x, y]" do
    assert FlatAnything.flatten([[[1]], [2]]) == [1, 2]
  end

  test "that [[w, x, [y]], z] is flattened to [w, x, y, z]" do
    assert FlatAnything.flatten([[1,2,[3]],4]) == [1, 2, 3, 4]
  end

  test "that [[w, x, [y, a, b, c]], z] is flattened to [w, x, y, a, b, c, z]" do
    assert FlatAnything.flatten([[1,2,[3, 4, 5, 6]], 7]) == [1, 2, 3, 4, 5, 6, 7]
  end

  test "that [[[w, [x, y]]], [z]] is flattened to [w, x, y, z]" do
    assert FlatAnything.flatten([[[1, [2, 3]]], [4]]) == [1, 2, 3, 4]
  end
end