my_list.ex

defmodule MyList do

  def flatten([]), do: []
  def flatten([head | tail]), do: flatten(head) ++ flatten(tail)
  def flatten(x), do: [x]

end

defmodule MyListTest do
  use ExUnit.Case
  doctest MyList

  test "preserves structure" do
    assert [1, 2, 3] == MyList.flatten([1, 2, 3])
  end

  test "flattens a shallow nested list" do
    assert [1,2,3,4] == MyList.flatten([1,2,[3],4])
  end

  test "flattens a deeply nested list" do
    assert [1,2,3,4] == MyList.flatten([[1,2,[3]],4])
  end
end
~