citrus_byte.ex

defmodule CitrusByte do
  @moduledoc """
  Provides the `flatten/1` function to flatten a List
  """

  @doc """
  Flattens a List

  ## Examples

      iex> CitrusByte.flatten([[1,2,[3]],4])
      [1,2,3,4]

  """
  @spec flatten(List.t) :: List.t
  def flatten([]), do: []
  def flatten([head|tail]), do: flatten(head) ++ flatten(tail)
  def flatten(head), do: [head]
end

citrus_byte_test.exs

defmodule CitrusByteTest do
  use ExUnit.Case
  doctest CitrusByte

  test "flatten" do
    assert [1,2,3,4] == CitrusByte.flatten([1,2,3,4])
    assert [1,2,3,4] == CitrusByte.flatten([[1,2,[3]],4])
    assert [1,2,3,4] == CitrusByte.flatten([[1,[2,[3]]],4])
    assert [] == CitrusByte.flatten([])
  end
end