Hello world in Elixir with some odd twists

hello_world.exs

defmodule HelloWorld do
  def greet("") do
    {:ok, "Hello World"}
  end

  def greet([]) do
    {:ok, "Hello World"}
  end

  def greet(names) when is_list(names) do
    name_list = Joiner.join(names,", ", " and ")
    {:ok, "Hello #{name_list}"}
  end

  def greet(name) do
    {:ok, "Hello #{name}"}
  end
end

defmodule Joiner do
  def join(list, sep, last_sep) do
    list |> do_join(sep, last_sep) |> Enum.join
  end

  defp do_join([], _, _), do: []
  defp do_join([x], _, _), do: [x]
  defp do_join([x,y], _, last_sep) do
    [x, last_sep, y]
  end

  defp do_join([head|rest], sep, last_sep) do
    [head, sep | do_join(rest, sep, last_sep)]
  end
end

ExUnit.start

defmodule HelloWorldTest do
  use ExUnit.Case, async: true

  test "returns hello world" do
    {_, greeting} = HelloWorld.greet("")
    assert greeting == "Hello World"
  end

  test "return hello <username>" do
    name = "Bruce"
    {_, greeting} = HelloWorld.greet(name)
    assert greeting == "Hello " <> name
  end

  test "returns hello world for no users" do
    names = []
    {_, greeting} = HelloWorld.greet(names)
    assert greeting == "Hello World"
  end

  test "returns hello user1, user2 ...." do
    names = ["Bruce", "Lora", "Carter"]
    {_, greeting} = HelloWorld.greet(names)
    assert greeting == "Hello Bruce, Lora and Carter"
  end
end

defmodule JoinerTests do
  use ExUnit.Case, async: true

  test "join many names" do
    value = Joiner.join(["a", "b", "c", "d"], ", ", " and ")
    assert value == "a, b, c and d"
  end

  test "join two names" do
    value = Joiner.join(["a", "b"], ", ", " and ")
    assert value == "a and b"
  end

  test "join one names" do
    value = Joiner.join(["a"], ", ", " and ")
    assert value == "a"
  end

  test "join empty names" do
    value = Joiner.join([], ", ", " and ")
    assert value == ""
  end
end