ExUnit Test for whether one element appears before another in a list

exunit_assert_order.exs

defmodule ExUnitAssertOrder do
  def assert_order(list, first, second) do
    assert_order(list, first, second, nil)
  end

  def assert_order([], first, second, first) do
    raise ExUnit.AssertionError, message: "Encountered #{first}, but never encountered #{second}"
  end

  def assert_order([], first, second, second) do
    raise ExUnit.AssertionError, message: "Encountered #{second}, but never encountered #{first}"
  end

  def assert_order([], _first, _second, nil) do
    raise ExUnit.AssertionError, message: "Never encountered either value"
  end

  def assert_order([first | _tail], first, second, second) do
    raise ExUnit.AssertionError, message: "Encountered #{first} after #{second}"
  end

  def assert_order([first | tail], first, second, nil) do
    assert_order(tail, first, second, first)
  end

  def assert_order([second | tail], first, second, nil) do
    assert_order(tail, first, second, second)
  end

  def assert_order([second | _tail], first, second, first) do
    true
  end

  def assert_order([_ | tail], first, second, state) do
    assert_order(tail, first, second, state)
  end
end