Elixir: Roman Numerals

roman_numeral.exs

defmodule RomanNumeral do
  @numerals [
    {50, "L"},
    {40, "XL"},
    {10, "X"},
    {9, "IX"},
    {5, "V"},
    {4, "IV"},
    {1, "I"}
  ]

  def convert(number) do
    convert(number, @numerals)
  end

  defp convert(0, _numerals), do: ""

  defp convert(number, [{arabic, roman} | tail]) when number >= arabic do
    roman <> convert(number - arabic, [{arabic, roman} | tail])
  end

  defp convert(number, [{arabic, _roman} | tail]) when number < arabic do
    convert(number, tail)
  end
end

roman_numeral_test.exs

defmodule RomanNumeralTest do
  use ExUnit.Case

  test "converts arabic numbers into roman numerals" do
    numeric_mapping = [
      {0,  ""},
      {1,  "I"},
      {2,  "II"},
      {3,  "III"},
      {4,  "IV"},
      {5,  "V"},
      {6,  "VI"},
      {9,  "IX"},
      {10, "X"},
      {19, "XIX"},
      {20, "XX"},
      {28, "XXVIII"},
      {40, "XL"},
      {49, "XLIX"},
      {50, "L"},
    ]
    Enum.each(numeric_mapping, fn({arabic, roman}) -> assert RomanNumeral.convert(arabic) == roman end)
  end
end