Roman Numerals Code Kata in Elixir

roman.exs

defmodule RomanNumeral do
  def convert(n) when n >= 1000, do: remove_and_continue(n, "M", 1000)
  def convert(n) when n >= 900,  do: remove_and_continue(n, "CM", 900)
  def convert(n) when n >= 500,  do: remove_and_continue(n, "D", 500)
  def convert(n) when n >= 400,  do: remove_and_continue(n, "CD", 400)
  def convert(n) when n >= 100,  do: remove_and_continue(n, "C", 100)
  def convert(n) when n >= 90,   do: remove_and_continue(n, "XC", 90)
  def convert(n) when n >= 50,   do: remove_and_continue(n, "L", 50)
  def convert(n) when n >= 40,   do: remove_and_continue(n, "XL", 40)
  def convert(n) when n >= 10,   do: remove_and_continue(n, "X", 10)
  def convert(9),                do: "IX"
  def convert(n) when n >= 5,    do: remove_and_continue(n, "V", 5)
  def convert(4),                do: "IV"
  def convert(n),                do: String.duplicate("I", n)

  defp remove_and_continue(total, roman, number) do
    String.duplicate(roman, div(total, number)) <> convert(rem(total, number))
  end
end

ExUnit.start

defmodule RomanNumeralTest do
  use ExUnit.Case

  defp is(number, roman) do
    assert RomanNumeral.convert(number) == roman
  end

  test 'convert 1 to "I"' do
    is(1, "I")
  end

  test 'convert 2 to "II"' do
    is(2, "II")
  end

  test 'convert 3 to "III"' do
    is(3, "III")
  end

  test 'convert 4 to "IV"' do
    is(4, "IV")
  end

  test 'convert 5 to "V"' do
    is(5, "V")
  end

  test 'convert 6 to "VI"' do
    is(6, "VI")
  end

  test 'convert 7 to "VII"' do
    is(7, "VII")
  end

  test 'convert 8 to "VIII"' do
    is(8, "VIII")
  end

  test 'convert 9 to "IX"' do
    is(9, "IX")
  end

  test 'convert 10 to "X"' do
    is(10, "X")
  end

  test 'convert 11 to "XI"' do
    is(11, "XI")
  end

  test 'convert 14 to "XIV"' do
    is(14, "XIV")
  end

  test 'convert 19 to "XIX"' do
    is(19, "XIX")
  end

  test 'convert 20 to "XX"' do
    is(20, "XX")
  end

  test 'convert 25 to "XXV"' do
    is(25, "XXV")
  end

  test 'convert 27 to "XXVII"' do
    is(27, "XXVII")
  end

  test 'convert 47 to "XLVII"' do
    is(47, "XLVII")
  end

  test 'convert 1990 to "MCMXC"' do
    is(1990, "MCMXC")
  end

  test 'convert 1954 to "MCMLIV"' do
    is(1954, "MCMLIV")
  end

  test 'convert 2,394 to "MMCCCXCIV"' do
    is(2394, "MMCCCXCIV")
  end
end

Nice! You used recursion where I used Enum.reduce. I'm still trying to develop a feel for idiomatic use of one vs. the other.

I know the feeling. I keep having to remind myself that recursion is ok. I'm so used to avoiding it.