Converting to Roman Numerals exercise; need help understanding code

I think its fine except the variable naming makes it a bit hard to see what’s happening. And the odd comment helps. Personally I think a recursive function like this is a good fit for the problem. Perhaps like this:

defmodule Roman do
  @roman_to_decimal_mappings [M: 1000, CM: 900, D: 500, CD: 400, C: 100, XC: 90, L: 50, XL: 40, X: 10, IX: 9, V: 5, IV: 4, I: 1]

  @doc """
  Convert the number to a roman number.
  """
  @spec numerals(pos_integer) :: String.t()
  def numerals(number) do
    numeral([], number, @roman_to_decimal_mappings)
  end

  # While the number is >= the decimal value at the head of the mappings,
  # accumulate the Roman numeral and decrement the number
  defp numeral(roman_acc, number, [{roman_numeral, decimal_value}| _t] = roman_to_decimal_mappings) 
      when number >= decimal_value do
    numeral([roman_numeral | roman_acc], number - decimal_value, roman_to_decimal_mappings)
  end

  # When the number is less than the decimal value at the head of the
  # mapping list, move to the next mapping pair
  defp numeral(roman_acc, number, [_h | remaining_mappings]) do
    numeral(roman_acc, number, remaining_mappings)
  end

  # And when the mapping list is exhausted, reverse the accumulated
  # Roman numerals and join then
  defp numeral(roman_acc, _n, [] = _remaining_mappings) do
    roman_acc
      |> Enum.reverse
      |> Enum.map_join("", &to_string/1)
  end
end

I’m not certain how readable this is for others, but it’s how I worked through the problem early in my Elixir adventures, and I can still explain the approach 6 years later. elixir_kata/roman/roman_numerals.exs at master · gvaughn/elixir_kata · GitHub

I iterate over the mappings and use Enum.reduce with a 2-tuple to keep track of the remainder of the number (after dividing out each Roman value) plus the accumulated Roman numeral string.

defmodule RomanNumerals do
  @spec to_roman(integer) :: String.t()
  def to_roman(number) when is_integer(number) and number > 0 do
    to_roman_recursive(number, "")
  end
  def to_roman(_), do: ""

  defp to_roman_recursive(0, acc), do: acc
  defp to_roman_recursive(number, acc) when number >= 1000, do: to_roman_recursive(number - 1000, acc <> "M")
  defp to_roman_recursive(number, acc) when number >= 900, do: to_roman_recursive(number - 900, acc <> "CM")
  defp to_roman_recursive(number, acc) when number >= 500, do: to_roman_recursive(number - 500, acc <> "D")
  defp to_roman_recursive(number, acc) when number >= 400, do: to_roman_recursive(number - 400, acc <> "CD")
  defp to_roman_recursive(number, acc) when number >= 100, do: to_roman_recursive(number - 100, acc <> "C")
  defp to_roman_recursive(number, acc) when number >= 90, do: to_roman_recursive(number - 90, acc <> "XC")
  defp to_roman_recursive(number, acc) when number >= 50, do: to_roman_recursive(number - 50, acc <> "L")
  defp to_roman_recursive(number, acc) when number >= 40, do: to_roman_recursive(number - 40, acc <> "XL")
  defp to_roman_recursive(number, acc) when number >= 10, do: to_roman_recursive(number - 10, acc <> "X")
  defp to_roman_recursive(number, acc) when number >= 9, do: to_roman_recursive(number - 9, acc <> "IX")
  defp to_roman_recursive(number, acc) when number >= 5, do: to_roman_recursive(number - 5, acc <> "V")
  defp to_roman_recursive(number, acc) when number >= 4, do: to_roman_recursive(number - 4, acc <> "IV")
  defp to_roman_recursive(number, acc) when number >= 1, do: to_roman_recursive(number - 1, acc <> "I")
end