Roman numerals/Decode - Rosetta Code might be interesting for you.

And

  def decode(<<x>>), do: to_value(x)

  def decode(<<h1, h2, rest::bytes>>) do
    case {to_value(h1), to_value(h2)} do
      {v1, v2} when v1 < v2 -> v2 - v1 + decode(rest)
      {v1, _} -> v1 + decode(<<h2, rest::bytes>>)
    end
  end

  def decode(""), do: 0

  defp to_value(?M), do: 1000
  defp to_value(?D), do: 500
  defp to_value(?C), do: 100
  defp to_value(?L), do: 50
  defp to_value(?X), do: 10
  defp to_value(?V), do: 5
  defp to_value(?I), do: 1

if you want to decode binaries instead of lists.

You could avoid subtraction entirely and rely on pattern-matching instead:

defmodule RomanParser do
  def parse(input, total \\ 0)

  def parse("M" <> rest, total), do: parse(rest, total + 1000)
  def parse("CM" <> rest, total), do: parse(rest, total + 900)
  def parse("D" <> rest, total), do: parse(rest, total + 500)
  def parse("CD" <> rest, total), do: parse(rest, total + 400)
  def parse("C" <> rest, total), do: parse(rest, total + 100)
  def parse("XC" <> rest, total), do: parse(rest, total + 90)
  def parse("L" <> rest, total), do: parse(rest, total + 50)
  def parse("XL" <> rest, total), do: parse(rest, total + 40)
  def parse("X" <> rest, total), do: parse(rest, total + 10)
  def parse("IX" <> rest, total), do: parse(rest, total + 9)
  def parse("V" <> rest, total), do: parse(rest, total + 5)
  def parse("IV" <> rest, total), do: parse(rest, total + 4)
  def parse("I" <> rest, total), do: parse(rest, total + 1)

  def parse("", total), do: total
end

RomanParser.parse("MMMDXVI")

(nimble_parsec can produce similar code with less typing, fwiw)

Both this solution and the other one up-thread have a similar bug/feature: they accept incorrect numbers and do their best to render them. For instance, "IIIIVM" is parsed to 1006 by both and "IM" is parsed to 1001 by this solution and 999 by the subtraction method.

We have is_map_key/2 guard now!:

defmodule Roman do

  @numerals %{
    "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}

  def parse(<<num::binary-size(2)>> <> rest) when is_map_key(@numerals, num), do: parse(rest) + @numerals[num]
  def parse(<<num::binary-size(1)>> <> rest) when is_map_key(@numerals, num), do: parse(rest) + @numerals[num]
  def parse(""), do: 0

end

Roman.parse("MMMDXVI") #=> 3516
Roman.parse("MMCMVI") #=> 2906