roman_numerals.ex

defmodule RomanNumerals do
  # Roman Numerals (Elixir)

  @digits %{"I" =>    1, 
            "V" =>    5, 
            "X" =>   10,
            "L" =>   50,
            "C" =>  100,
            "D" =>  500,
            "M" => 1000}

  @doc """
  Integer value of a String of Roman Numerals.
  """
  def value(s) do
    String.codepoints(s)
    |> Enum.map(fn d -> Map.get(@digits, d, 0) end)
    |> Enum.chunk(2, 1, [0])
    |> Enum.reduce(0, &RomanNumerals.accumulate/2)    
  end

  def accumulate([a, b], acc) do
    if a >= b do
      acc + a
    else
      acc - a
    end
  end

end

With pattern matching:

defmodule RomanNumerals do

  def value("I") do    1 end
  def value("V") do    5 end
  def value("X") do   10 end
  def value("L") do   50 end
  def value("C") do  100 end
  def value("D") do  500 end
  def value("M") do 1000 end

  @doc """
  Integer value of a String of Roman Numerals.
  """
  def value(s) do
    s
    |> String.codepoints
    |> Enum.map(fn d -> value(d) end)
    |> Enum.chunk(2, 1, [0])
    |> Enum.reduce(0, &RomanNumerals.accumulate/2)    
  end

  def accumulate([a, b], acc) do
    if a >= b do
      acc + a
    else
      acc - a
    end
  end

end