Here is my solution to Exercism's Roman Numerals exercise.

roman_numerals.exs

defmodule RomanNumerals do
  import IO, only: [iodata_to_binary: 1]

  @doc """
  Convert the number to a roman number.
  """
  @spec numeral(pos_integer) :: String.t()
  def numeral(0),               do: ""
  def numeral(1),               do: "I"
  def numeral(2),               do: "II"
  def numeral(3),               do: "III"
  def numeral(4),               do: "IV"
  def numeral(5),               do: "V"
  def numeral(6),               do: "VI"
  def numeral(7),               do: "VII"
  def numeral(8),               do: "VIII"
  def numeral(9),               do: "IX"
  def numeral(10),              do: "X"
  def numeral(50),              do: "L"
  def numeral(100),             do: "C"
  def numeral(500),             do: "D"
  def numeral(1000),            do: "M"
  def numeral(n) when n < 40,   do: [numeral(10), numeral(n - 10)]                  |> iodata_to_binary()
  def numeral(n) when n < 50,   do: [numeral(10), numeral(50), numeral(n - 40)]     |> iodata_to_binary()
  def numeral(n) when n < 90,   do: [numeral(50), numeral(n - 50)]                  |> iodata_to_binary()
  def numeral(n) when n < 100,  do: [numeral(10), numeral(100), numeral(n - 90)]    |> iodata_to_binary()
  def numeral(n) when n < 400,  do: [numeral(100), numeral(n - 100)]                |> iodata_to_binary()
  def numeral(n) when n < 500,  do: [numeral(100), numeral(500), numeral(n - 400)]  |> iodata_to_binary()
  def numeral(n) when n < 900,  do: [numeral(500), numeral(n - 500)]                |> iodata_to_binary()
  def numeral(n) when n < 1000, do: [numeral(100), numeral(1000), numeral(n - 900)] |> iodata_to_binary()
  def numeral(n) when n < 5000, do: [numeral(1000), numeral(n - 1000)]              |> iodata_to_binary()
end