Roman Numerals in Haskell

gistfile1.hs

module Main where

import Data.List

data Numeral = Numeral Int String

numerals :: [Numeral]
numerals = reverse [
    Numeral 1 "I",
    Numeral 4 "IV",
    Numeral 5 "V",
    Numeral 9 "IX",
    Numeral 10 "X"
  ]

numeral :: Int -> Maybe (String, Int)
numeral x = fmap next $ find predicate numerals
  where
    predicate (Numeral value _) = x >= value
    next (Numeral value numeral) = (numeral, x - value)

romanNumeral :: Int -> String
romanNumeral x = concat $ unfoldr numeral x