Given a positive Integer less than 4999, produce the Roman numeral representation.

Roman.hs

{-# OPTIONS_GHC -Wall -Werror #-}
module Roman( numerals ) where
import Data.List( unfoldr )

numerals :: Integer -> String
numerals = concat . unfoldr getNextRoman
    where       
      getNextRoman n | n >= 1000 = Just ("M", n - 1000)
                     | n >= 900 = Just ("CM", n - 900)
                     | n >= 500 = Just ("D", n - 500)
                     | n >= 400 = Just ("CD", n - 400)
                     | n >= 100 = Just ("C", n - 100)
                     | n >= 90 = Just ("XC", n - 90)
                     | n >= 50 = Just ("L", n - 50)
                     | n >= 40 = Just ("XL", n - 40)
                     | n >= 10 = Just ("X", n - 10)
                     | n >= 9 = Just ("IX", n - 9)
                     | n >= 5 = Just ("V", n - 5)
                     | n >= 4 = Just ("IV", n - 4)
                     | n >= 1 = Just ("I", n - 1)
                     | otherwise = Nothing