Adding together two roman numerals to get a third one. See at: http://programmingpraxis.com/2009/03/06/roman-numerals/

pp_addRomans.hs

import Data.Map (fromList, (!))
 
romans = zip (words "M CM D CD C XC L XL X IX V IV I") [1000,900,500,400,100,90,50,40,10,9,5,4,1]
romMap = fromList romans
 
toRoman = go romans where
    go _ 0 = ""
    go a@((r, c):rs) n | c <= n = r ++ go a (n - c)
                       | otherwise = go rs n
 
toArab n = c + sum [b - 2 * a * fromEnum (b > a) | (a, b) <- zip conv cs] where
    conv@(c:cs) = map ((romMap!) . (:[])) n
 
addRomans a b = toRoman (toArab a + toArab b)
 
main = print $ addRomans "IV" "XXIV"
 
 --Slower but terser conversions with no Data.Map (about half as fast)
 
--toRoman 0 = ""
--toRoman n = r ++ toRoman (n - c) where
--    (r, c) = head $ filter ((<=n) . snd) romans
 
--toArab n = c + sum [b - 2 * a * fromEnum (b > a) | (a, b) <- zip conv cs] where
--    conv@(c:cs) = map (fromJust . (`lookup` romans) . (:[])) n