Roman Notation

roman.hs

trivial :: [(Int, String)]
trivial = [
            {- Bridge values -}
            (1, "I"), (5, "V"), (10, "X"), (50, "L"), 
            (100, "C"), (500, "D"), (1000, "M"),

            {- Subtractions -}
            (4, "IV"), (9, "IX"), (40, "XL"), (90, "XC"),
            (400, "CD"), (900, "CM")
         ]

decimals :: [Int]
decimals = map fst trivial

mapTrivial :: Int -> String
mapTrivial n = snd . head $ filter (\ (dec, _) -> dec == n) trivial

toRoman :: Int -> String
toRoman n | n `elem` decimals = mapTrivial n
          | otherwise = mapTrivial nextRoman ++ toRoman (n - nextRoman) 
                where nextRoman = maximum $ filter (< n) decimals