Conversion between Roman and decimals numbers

Roman.hs

import System.Environment
import qualified Data.Map as M
import Data.Maybe
import Data.Char

letters = "MDCLXVI"
values = [1000, 500, 100, 50, 10, 5, 1]


-- convert in to Roman
toRoman n = concat $ convert (reverse letters) (coeffs values n []) []
    where coeffs (v : vs) n buf = let (val, n') = divMod n v in coeffs vs n' (val:buf)
          coeffs [] n buf = buf
          convert (s1:(s2:ss@(s3:_))) (4:(1:(vs))) buf = convert ss vs ([s1, s3]:buf)
          convert (s1:(s2:ss)) (4:(0:vs)) buf = convert ss vs ([s1, s2]:buf)
          convert (s1:ss) (v1:vs) buf = convert ss vs ((replicate v1 s1):buf)
          convert [] _ buf = buf


-- convert Roman to Int
parseRoman s =
    let mapping = M.fromList $ zip letters values
        convert = fromJust . (flip M.lookup mapping)
        parsedDigits = (map convert s) in
        sum $ parse parsedDigits []
            where parse [] buf = buf
                  parse (x:y:xs) buf | x < y = parse xs ((y-x): buf)
                  parse (x:xs) buf = parse xs (x:buf)


convert s
    | all (flip elem letters) s = show $ parseRoman s
    | all isDigit s             = toRoman $ read s
    | otherwise                 = "error"


main = getArgs >>= putStrLn . convert . (map toUpper) . head