Roman.hs

import System.Environment

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

-- Convert an int to Roman representation
toRoman n = concat $ norm (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
          norm (s1:(s2:ss@(s3:_))) (4:(1:(vs))) buf = norm ss vs ([s1, s3]:buf)
          norm (s1:(s2:ss)) (4:(0:vs)) buf = norm ss vs ([s1, s2]:buf)
          norm (s1:ss) (v1:vs) buf = norm ss vs ((replicate v1 s1):buf)
          norm [] _ buf = buf

main = getArgs >>= putStrLn . toRoman . read . head