Roman numeric converter in Haskell

roman.hs

module Roman
where

roman' n (one:five:ten:[]) | n == 0    = []
                           | n == 4    = [one, five]
                           | n == 5    = [five]
                           | n == 9    = [one, ten]
                           | n  > 5    = five : (roman' (n-5) (one:five:ten:[]) )
                           | otherwise = (roman' (n-1)  (one:five:ten:[]) ) ++ [one]

--roman::Integral a=>a->String
roman n |    1 <= n && n <   10 =  roman'           n  $ part 0 
        |   10 <= n && n <  100 =  roman' (div n   10) (part 2)  ++  rest (mod n   10)
        |  100 <= n && n < 1000 =  roman' (div n  100) (part 4)  ++  rest (mod n  100)
        | 1000 <= n && n < 4000 =  roman' (div n 1000) (part 6)  ++  rest (mod n 1000)
        where rest n | n == 0 = ""
                     | otherwise = roman n
              part n = take 3 $ drop n "IVXLCDM--"

chkroman n | n < 1     = Nothing
           | n < 4000  = Just $ roman n
           | otherwise = Nothing

deroman s = head $ filter (\n -> s == roman n) [1..3999]