open number string math

digit x y z k =
  [[x],[x,x],[x,x,x],[x,y],[y],[y,x],[y,x,x],[y,x,x,x],[x,z]] :
  (toInt k - 1)

toRoman 0 = ""
toRoman x | x < 0     = fail "Negative roman numeral"
          | x >= 1000 = 'M' :: toRoman (x - 1000)
          | x >= 100  = let (q,r) = x `divrem` 100 in
                        digit 'C' 'D' 'M' q ++ toRoman r
          | x >= 10   = let (q,r) = x `divrem` 10 in
                        digit 'X' 'L' 'C' q ++ toRoman r
          | else = digit 'I' 'V' 'X' x

map (join "" << toRoman) [1999,25,944]