Task

Create a function taking a positive integer as its parameter and returning a string containing the Roman numeral representation of that integer. Modern Roman numerals are written by expressing each digit separately, starting with the left most digit and skipping any digit with a value of zero.



In Roman numerals:

 1990 is rendered: 1000=M, 900=CM, 90=XC; resulting in MCMXC
 2008 is written as 2000=MM, 8=VIII; or MMVIII
 1666 uses each Roman symbol in descending order: MDCLXVI


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]