Not sure where this problem came from, but it was in an old folder

roman_translation.hs

import Control.Monad

romanValues = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1]
romanDigits = words "m cm d cd c xc l xl x ix v iv i"
theRomans   = zip romanValues romanDigits

toRoman' :: Int -> String
toRoman' n = fst $ foldl romFold ("", n) theRomans

romFold  :: ([a], Int) -> (Int, [a]) -> ([a], Int)
romFold (acc, n) (romValue, romDigit) =
  let (result, remain) = divMod n romValue
      newDigits = concat $ replicate result romDigit
  in (acc ++ newDigits, remain)


romanRangeCheck :: Int -> Bool
romanRangeCheck n
  | 0 < n && n < 4000 = True
  | otherwise = error "out of bounds [0 < n < 4000]"

toRoman :: Int -> String
toRoman n = do
  guard (romanRangeCheck n)
  toRoman' n