import Control.Monad.Except
import Data.Char
data ParseError = ParseError {location::Int, reason::String}

type ParseMonad = Either ParseError

parseDig :: Char -> Integer -> ParseMonad Integer
parseDig x pos = do
  if isHexDigit x
  then return $ toInteger $ digitToInt x
  else throwError $ ParseError (fromInteger pos) (x:": Invalid digit")




parseHex' :: String -> Integer -> Integer -> ParseMonad Integer
parseHex' [x] pos acc = do
  b <- (parseDig x pos)
  return $ acc * 16 + b


parseHex' (x:xs) pos acc = do
  b <- parseDig x pos
  c <- parseHex' xs (pos + 1) (acc * 16 + b)
  return c


parseHex :: String -> ParseMonad Integer
parseHex x  = parseHex' x 1 1

printError :: ParseError -> ParseMonad String
printError pe = Right $ "At pos " ++ (show (location pe)) ++ ": " ++ (reason pe)

test s = str where
  (Right str) = do
      n <- parseHex s
      return $ show n
    `catchError` printError