import Data.Char

charSucc :: Char -> (Char, Bool)
charSucc c
  | isDigit c = charSucc' '0' '9' c
  | isUpper c = charSucc' 'A' 'Z' c
  | isLower c = charSucc' 'a' 'z' c
  | otherwise = (c, True)
  where
    charSucc' low high c
      | c == high = (low, True)
      | otherwise = (succ c, False)

successor :: String -> String
successor = finish . foldr step ("", True)
  where
    step c (rest, False) = (c : rest, False)
    step c (rest, True ) = (s : rest, carry)
      where (s, carry) = charSucc c
    finish (s, False) = s
    finish (s@('0':_), True) = '1' : s
    finish (s@('a':_), True) = 'a' : s
    finish (s@('A':_), True) = 'A' : s
    finish (s, _) = s