Small example about decoding values

Korttipeli.hs

{-# LANGUAGE RecordWildCards #-}
module Korttipeli where

import Control.Monad.State

decodeValue :: Integer -> State Integer Integer
decodeValue n = state $ \s -> let (s', ord) = divMod s n
                               in (ord, s')

decodeValues :: Traversable t => t Integer -> Integer -> Maybe (t Integer)
decodeValues xs a = case runState (mapM decodeValue xs) a of
  (out, 0) -> Just out
  _        -> Nothing