ByteString: hex integers

HesBs.hs

module HexBs where
import qualified Data.ByteString.Lazy.Char8 as B
import Data.Char

itox :: Integer -> B.ByteString
itox 0 = B.singleton '0'
itox n | n > 0 = B.reverse $ B.unfoldr unhex n
                 where unhex i = if i == 0 then
                                   Nothing
                                 else
                                   Just (intToDigit $ fromIntegral b, a)
                                   where (a, b) = divMod i 16


xtoi :: B.ByteString -> Integer
xtoi bs = B.foldl' hex 0 bs
          where hex i c = i*16 + (toInteger $ digitToInt c)