Transform ByteString and Integral Num back and forth

gistfile1.hs

positiveDecimalBytesToIntegral :: (Integral num, Num num) => L.ByteString -> num
positiveDecimalBytesToIntegral = collect0
  where
    collect0 :: (Integral num, Num num) => L.ByteString -> num
    collect0 bs = case L.uncons bs of
      Just (byte, bs') ->
        if 0x30 <= byte && byte <= 0x39
          then collect1 (fromIntegral $ byte-0x30) bs'
          else collect0 bs'
      Nothing -> 0

    collect1 :: (Integral num, Num num) => num -> L.ByteString -> num
    collect1 acc bs = case L.uncons bs of
      Just (byte, bs') ->
        if 0x30 <= byte && byte <= 0x39
          then collect1 (acc * 10 + fromIntegral (byte-0x30)) bs'
          else acc
      Nothing -> acc

positiveIntegralToDecimalBytes :: (Integral num, Num num) => num -> L.ByteString
positiveIntegralToDecimalBytes = L.pack . go []
  where
    go :: (Integral num, Num num) => [Word8] -> num -> [Word8]
    go after num
      | num <= 9 = (0x30 + fromIntegral num) : after
      | otherwise =
        let (a, b) = num `divMod` 10
        in go ((0x30 + fromIntegral b) : after) a