byteSum :: [Word8] -> [Word8] -> [Word8]
byteSum = byteSum' 0
  where
    byteSum' 0 [] [] = []
    byteSum' 1 [] [] = [1]
    byteSum' carry (x:xs) (y:ys) =
        let v = x + y + carry
        in v : byteSum' (if v < x || v < y then 1 else 0) xs ys

main = do
    let bs1 = ...
    let bs2 = ...
    let sum = pack $ byteSum (unpack bs1) (unpack bs2)