RLE.hs

import Data.Word (Word8)

rleBytes :: [Word8] -> [Word8]
rleBytes = concatMap (\(n, x) -> [n, x]) . break8 . rle

rle :: Eq a => [a] -> [(Int, a)]
rle = map len . runs
  where
    len (x, xs) = (length xs + 1, x)
    runs [] = []
    runs (x:xs) = (x, takeWhile (== x) xs) : runs (dropWhile (== x) xs)

break8 :: [(Int, a)] -> [(Word8, a)]
break8 = concatMap break
  where
    break (n, x)
        | n > 255 = (255, x) : break (n-255, x)
        | otherwise = [(fromIntegral n, x)]