main = putStrLn $ rle "wwwwaaadexxxxxx"

split :: [Char] -> [[Char]]
split str = reverse $ go [] (head str) str
  where go acc _ ""    = acc
        go acc cur str = let newAcc = (takeWhile (==cur) str):acc
                             newStr = dropWhile (==cur) str
                             newCur = head newStr
                         in
                           go newAcc newCur newStr

rle :: [Char] -> [Char]
rle "" = ""
rle x  = foldl (\acc x -> acc ++ (head x:(show $ length x))) "" . split $ x