https://www.hackerrank.com/challenges/string-compression

StringCompression.hs

group :: Eq a => [a] -> [(a, Int)]
group = foldr step []
    where step c []       = [(c, 1)]
          step c whole@((x, n) : xs)
              | c == x    = (x, n + 1) : xs
              | otherwise = (c, 1) : whole

compress :: String -> String
compress =
    concatMap f . group
        where f (x, 1) = [x]
              f (x, n) = [x] ++ show n

main :: IO ()
main = do
    input <- getLine
    putStrLn $ compress input