https://codeiq.jp/ace/yuki_hiroshi/q839 の回答

spacetalky.hs

import Control.Applicative ((<$>))
import Data.Char (ord)

main :: IO ()
main = do
    ws <- lines' <$> getContents
    mapM_ putStrLn $ solve ws

solve :: [String] -> [String]
solve ws = do
    w <- ws
    return . maybe (glue w "X") (glue w) $ spacetalky w
  where
    glue xs ys = ys ++ ":" ++ xs

spacetalky :: String -> Maybe String
spacetalky output
    | odd $ length output = Nothing
    | otherwise           = go output
  where
    len x = ord x - ord 'a' + 1
    go (x0:x1:x2:xs)
        | x0 == x2 && x1 /= 'z' = Nothing
        | otherwise = do
            enc <- go (x2:xs)
            Just $ take (len x1) (repeat x0) ++ enc
    go [x0, x1] = Just $ take (len x1) (repeat x0)
    go _ = Nothing -- ここには来ないはず

-- "\r\n" 用
lines' :: String -> [String]
lines' = lines . filter (/= '\r')

始めのバージョンは不正解だったので修正した。