Week 12 code dojo with @lamarqua

Week12.hs

import Control.Applicative
import Control.Monad
import System.IO
import Data.List (foldl')
import Data.Maybe

main :: IO ()
main = do
    t   <- fmap read getLine :: IO Int
    lns <- mapM (const getLine) [1..t]
    mapM_ putStrLn (map valid lns)   

matches o c = (o:c:[]) `elem` ["{}", "[]", "()"]

step Nothing c      = Nothing
step (Just stack) c = case (c `elem` "({[", stack) of
    (True, _)     -> Just (c:stack)
    (False, [])   -> Nothing
    (False, x:xs) -> if matches x c then Just xs else Nothing

valid = maybe "NO" (\o -> if o=="" then "YES" else "NO") . foldl' step (Just [])