Solution for a classic balanced-parens problem

brackets.hs

{-
 Bal ::= (Bal)Bal | [Bal]Bal | ""
 -}

import Control.Monad

brackets = [('[',']'), ('(',')')]

isObr b = elem b (map fst brackets)
isCoBr b1 b2 = elem (b1,b2) brackets || elem (b2,b1) brackets

balanced (x:xs) | isObr x = 
  balanced >=> closing x >=> balanced $ xs
  where 
    closing x (y:ys) | isCoBr x y = return ys
    closing _ _ = mzero
balanced x = return x
                      
check = maybe False null . balanced

main = do
  s <- getLine
  print $ check s