DouKaku?

reverseString2 :: String -> String
reverseString2 str = let (e, s) = foldl f ([],[]) str in foldl (\s1 (s2,c) -> s2++c:s1) s e
    where f (e, s) c
              | c `elem` "({[" = (([], c):e, s)
              | c `elem` ")}]" = case e of
                                   (es1, p1):(es2, p2):rest
                                        | c == opposite p1 -> ((p1:es1++c:es2, p2):rest, s)
                                        | otherwise -> ((c:es1++p1:es2, p2):rest, s)
                                   [(es, p)]
                                       | c == opposite p -> ([], p:es++c:s)
                                       | otherwise -> ([], c:es++p:s)
                                   [] -> (e, c:s)
              | otherwise = case e of
                              (es, p):rest -> ((c:es, p):rest, s)
                              otherwise    -> (e, c:s)
          opposite '(' = ')'
          opposite ')' = '('
          opposite '{' = '}'
          opposite '}' = '{'
          opposite '[' = ']'
          opposite ']' = '['
          opposite c = c