brackets.hs

module Demo where

check :: String -> Bool
check = go []
  where
    go ('[':xs) (']':ys) = go xs ys
    go ('{':xs) ('}':ys) = go xs ys
    go ('(':xs) (')':ys) = go xs ys
    go xs (y:ys)
      | y `elem` "[{(" = go (y:xs) ys
      | otherwise      = y `notElem` "]})" && go xs ys
    go xs [] = null xs

MegaparseVersion.hs

module Demo where

import Text.Megaparsec
import Text.Megaparsec.Char

type Parser = Parsec (ErrorItem Char) String

demo :: String -> IO ()
demo = parseTest parser

parser :: Parser ()
parser = parserN *> eof

-- | Parse many non-bracket characters and bracketed subexpressions.
parserN :: Parser ()
parserN = skipMany parser1

-- | Parse a single non-bracket character or a whole bracketed subexpression.
parser1 :: Parser ()
parser1 = aux '(' ')' <|>
          aux '[' ']' <|>
          aux '{' '}' <|>
          () <$ noneOf ")]}"
  where
    aux x y = () <$ between (char x) (char y) parserN