Checking if a string consists of balanced parenthesis

Question

I wrote the following program to check strings for balanced parenthesis:

isBalanced xs = isBalanced\' xs []

isBalanced\' [] [] = True
isBalanced\' [] _  = False         


        

Answer 1

As Henning said, parser combinators would work for this. Here's an example using Parsec:

import Text.Parsec

grammar = many braces >> return ()
    where braces = choice [ between (char '(') (char ')') grammar
                          , between (char '[') (char ']') grammar
                          , between (char '{') (char '}') grammar
                          ]

isBalanced :: String -> Bool
isBalanced input = case parse (grammar >> eof) "" input of
                       Left  _ -> False
                       Right _ -> True