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

Using a left fold

import Data.List (foldl')

isBalanced xs = null $ foldl' op [] xs
  where
    op ('(':xs) ')' = xs
    op ('[':xs) ']' = xs
    op ('{':xs) '}' = xs
    op xs x = x:xs

The fold builds up a stack of previously encountered characters, stripping away any matches as it finds them. If you end up with an empty list, the string is balanced.

Using a left fold in the Maybe monad

The disadvantage of using a left fold is that the entire string must always be scanned through. It would be nice to abort the operation with a failure should a closing brace be found without a matching opening brace. Here's a version that does just that.

import Control.Monad (foldM)

isBalanced' xs = maybe False null $ foldM op [] xs
  where
    op ('(':xs) ')'          = Just xs
    op ('[':xs) ']'          = Just xs
    op ('{':xs) '}'          = Just xs
    op xs x | x `elem` ")]}" = Nothing
            | otherwise      = Just (x:xs)