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

I think hammar's answer is the best, but here are smaller steps you can do - use null and lookup:

{-# LANGUAGE PatternGuards #-}
isBalanced xs = isBalanced' xs []

isBalanced' [] x = null x

isBalanced' (c:xs) ys | Just d <- lookup c par = isBalanced' xs (d:ys)
  where par = [('(',')'), ('[',']'),('{','}')]

isBalanced' _  [] = False

isBalanced' (x:xs) (y:ys) = x == y && isBalanced' xs ysl

Your example positives and negatives data should definitely use map, or even all.