Question

I need to write a function par :: String -> Bool to verify if a given string with parentheses is matching using stack module.

Ex:

par "(((()[()])))" = True
par "((]())" = False

Here’s my stack module implementation:

module Stack (Stack,
              push, pop, top,
              empty, isEmpty)
    where

data Stack a = Stk [a]
             deriving (Show)

push :: a -> Stack a -> Stack a
push x (Stk xs) = Stk (x:xs)

pop :: Stack a -> Stack a
pop (Stk (_:xs)) = Stk xs
pop _ = error "Stack.pop: empty stack"


top :: Stack a -> a
top (Stk (x:_)) = x
top _ = error "Stack.top: empty stack"

empty :: Stack a
empty = Stk []

isEmpty :: Stack a -> Bool
isEmpty (Stk [])= True
isEmpty (Stk _) = False

So I need to implement a par function that would test a string of parentheses and say if the parentheses in it are balanced or not. How can I do that using a stack?

Answer 1

module Parens where

import Data.Map (Map)
import qualified Data.Map as Map

matchingParens :: Map Char Char
matchingParens = Map.fromList [
    ('(', ')')
  , ('{', '}')
  , ('[', ']')
  ]

isOpening :: Char -> Bool
isOpening c = maybe False (const True) $ Map.lookup c matchingParens

type Stack a = [a]

balanced :: String -> Bool
balanced = balanced' []

balanced' :: Stack Char -> String -> Bool
balanced' [] ""     = True
balanced' _  ""     = False
balanced' [] (c:cs) = balanced' [c] cs
balanced' (o:os) (c:cs)
  | isOpening c = balanced' (c:o:os) cs
  | otherwise   = case Map.lookup o matchingParens of
      Nothing -> False
      Just closing -> if closing == c
        then balanced' os cs
        else False