Stack-based bracket matcher in Haskell

brachetmatcher.hs

module Main
where

data Node = Node { chr :: Char
                 , next :: Maybe Node }
                 deriving (Show)

prepareInput :: String -> String
prepareInput str = filter (\c -> c `elem` ['(', ')']) str

processChar :: Char -> String -> Maybe Node -> Maybe Node
processChar '(' "" Nothing  = Just (Node '(' Nothing) -- init case
processChar ')' r  (Just n) = process r (next n)
processChar ')' _  Nothing  = Just (Node ')' Nothing) -- error, generate a dummy
processChar '(' r  stack    = process r (Just (Node '(' stack))
-- Todo: Integrate this function into the `process` function as soon as it
-- works.

process :: String -> Maybe Node -> Maybe Node
process []  Nothing = Nothing
process []  stack   = stack
process str stack = processChar (head str) (tail str) stack
-- We actually don't care when `stack` is Nothing and pass it to the
-- `processChar` function

answer :: Maybe Node -> String
answer Nothing  = "OK"
answer _        = "Nup"

main :: IO ()
main = do
    str <- getLine
    putStrLn $ answer $ process (prepareInput str) Nothing

Compare with: https://gist.github.com/Heimdell/3f483c02226d95faf1ee

Use specialized solutions for parsing - Parsec/Attoparsec/ReadP/Whatever.