parens.hs

{-# LANGUAGE LambdaCase #-}

import Control.Monad.Trans.Reader (runReader, local, ask, Reader)
import Data.Maybe (mapMaybe)

data Paren = LParen | RParen

balanced :: [Paren] -> Bool
balanced xs = flip runReader 0 $ go xs
  where
    go :: [Paren] -> Reader Int Bool
    go [] = do
      n <- ask
      pure $ n == 0
    go (par:pars) = do
      curr <- ask
      if curr < 0
      then pure False
      else local (+ count par) $ go pars

count :: Paren -> Int
count LParen = 1
count RParen = -1

fromChar :: Char -> Maybe Paren
fromChar = \case
  '(' -> Just LParen
  ')' -> Just RParen
  _ -> Nothing

parensOnly :: String -> [Paren]
parensOnly = mapMaybe fromChar