StateMonad vs Folding

Main.purs

module Main where

import Prelude (Unit, negate, (==), (>>>), (-), (>), (+), ($), bind)
import Data.Foldable (traverse_, foldl)
import Data.String (toCharArray)
import Control.Monad.State (State, execState, modify)
import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (CONSOLE, logShow)

match :: Int -> Char -> Int
match lefts '(' = lefts + 1
match lefts ')' = if lefts > 0 then lefts - 1
                  else -1
match lefts _ = lefts

matching' :: Int -> Array Char -> Int
matching' = foldl match

matching :: Array Char -> State Int Unit
matching = traverse_ \p -> modify \lefts -> match lefts p

testParens :: String -> Boolean
testParens = toCharArray >>> matching >>> (\n -> execState n 0) >>> (==) 0

testParens' :: String -> Boolean
testParens' = toCharArray >>> (\lefts -> matching' 0 lefts) >>> (==) 0

main :: forall e. Eff (console :: CONSOLE | e) Unit
main = do
  logShow $ testParens "((()))" -- true
  logShow $ testParens "((())" -- false
  logShow $ testParens ")" -- false

  logShow $ testParens' "((()))" -- true
  logShow $ testParens' "((())" -- false
  logShow $ testParens' ")" -- false