module Codewars.Kata.Braces where

import Control.Monad ((>=>))
import Control.Applicative ((<|>))

newtype Validator = Validator { check :: String -> Maybe String }

validChar :: Char -> Validator
validChar c = Validator  v
    where
    v (c' : rest)  | c' == c = Just rest
    v _            = Nothing

fail :: Validator
fail = Validator $ const Nothing

pass :: Validator
pass = Validator Just

andThen :: Validator -> Validator -> Validator
v1 `andThen` v2 = Validator $ check v1 >=> check v2

orElse :: Validator -> Validator -> Validator
v1 `orElse` v2 = Validator $ \s -> check v1 s <|> check v2 s

some :: Validator -> Validator
some v = v `andThen` many v

many :: Validator -> Validator
many v = some v `orElse` pass

wrapped :: Char -> Char -> Validator -> Validator
wrapped l r v = validChar l `andThen` v `andThen` validChar r

wrappedRound :: Validator -> Validator
wrappedRound = wrapped '(' ')'

wrappedSquare :: Validator -> Validator
wrappedSquare = wrapped '[' ']'

wrappedCurly :: Validator -> Validator
wrappedCurly = wrapped '{' '}'

balancedOnce :: Validator
balancedOnce =
       wrappedRound pass
       `orElse` wrappedSquare pass
       `orElse` wrappedCurly pass
       `orElse` wrappedRound more
       `orElse` wrappedSquare more
       `orElse` wrappedCurly more
   where more = some balancedOnce

validBraces :: String -> Bool
validBraces = maybe False (== []) . check (many balancedOnce)