{-# LANGUAGE OverloadedStrings #-}

module Exercise04 where


import qualified Data.Text as L
import Data.Char(isSpace)
import Data.List(isPrefixOf)

{-WETT-}
xmlLight :: String -> Bool
xmlLight string = not ( ">" `isPrefixOf` string) && L.null (head tags) && tagsmatching [] (tail tags)
    where tags = L.stripEnd . L.dropEnd 1 . L.dropWhileEnd (/='>') <$> L.splitOn "<" (L.pack string)
          tagsmatching [] [] = True
          tagsmatching _ [] = False
          tagsmatching [] (tag:rest) = validTag tag && not ( L.isPrefixOf "/" tag) && tagsmatching [tag] rest
          tagsmatching (top:stack) (tag:rest)= validTag tag && 
            if L.isPrefixOf "/" tag
                then top == L.tail tag && tagsmatching stack rest
                else tagsmatching (tag:top:stack) rest
          validTag tag = not $ L.null tag || L.any isSpace tag


tags string = L.stripEnd . L.dropEnd 1 . L.dropWhileEnd (/='>') <$> L.splitOn "<" (L.pack string)

{-TTEW-}

main = interact (show . xmlLight)