module Exercise04 where

{-WETT-}
import Data.List
xmlLight :: String -> Bool
xmlLight = xmlAux []

xmlAux :: [String] -> String -> Bool
xmlAux yss ('<':xs)  
    | '>' `notElem` xs = False
    | '/' /= head xs = xmlAux (tag : yss) rest 
    | ' ' `elem` strip tag || '<' `elem` tag || null yss = False
    | otherwise = strip (tail tag) == strip (head yss) && xmlAux (tail yss) rest
        where   i = head (elemIndices '>' xs)
                tag = take i xs
                rest = drop (i+1) xs
                strip zs = dropWhile (== ' ') $ reverse zs
xmlAux yss (x:xs) = '>' /= x && xmlAux yss xs
xmlAux yss [] = null yss
{-TTEW-}