longestWord04.hs

import Text.Regex
import Control.Monad.State

longestWord' :: [String] -> State (String, Int) ()
longestWord' []     = return ()
longestWord' (x:xs) = do x1@(word, idx) <- get
                         put (x, idx+1) 
                         longestWord' xs
                         x2@(word', idx') <- get
                         if length word < length word' then put x2 else put x1

longestWord (x:xs) = execState (longestWord' xs) (x, 0)

main = print $ longestWord $ splitRegex (mkRegex " ") "The quick brown fox"