longestWord03.hs

import Text.Regex

type State s a = s -> (a, s)

comb :: State s a -> (a -> State s b) -> State s b
comb m k = \s -> let (x1, s') = m s in k x1 s'

comb_ :: State s a -> State s b -> State s b
comb_ m n = m `comb` \_ -> n

ret :: a -> State s a
ret x = \s -> (x, s)

get :: State s s
get = \s -> (s, s)

put :: s -> State s ()
put x = \_ -> ((), x)

execState :: State s a -> s -> s
execState s s0 = snd $ s s0

longestWord' :: [String] -> State (String, Int) ()
longestWord' [] = ret ()
longestWord' (x:xs) = get             `comb` \x1@(word, idx) ->
                      put (x, idx+1)  `comb_` 
                      longestWord' xs `comb_` 
                      get             `comb` \x2@(word', idx') ->
                      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"