longestWord05.hs

import Control.Monad.State

type Word  = String  -- 最長の文字列
type Idx   = Int     -- 最長の文字列のインデックス
type Index = Int     -- ループで使うインデックス

longestWord' :: [String] -> State Index (Word, Idx)
longestWord' (x:xs) = foldl f (return (x, 0)) xs
    where
      f :: State Index (Word, Idx) -> String -> State Index (Word, Idx)
      f s w = do (word, idx) <- s
                 wordIdx <- if length word < length w
                            then return (w, idx+1)
                            else return (word, idx)
                 State $ \i -> (wordIdx, i+1)

longestWord xs = let (wordIdx, _) = runState (longestWord' xs) 0 in wordIdx
                            
main = print $ longestWord $ words "The quick brown fox"