In [1]:

    

import Data.List
import Data.Maybe
import qualified Data.Set as Set


    

In [2]:

    

isMadeOf :: String -> Set.Set String -> Bool
isMadeOf word otherWords =
    let
        allSplits = tail $ init $ zip (inits word) (tails word) 
        isValidSplit (left, right) =
            left `Set.member` otherWords &&
            (right `Set.member` otherWords || right `isMadeOf` otherWords)
    in
        any isValidSplit allSplits


    

In [3]:

    

longestWord :: [String] -> Maybe String
longestWord xs =
    listToMaybe $ filter (`isMadeOf` Set.fromList xs) $ reverse $ sortOn length xs


    

In [4]:

    

longestWord ["cat", "banana", "dog", "nana", "walk", "walker", "dogwalker"]


    

    





Just"dogwalker"