bfs

bfsaux :: (Ord a) => Set.Set a -> Set.Set a -> Int -> (a -> Set.Set a) -> a -> Maybe Int
bfsaux closed opened current neighbours target
    | opened == Set.empty = Nothing
    | Set.member target opened = Just current
    | otherwise = bfsaux newclosed newopened (current + 1) neighbours target
    where
        newclosed = Set.union closed opened
        newopened = Set.difference (Set.unions $ Set.toList $ Set.map neighbours opened) closed

-- neighbours function, start, target
bfs :: (Ord a) => (a -> Set.Set a) -> a -> a -> Maybe Int
bfs neighbours start target = bfsaux Set.empty (Set.singleton start) 0 neighbours target