suggestedPostsFor userId = do
    friendIds <- friendsOf userId                     -- Get the friends of the current user
    topics <- favoriteTopicsOf userId                 -- Get the favorite topics of the current user
    friendsPosts <- forM friendIds $ \friendId -> do  -- For each friend
        posts <- lastPostsOf friendId                 -- * Get their last posts
        return (filter (isAbout topics) posts)        -- * Keep those whose topic is relevant
    return (mostLiked 3 friendsPosts)                 -- Rank them to keep only the bests