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select :: IRequest req a => req a -> AsyncFetch req a
select req = AsyncFetch $ \cacheVar -> do
  cache <- takeMVar cacheVar
  case lookupCache req cache of
    Just var -> do
      putMVar cacheVar cache
      pure var
    Nothing -> do
      var <- fetch req
      putMVar cacheVar (insertCache req var cache)

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newtype RequestCache req =
  RequestCache (forall a. HashMap (req a) (MVar a))

emptyCache :: RequestCache req
lookupCache :: IRequest req a => req a -> RequestCache req -> Maybe (MVar a)
insertCache :: IRequest req a => req a -> MVar a -> RequestCache req -> RequestCache req

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class Fetchable req where
  fetch :: req a -> IO (MVar a)

type Cachable req a = (Eq (req a), Hashable (req a))
type IRequest req a = (Fetchable req, Cachable req a)

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class Monad m => WithProfileInfo m where
  friendsOf :: ProfileId -> m [ProfileId]
  favoriteTopicsOf :: ProfileId -> m (Set Topic)
  lastPostsOf :: ProfileId -> m [BlogPost]

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suggestedPostsFor :: (WithProfileInfo m) => ProfileId -> m [BlogPost]
suggestedPostsFor userId = do
    friendIds <- friendsOf userId
    topics <- favoriteTopicsOf userId
    friendsPosts <- forM friendIds $ \friendId -> do
        posts <- lastPostsOf friendId
        return (filter (isAbout topics) posts)
    return (mostLiked 3 (concat friendsPosts))

mostLiked :: Int -> [BlogPost] -> [BlogPost]

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algorithm1.suggestedPostsFor(new ProfileId(1)); // First algorithm
algorithm2.suggestedPostsFor(new ProfileId(2)); // Second algorithm

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algorithm1.suggestedPostsFor(new ProfileId(1));
algorithm1.suggestedPostsFor(new ProfileId(2));

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class SuggestPopularFriendPost {
    private final IAccessProfileInfo profileInfo;
    private final ExecutorService executor; // Where would it come from?

    private Future<Iterable<ProfileId> friendsOf(ProfileId profileId) {
        return executor.submit(() -> profileInfo.friendsOf(profileId));
    }

    // ... remaining code ...
}

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template<typename T>
struct Node {
  T content;
  Node<T>* next;
};

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public interface IAccessProfileInfo {
    Future<Map<ProfileId, Iterable<ProfileId>>> friendsOf(Iterable<ProfileId> profileIds);
    Future<Map<ProfileId, Set<Topic>>> favoriteTopicsOf(Iterable<ProfileId> profileIds);	
    Future<Map<ProfileId, Iterable<PostSummary>>> lastPostsOf(Iterable<ProfileId> profileIds);
}