# Pastebin ml4glWWt
-- type SourceEnv t = (Text, Behavior t (Map (Int, SomeTypeRep) Bool))

instance (MonadIO m, MonadIO (HostFrame t), ReflexHost t, Reflex t, Ref m ~ IORef) => HasSource t js (
  SourceT t js (HydratableT (PostBuildT t (StaticDomBuilderT t (PerformEventT t m)) ))
  ) where

  fetchData :: forall query. (Typeable query, Fetch query, Hashable (Args query), FromJSON query) => Event t (Args query) -> SourceT t js (HydratableT (PostBuildT t (StaticDomBuilderT t (PerformEventT t m)) )) (Event t (Either String query))
  fetchData queryE = do

    (_endpoint, cache) <- ask

    lookupCache :: Map (Int, SomeTypeRep) SomeTypeRep <- sample cache

    performEvent $ do
      coincidence $ ffor queryE $ \(args) -> do
        case Map.lookup (hash args, someTypeRep (Proxy :: Proxy query)) lookupCache of
          Just (rep) -> case typeRep @query `eqTypeRep` (typeOf rep) of
            Just HRefl -> (pure $ Right rep) <$ queryE
            Nothing    -> fetch xhrFetch <$> queryE
          Nothing         -> fetch xhrFetch <$> queryE

    where

      xhrFetch queryBS = do

        runReq defaultHttpConfig $ do
          r <-
            req
            POST -- method
            (http "graphql.localhost") -- safe by construction URL
            (ReqBodyLbs queryBS) -- use built-in options or add your own
            lbsResponse -- specify how to interpret response
            (port 3000) -- query params, headers, explicit port number, etc.
          pure $ responseBody r