Haskell wai middleware - how to run application if 404 is returned from the first application?

Question

I'm trying to 'join' two wai Applications together. Essentially the first app will either serve a 404 Not found response or a response that is not a 404 Not found response. I'd like to add a second application that will attempt to process the request if the first case (first application returns a 404) is present.

However looking at the type of Application:

type Application = Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived

It's not clear how to inspect the response? As wouldn't I just end up with ResponseRecived as the end result - which does not seem to contain any response code. As seen below:

xyzMiddle :: (Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived) ->
             (Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived)
xyzMiddle app req respond = do
  zzz <- (app req respond)
  pure zzz

How could I implement this? And or, what am I failing to understand in the above?

Answer 1

I overlooked this (Response -> IO ResponseReceived) as that is in the IO context - this is where we can run the second application. Which means, to inspect the Response we can do something like:

xyzMiddle :: (Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived) ->
             Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived
xyzMiddle app req respond = app req (\r -> do
  print $ show $ responseStatus r
  respond r)