f.hs

exceptionJust :: Exception e 
              => (e -> Maybe b)
              -> (b -> IO n)
              -> (r -> IO n)
              -> IO r
              -> IO n           
exceptionJust p e h a = either pure h =<< catchJust p (fmap Right a) (fmap Left <$> e)

handler :: Int -> PendingConnection -> IO ()
handler pid pending = handle uncaught $ do
    exceptionFileNotExist manifestNotFound
                          acceptWithManifest
                          (decodeManifest pid)
  
  where
    acceptWithManifest manifest = handleManifest manifest =<< acceptRequest pending
    manifestNotFound _ = rejectRequestWith defaultRejectRequest{ rejectCode = 404 }
    uncaught _ = rejectRequestWith defaultRejectRequest{ rejectCode = 500 }
    exceptionFileNotExist =  
      exceptionJust (\f -> if isDoesNotExistError f then Just f else Nothing)

handleManifest :: Manifest -> Connection -> IO ()
handleManifest = ....