Application package:yesod-core

type Application = Request -> Response -> IO ResponseReceived -> IO ResponseReceived
yesod-core Yesod.Core
The WAI application. Note that, since WAI 3.0, this type is structured in continuation passing style to allow for proper safe resource handling. This was handled in the past via other means (e.g., ResourceT). As a demonstration: 
app :: Application
app req respond = bracket_
(putStrLn "Allocating scarce resource")
(putStrLn "Cleaning up")
(respond $ responseLBS status200 [] "Hello World")
sendWaiApplication :: MonadHandler m => Application -> m b
yesod-core Yesod.Core.Handler
Switch over to handling the current request with a WAI Application.