On Jan 25, 2011, at 12:10 PM, Michael Snoyman wrote:
7. the ServerPartT monad has a built-in FilterT monad which makes it easy to apply a function (Response -> Response) to modify the response.
In all honesty, this is an argument *against* frameworks, not for them: at a WAI level, we don't need a FilterT monad, since an application is just a function.
(And I can't believe I'm giving arguments against frameworks here, I obviously think they're a good thing. I'm just trying to be fair.)
Sure, you can do everything at the low-level. The idea is to add value. Ultimately happstack calls this function, simpleHTTP'' :: (ToMessage b, Monad m, Functor m) => ServerPartT m b -
Request -> m Response
which is basically a function to convert a ServerPartT into a WAI- like, type Application = Request -> IO Response. :) The value added by the FilterT monad is that it allows you to apply a transformation to the Response before you actually have created the Response. For example, I may have a function that adds a session cookie. But I don't have to structure my code so that the function is run after I have generated the response. For example, lets say I have a function that sets a cookie and returns the response body as HTML: myPart :: ServerPart Html myPart = do addCookie someCookie return $ <html>...</html> I can't add the cookie directly to the Response yet, because this function returns Html not a Response. The cookie is added later after the Html is converted to a Response. I assume the WriterT (Endo [Header]) thing in yesod serves a similar purpose ? - jeremy