[web-devel] Hoogle Advice

Michael Snoyman michael at snoyman.com
Tue Jan 25 19:45:44 CET 2011

On Tue, Jan 25, 2011 at 8:35 PM, Jeremy Shaw <jeremy at n-heptane.com> wrote:
> 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
> ?

Yes, precisely. As I see it, the difference between the two approaches would be:

* Yesod's is a little bit faster. And I mean a *little* bit.
* Happstack's is more powerful, in that you can do more than simply
append a header.

Would you concur?


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