[web-devel] Yesod performance

Aur Saraf sonoflilit at gmail.com
Fri Apr 15 02:34:48 CEST 2011

First, the nice people at #haskell (specifically it was <sm>, but
there were many other helpful people there like <ksf> and others whose
names I won't remember) made me swtich from fastcgi to warp. I didn't
think it would be relevant, since my performance problems were all
memory-consumption-related, but it turned out to speed my code by
orders of magnitude. Everybody switch to warp! :-)

Second, I've been thinking about the performance bounds of a Yesod
app. As in, given an app that has no leaks etc', where can I expect it
to perform better than imperative languages I'm familiar with, where
can I expect it to perform worse?

The biggest limits imposed by Yesod that I thought about were:

* [Char] - not in Yesod 0.8
* Persistent always takes a full row - shouldn't matter much in
practice, especially if persistent is lazy and doesn't parse columns I
don't care about
* No joins in DB - being solved as we speak
* All page must be generated before it is sent if we plan to use Widgets --

  Well, this one is interesting. It seems to be very much against
Haskell nature, that is so lazy and nice, and I think in my code it
might actually make performance difference. So I thought -- isn't it
possible to extend Hamlet to work in an Iteratee too?

  What do I mean? Something like:

  rows <- selectEnum ...
  hamletAction [$hamlet|
    $for row <- rows
      <p> #{rowDescription row}
    <p .fineprint> (c) Aur Saraf etc' etc'

  That would expand to a monadic action that creates chunks of HTML
while reading the rows Enumerable.

  Is there any serious technological problem with this approach? Or
just choosing a nice extension of Hamlet that would allow running
monadic actions in it like functions are allowed today, without
affecting the simplicity of the API?


PS Michael, is it possible in Yesod to have two selectEnum-s "running"
at the same time? Otherwise you'd only be able to work with one query
at a time in this way...

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