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<div class="moz-cite-prefix">Hello,<br>
<br>
Thanks for the tip!<br>
<br>
I'm in fact using dom-selector:<br>
<a class="moz-txt-link-freetext" href="http://hackage.haskell.org/package/dom-selector">http://hackage.haskell.org/package/dom-selector</a><br>
which is based on xml-conduit and html-conduit. The reason being
that it offers CSS selectors and is generally much higher-level
than what I would do with parsec.<br>
<br>
So I'm not sure whether what you wrote applies.<br>
Actually your function doing the parsing here is not pure as such,
it's a do block and ordered. What I have done so far is that
dom-selector gives me the DOM structure of the page (so that
parsing part is done for me), and then I give to my function that
DOM structure and the examination of that DOM structure is
completely without a do block, it's not ordered, it's pure. In
that way my "parsing" (really examination of the DOM tree) is
completely split of any IO or other monad.<br>
<br>
I think when you are within parsec as you mentioned, you are
within the parsec monad (bear in mind I don't really understand
all of this for now), and to do IO you need to go to the IO monad,
and for that you use liftIO. In that case that's another problem
than the one I'm having.<br>
<br>
Emmanuel<br>
<br>
On 12.10.2012 15:39, David McBride wrote:<br>
</div>
<blockquote
cite="mid:CAN+Tr43_Dt8Ex54zKzhpdMi0hoPbNS0+BCep469UErb8CpuTAA@mail.gmail.com"
type="cite">There's a better option in my opinion. Use the monad
transformer capability of the parser you are using (I'm assuming
you are using parsec for parsing).<br>
<br>
If you check the hackage docs for parsec you'll see that the
ParsecT is an instance of MonadIO. That means at any point during
the parsing you can go liftIO $ <any IO action> and use the
result in your parsing. Here's an example of what that would
might look like.<br>
<br>
import Control.Monad.IO.Class<br>
import Control.Monad (when)<br>
import Text.Parsec<br>
import Text.Parsec.Char<br>
<br>
parseTvStuff :: (MonadIO m) => ParsecT String u m (Char,Maybe
())<br>
parseTvStuff = do<br>
string "tvshow:"<br>
c <- anyChar<br>
morestuff <- if c == 'x'<br>
then fmap Just $ liftIO $ putStrLn "run an http request, parse
the result, and store the result in morestuff as a maybe"<br>
else return Nothing<br>
return (c,morestuff)<br>
<br>
So you will run an http request if you get back something that
seems like it could be worth further parsing. Then you just parse
that stuff with a separate parser and store it in your data
structure and continue parsing the rest of the first page with the
original parser if you wish.<br>
<br>
<div class="gmail_quote">On Fri, Oct 12, 2012 at 9:28 AM, Emmanuel
Touzery <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:etouzery@gmail.com" target="_blank">etouzery@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi,
<div class="im"><br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
when parsing the string representing a page, you could<br>
save all the links you encounter.<br>
<br>
After the parsing you would load the linked pages and
start<br>
again parsing.<br>
<br>
You would redo this until no more links are returned or a<br>
maximum deepness is reached.<br>
</blockquote>
<br>
</div>
Thanks for the tip. That sounds much more reasonable than what
I mentioned. It seems a bit "spaghetti" to me though in a way
(but maybe I just have to get used to the Haskell way).<br>
<br>
To be more specific about what I want to do: I want to parse
TV programs. On the first page I have the daily listing for a
channel. start/end hour, title, category, and link or not.<br>
To fully parse one TV program I can follow the link if it's
present and get the extra info which is there (summary,
pictures..).<br>
<br>
So the first scheme that comes to mind is a method which takes
the DOM tree of the daily page and returns the list of
programs for that day.<br>
<br>
Instead, what I must then do, is to return the incomplete
programs: the data object would have the link filled in, if
it's available, but the summary, picture... would be empty.<br>
Then I have a "second pass" in the caller function, where for
programs which have a link, I would fetch the extra page, and
call a second function, which will fill in the extra data
(thankfully if pictures are present I only store their URL so
it would stop there, no need for a third pass for pictures).<br>
<br>
It annoys me that the first function returns "incomplete"
objects... It somehow feels wrong.<br>
<br>
Now that I mentioned my problem with more details, maybe you
can think of a better way of doing that?<br>
<br>
And otherwise I guess this is the policy when writing Haskell
code: absolutely avoid spreading impure/IO tainted code, even
if it maybe negatively affects the general structure of the
program?<br>
<br>
Thanks again for the tip though! That's definitely what I'll
do if nothing better is suggested. It is actually probably the
best way to do that if you want to separate IO from "pure"
code.<span class="HOEnZb"><font color="#888888"><br>
<br>
Emmanuel</font></span>
<div class="HOEnZb">
<div class="h5"><br>
<br>
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</blockquote>
</div>
<br>
</blockquote>
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