[Haskell-cafe] Backtracking in HXT

Tue Jan 1 02:08:00 CET 2013

(">>>" is my GHCi prompt, might be a bit confusing for an arrow example.)

On Tue, Jan 1, 2013 at 2:05 AM, dag.odenhall at gmail.com <
dag.odenhall at gmail.com> wrote:

> Use arrow notation and zeroArrow, like so:
>
> {-# LANGUAGE Arrows #-}
> import Text.XML.HXT.Core
> getA =
>   hasName "a" >>> proc elem -> do
>     text <- getText <<< getChildren -< elem
>     if text == "Hello Two!"
>       then getAttrValue "href" -< elem
>       else zeroArrow -< ()
>
> >>> runX $ readString  [] "<html><a href='example.com/somelink.html'>Hello
> One!</a><a href='example.com/anotherlink.html'>Hello Two!</a></html>" >>>
> deep getA
> ["example.com/anotherlink.html"]
>
>
>
> On Mon, Dec 31, 2012 at 3:36 PM, Mateusz Kowalczyk <
> fuuzetsu at fuuzetsu.co.uk> wrote:
>
>> Hello,
>> I'm not sure if this is the right place to ask this but here goes.
>>
>> I'm currently working with HXT and I quite like it. There's one issue I
>> can't solve (at least now without some dirty, dirty hacking) and I'm
>> sure it's fairly simple.
>>
>> Consider the following mark-up
>>
>> <a href="example.com/somelink.**html <http://example.com/somelink.html>">Hello
>> One!</a>
>> <a href="example.com/anotherlink.**html<http://example.com/anotherlink.html>">Hello
>> Two!</a>
>>
>> Now, what I'm trying to achieve is to get the href based on the text. I
>> can test for what the text is by traversing <a>s, then using the
>> getChildren >>> getText arrow. What I can't figure out is how to check
>> the text and then get the href as by the time I get to the text, I'll
>> further down the tree!
>>
>> I imagine it would look something along the lines of
>>  (getChildren >>> getText <?*> "Hello Two") `guards` getAttrValue
>> "href". <?*> is just a random operator I made up for illustration
>> purposes that works as a predicate over arrows.
>>
>> Is this the right approach? Is there a built-in that already does what I
>> want? It
>> seems like a common task...
>>
>> Mateusz Kowalczyk
>>
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>>
>
>
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