[Haskell-cafe] Backtracking in HXT

Tue Jan 1 14:25:11 CET 2013

Ah, zeroArrow looks exactly like what I need here!

Thanks, this helps.

Mateusz Kowalczyk

On 01/01/13 01:08, dag.odenhall at gmail.com wrote:
> (">>>" 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 
> <mailto:dag.odenhall at gmail.com> <dag.odenhall at gmail.com 
> <mailto: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
>     <http://example.com/somelink.html>'>Hello One!</a><a
>     href='example.com/anotherlink.html
>     <http://example.com/anotherlink.html>'>Hello Two!</a></html>" >>>
>     deep getA
>     ["example.com/anotherlink.html <http://example.com/anotherlink.html>"]
>
>
>
>     On Mon, Dec 31, 2012 at 3:36 PM, Mateusz Kowalczyk
>     <fuuzetsu at fuuzetsu.co.uk <mailto: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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