Re: [Haskell-cafe] Backtracking in HXT

1 Jan 2013

      Ah, zeroArrow looks exactly like what I need here!

Thanks, this helps.

Mateusz Kowalczyk

On 01/01/13 01:08, dag.odenhall@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@gmail.com 
mailto:dag.odenhall@gmail.com mailto:dag.odenhall@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
    mailto:fuuzetsu@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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