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Re: XML Encoding of XPath: Examples

• From: Wayne Steele <xmlmaster@h...>
• To: vdv@d..., xml-dev@l...
• Date: Thu, 15 Mar 2001 11:16:17 -0800

>From: Eric van der Vlist <vdv@d...>
>To: xml-dev@l...
>Subject: Re: XML Encoding of XPath: Examples
>Date: Thu, 15 Mar 2001 17:58:32 +0100
>
>Wayne Steele wrote:
> >
> > I propose an optional XML Encoding for XPath expressions.
> > I just posted a DTD; here's some usage examples to help clarify things.
> >
> > Questions/Comments/Errata welcome.
>
>It's a neat proposal!

Thank you.

>I have some questions:
>
>Having used HP polish notation a lot in my early days, I would have
>thought of an XPath expression as a stack and would have embedded the
>location steps instead of keeping them at the same level...

That was my original thought.
Modeling Location Steps as functions, and using a prefix notation.
I ended up with something like
   child::foo/attribute::bar
looking like
   <attribute name="bar">
      <child name="foo">
         <contextNode />
      </child>
   </attribute>

I didn't like the need to specify where the initial context is resolved, 
somewhere deeply nested in the XML.

I couldn't think of a clean way to handle predicates.

I also noticed that the 'child' node above would never have any siblings, so 
nesting didn't seem to really express the logical structure of XPath.

XPath defines a Location Path expression as a _series_ of location steps.
Each one operates on the output of the previous one.

A flat sequence of elements seems like a good way to express that, as well 
as keeping the 'look' of XPath.

It also lets me reserve child elements for predicates, without having to 
create a <predicate> element to muddy the waters.


>Can you give examples of location steps within conditions ?
>(such as child::chapter[section/title="XML Encoding of XPath"])

I think you've got me there.
The examples I gave all have (at most) a single-step path expression inside 
a predicate.

Perhaps I need a wrapper element for an XPath Location Path.
Then I could do:
<location-path>
   <child name="chapter">
     <eq>
        <location-path>
           <child name="section"/>
           <child name="title"/>
        </location-path>
        <literal>XML Encoding of XPath</literal>
      </eq>
   </child>
</location-path>

This would also give you something to use for the documentElement of an 
XPath expression, so they can be always well-formed.


>Wouldn't it be a good idea to extend it to encode XPointer ?

Yes.
One thing at a time.

>Eric
>
> > -Wayne Steele
> >

Thanks for your comments.

I dare anyone to give me an XPath expression I cannot encode this way.
Or alternately, an XML Encoded XPath document that doesn't have an 
unambiguous XPath syntax.

-Wayne Steele
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