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Re: DOM Xpath comments

From: Curt Arnold <carnold@houston.rr.com>
Date: Thu, 1 Nov 2001 20:53:32 -0600
Message-ID: <001701c16349$8fd38110$7600a8c0@CurtMicron>
To: <www-dom@w3.org>
> Actually EntityReferences doesn't exist in the XPath data model, so I
> it's puposly left out. However the entityreferences children are inlined
> (and merged with andjecent textnodes) so _they_ still exist in the tree.
> EntityReference it self doesn't exist though.

Entity References are normalized out (as are CDATASections) of the XPath
data model, but they may be represented as distinct nodes in a DOM.  If I
have a fragment like:

<Description>&boilerplate;<!CDATA[[ >>>]]>Some Text</Description>

and entity references and CDATASections are not normalized, then I don't
have a single node that corresponds to Description/text(), so DOM XPath
allows any of child nodes of Description to represent the entire XPath text
node.  However, Entity Reference's were not listed in the list of acceptible
context nodes, so if you passed descriptionElement.firstChild as the context
node, the query would execute if entity references were expanded, but fail
if they were preserved.

> This is exactly how TransforMiiX is designed (the XSLT/XPath engine used
> mozilla). We have one common interface similar to XPathResult and separate
> implementations for each result type.
> What we have had to do (Peter van der Beken has made a preliminary
> implementation of DOM XPath) is to have a wrapper that holds an internal
> object which can be of any result type. However this defeats the entire
> purpose of reuse it's only the wrapper that gets reused, the internal
> is dropped and recreated on every evaulation.
> One way to help this situation without dropping support for reuseing of
> XPathResult objects would be to allow the implementations to return a new
> XPathResult if it is not able to reuse the supplied XPathResult. OTOH this
> might create interopability problems.

I've just got to think that the true performance benefit of explicit
XPathResult recycling is either negligible or imaginary.

If you have a preliminary implementation, we could look at starting the test
suite.  Is it accessible from ECMAScript in your experiemental build.

> I think that this sounds like a really good idea. We are just about to
> our XPath engine to produce sorted nodeset instead of unsorted ones, since
> much time can be saved if the nodeset is sorted during evaluation rather
> then after. This save would be totally lost if we are not told if the
> nodeset is to be sorted until after evaluation.

The ORDERED / UNORDERED were about preserving document order in a query
since fabricating a document order in a datastore that doesn't have a
structural order (like a query against a database) might be expensive or
multiple threads could be querying the tree and the first found might not be
the first in document order.

I had mentioned in an earlier post but forgotten that a sort parameter
(along the lines of xsl:sort) in an XPathExpression would be greatly
desired.  If such a sort parameter were added, then UNORDERED would skip
evaluation of the sort clause.

> While I don't like the getSetIterator or getSetSnapshot functions either,
> don't think putting it all in the same interface is ideal either. I
> that XPathSetSnapshot and XPathSetIterator iterator inherit XPathResult:

There seemed to be a design philosophy to avoid interface coersion, hence
the putting all the result types in one monolythic XPathResult set.  If
avoiding interface casting was not a design goal, then the appropriate
approach is to put all type specific functions into distinct interfaces,
like :

interface XPathResult
readonly attribute short       type;

interface XPathResultNumber : XPathResult
readonly attribute double numberValue;

interface XPathResultString : XPathResult
readonly attribute DOMString stringValue;

interface XPathResultBoolean : XPathResult
readonly attribute boolean booleanValue;

> interface XPathSetIterator : XPathResult {
>   Node               nextNode()
>                                         raises(DOMException);
     readonly attribute boolean valid;
> };
> interface XPathSetSnapshot : XPathResult {
>   Node               item(in unsigned long index);
>   readonly attribute unsigned long   length;
> };

It seems like an interface coercion has to be cheap compared to an XPath
evaluation, so the desire to have one big interface may be misguided.

> On a separate issue, I think that XPathSetIterator need some way to detect
> if it still is valid without causing an exception to to be thrown. I
> that an isStillValid() function is added to the iterator. The function
> returns a boolean indicating if the nodeset is still valid. The function
> returns false if a call to nextNode() would throw an INVALID_STATE_ERR
> exception. The function does not indicate if a subsequent call to
> would return a non-null value or not.

I'd agree on that, though probably a "valid"  boolean attribute on
XPathSetIterator would be better than a method.
Received on Thursday, 1 November 2001 21:53:45 GMT

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