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Re: Document Object Model (DOM) Level 3 XPath Specification

From: Ray Whitmer <rayw@netscape.com>
Date: Wed, 27 Jun 2001 07:52:47 -0700
Message-ID: <3B39F33F.7050803@netscape.com>
To: Bob Foster <bob.foster@webgain.com>
CC: www-dom@w3.org
Bob Foster wrote:

>Very nice to see this! As an implementor of a DOM-based XPath, it is always
>nice to think that someday I will be able to stop maintaining it. I tried to
>make my comments brief, but there are 10 of them.
>
>1) [1.2.1 Text Nodes] This is very imprecise. Does the section describe the
>behavior of the text() node test or are there other incompatibilities? Each
>incompatibility should be specifically identified.
>
I do not understand your confusion.  As I attempt to respond, I write 
exactly what is there, so I don't know how I can clarify (without 
further feedback).  We are talking about the actual DOM nodes returned 
from an xpath expression, and what to do when XPath says there should be 
a single node, having discarded information, but DOM has fragmented it 
across multiple nodes.  Answer: in case of fragmentation, return just 
the first node.

>2) Irrespective of the data model differences the XPath text() node test is
>defined as true for all text children, not for "the text child" and
>certainly not for "the first text child". Arbitrarily redefining text() in
>this way fixes nothing and introduces further incompatibilities. If the
>definition of text() is left alone a normalized DOM document written without
>CDATA sections should be processed compatibly.
>
I do not think we are talking about a boolean XPath function here.  We 
are talking about the actual nodes enumerated in the returned set, how 
to get 1:1 mapping when there is no 1:1 mapping in the nodes themselves.

>3) If you are going to add a method to the DOM, it would be far better to
>introduce a variant of normalize() that coalesces adjacent text (that is,
>Text and CDATA) nodes exactly as described in the XPath specification.
>
While such a normalize function is a good idea, it is not sufficient in 
cases where stripping out all CDATASections and EntityReferences is not 
an option.  It is also questionable whether XPath inquiries should only 
function correctly on a completely normalized tree.

A big part of the use of XPath in DOM is to find nodes in the document 
so that they can be manipulated and later written out.  One of the big 
differences between DOM and XPath is that XPath affords itself the 
luxury of considering the document read-only, so CDATA or Entity 
References are not preserved.  DOM, on the other hand, has to worry in 
many cases about writing the document out.  Some users care whether 
CDATASection and EntityReference nodes are lost.  Otherwise, they never 
would have been included in DOM, and we could stop worrying about them.

IMHO those specifications which rely on the XPath data model should 
expect this to only be the tip of the iceberg of issues if they suddenly 
want to mutate the data (for example updating a database that has been 
queried) since XPath's model has never worried about modifying the 
hierarchy nodes.  This is where ActiveNodeSet and a number of other DOM 
things come into play.

>4) [Interface XPathEvaluator] There should be a single evaluate() method
>returning an Object. The number of possible types from XPath 2.0 expressions
>will make enumerating them unproductive; might as well get it right the
>first time. A general-purpose method may have no way to determine the type
>beforehand, and would need an inelegant switch statement to make use of the
>type if it knew it. I did it wrong in a similiar way myself thinking it
>would be a convenience for programmers before discovering that a) the
>Object-returning variant is needed in any case and b) in most cases the
>convenience amounts to the absence of a cast and, for nodeset values, a call
>to getFirst(). Programmers can provide this level of sugar for themselves.
>
While enumerating all types is impossible for XPath 2.0, I still think 
there is no reason to in the common cases of String, integer, or boolean 
to force users to muck with untyped object returns and native coercion, 
when well-defined system primatives can be supported.

For XPath 2.0, we should probably consider adding:

Object evaluateAs(<type>, ...)

It is good to be able to ask the XPath evaluator to supply any available 
XPath coercion to whatever type is desired.  We will likely be adding a 
method to evaluate the type, so coercion can be avoided and switch 
statements are possible if people really want it.

>6) Some responses seem to think that the Node-returning variant is meant as
>a hint to XPath that at most one node need be returned. If this is the sly
>intention ("There is nothing to stop an XPath implementor from taking
>advantage...") it should be made explicit. I agree that this is a common
>case and a useful optimization (you can slap a /.[1] at the end of any node
>locator, but you can't stop most XPath implementations from grinding out and
>testing all n nodes). It just shouldn't sneak in the back door.
>
The call is not a "hint", but a very clear statement that one node is 
requested.  The implementation may or may not be "sly" about only 
computing the first node, just as it may be sly enough to evaluate 
ActiveNodeSets incrementally as the caller requests additional nodes, in 
which case this method wasn't needed to avoid computation.  There are 
quite a few gains, and some of the best ones have to do with not 
returning a NodeSet, and are just as true of evaluateAsString, for 
example.  It is not clear to me that we should spend a lot of time in 
the specification discussing these implementation optimizations which 
may or may not be available.  Implementations will choose how sly they 
should be.

>7) [Interface ActiveNodeSet] For simplicity and concurrency reasons,
>ActiveNodeSet should be eliminated entirely in favor of StaticNodeSet.
>Without explicit synchronization of access to the DOM, the useful lifetime
>of an ActiveNodeSet cannot be determined. It is possible a returned instance
>might already be invalid.
>
>Obviously, if this were done some methods should transfer to StaticNodeSet,
>esp. getDocumentOrderedSet().
>
I disagree that it is simpler or better for concurrency to use 
StaticNodeSet.  It is often extremely complex to have to worry about 
what if the node set no longer matches the hierarchy.  With 
ActiveNodeSet, you can guarantee in the API that either it does or there 
is a programming exception.  This provides a much simpler failure than 
what results if you assume they match and really they do not.

I would eliminate StaticNodeSet before ActiveNodeSet, because the user 
can copy the list before resuming processing if he needs a static copy, 
rather than forcing implementations to do it on every call.  Static is 
just convenience (but very convenient for whenever the caller may be 
using the list to mutate the hierarchy).

Without some idea of how your program is synchronized, you cannot do 
much that is useful or reliable with the DOM.  How can you do an XSL 
transform, for example, if you don't know the document is not shifting 
underneath you.  The ActiveNodeSet guarantees that you are not 
processing a skewed static view of the data.  If you want a skewed 
static view of the data, then you request a static set, and you take 
responsibility for the additional upfront processing and allocation that 
may be required for a StaticNodeSet, and then you have to avoid making 
assumptions about the state or location of the nodes you are manipulating.

>8) [Interface StaticNodeSet] A getFirst() method defined as returning the
>first element or null would be a handy addition. A getOnly() method defined
>as getFirst() but throws if there is more than one node in the set might be
>even better.
>
I think it is clear that being able to get a single node is good.  It is 
not obvious to me why I would want to have a node set just to guarantee 
that I have a single node.  If we determined that raising an exception 
was a common use case where only 1 node was desired and more than one 
was returned, then perhaps what we really need is a flag on 
evaluateAsNode, saying whether to raise an exception if more than one 
node are matched.

Ray Whitmer
rayw@netscape.com
Received on Wednesday, 27 June 2001 10:48:39 GMT

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