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RE: Question regarding order in XPath

From: Murali Mani <mani@CS.UCLA.EDU>
Date: Fri, 21 May 2004 22:38:10 -0700 (PDT)
To: Michael Kay <mhk@mhk.me.uk>
Cc: www-ql@w3.org
Message-ID: <Pine.GSO.4.58.0405212225270.12315@panther.cs.ucla.edu>


Thanks. I like especially the fact that you can really get to what I am
trying to say, and correct some XQuery specific syntax issues..

Anyways, I see your reasoning, and I do understand them (I believe..). I
think document order makes it easier.. we can probably add other order
using other constructs (probably outside XPath)..

However, I am not sure why we should always try to translate // to
something like descendant-or-self::node()/child; why not define semantics
for p//text () as -- all nodes in the document in document order with 2
predicates -- check if they are descendants of p, check if they are text
nodes.. then we will not switch off the computer !!??

Anyways, it is good to note that the WG considered both "document-order"
and "sequence-mapping"..

best, murali.

On Fri, 21 May 2004, Michael Kay wrote:

>
> >
> > My concern is regarding the statement that "even reverse axes
> > will return
> > results in document order". Let us consider something like:
> >
> > a [2 TO 5]/b
>
> This query probably doesn't mean what you think it means. Firstly, the
> keyword has to be lower case. The value of the expression (2 to 5) is the
> sequence (2, 3, 4, 5), and the effective boolean value of a sequence of four
> integers is always true. I suspect you mean
>
> a[position() = 2 to 5]/b
>
> where the predicate is true for any a whose position is equal to one of the
> values in the sequence (2,3,4,5).
> >
> > Now suppose "a" uses a reverse axis. Now because of the range
> > predicate,
> > we will get only results that are produced by a, but have
> > positions 2 TO 5
> > in reverse document order, right??
>
> Yes.
> >
> > Now we again start getting results in document order when we start
> > navigating etc.. right??
>
> Yes.
> >
> > My question is: Why do we define order based on document order, rather
> > than the order of previous step??
>
> There are two separate questions here.
>
> Firstly, any path expression (that is, any expression using the "/"
> operator) returns results in document order, regardless of the order of the
> sequences in its operands.
>
> Secondly, an axis step (such as a[position() = 2 to 5]) always produces
> results in document order, regardless of the axis.
>
> The rationale is a long story. It is partly done this way in XPath 2.0 to
> provide compatibility with 1.0. But it is also done this way because if you
> do it differently you get some very strange anomalies especially for
> expressions that use the recursive axes such as descendant. There is some
> discussion of these points in Don Chamberlin's chapter of the "XQuery from
> the Experts" book.
>
> A lot of people from the database side of the fence wanted to make "/" into
> a sequence mapping operator, but this fails on documents with recursive
> structures. Recursive queries have traditionally been really difficult with
> languages based on predicate calculus, but they are routine with hierarchic
> data and with narrative XML in particular.
> >
> > The thing is document order and order based on previous step
> > are identical
> > for forward axes, reverse axis with a position predicate
> > (rather than a
> > range predicate) etc..
> >
> > Were these options for defining order discussed by the WG? and any
> > insights into the reasoning process would be greatly appreciated.
>
> They were discussed at immense length.
> >
> > best, murali.
> >
> > Note: Also, I cannot think of any use case where after a step using a
> > reverse axis, we want to revert back to document order (if
> > document order
> > and the "previous-step-order" yield different results)..
> >
> It's too late on a Friday night to go into all the detail. As I say, part of
> it is simply backwards compatibility. At XSLT 1.0 people quite reasonably
> write:
>
> <xsl:for-each select="ancestor::*">
>   <xsl:value-of select="name()"/>
>   <xsl:if test="position()!=last()">/</xsl:if>
> </xsl:for-each>
>
> and one can't really change the meaning of such an everyday construct in
> version 2 of a language.
>
> But you also get into all sorts of problems if you try to treat "/" as a
> sequence mapping operator, because of the recursive axes. The classic
> example was if you start with the element:
>
> <p>Do <b>not</b> switch off the computer, it will <b>explode</b></p>
>
> and then do
>
> <p><xsl:copy-of select="p//text()" /></p>
>
> then the result if you don't sort into document order is
>
> <p>Do switch off the computer, it will not explode</p>
>
> The WG also looked for ways of getting round this problem by making
> p//text() mean something other than
> child::p/descendant-or-self::node()/child::text(), but no alternatives
> actually worked.
>
> Michael Kay
>
Received on Saturday, 22 May 2004 01:38:45 GMT

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