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Single Node XPath Evaluation Is Faulty

From: Dominic Chambers <dominic.chambers@bigfoot.com>
Date: Wed, 6 Nov 2002 10:35:19 +0100
Message-ID: <001401c285cf$b66fcd20$0100a8c0@hp>
To: "WWW DOM" <www-dom@w3.org>

This is a repost since the original mail apperars to have slipped through
unoticed. But I think this issue could be serious, so I am reposting.

> The DOM Level 3 XPath module allows single nodes to be selected,
> where the returned node can be either the first node in document
> order, or any node from the result set. The ability to select
> the first node in document order is certainly useful, but the
> second definition is definately not.
> The reason for this is that when a single node is selected
> from a node set, there is almost always a particular node in
> mind. If the node returned varies depending on the particular
> implementation used, then the method becomes worthless for
> implementation agnostic code.
> XPath itself does not specify how single nodes should be
> returned, since it does not cater for single node selections.
> XSLT and MSXML, however, both allow single node selections, and
> both provide them in a consistent manner.
> The node returned is always the first node in evaluation order.
> For example (in msxml), if I wanted to find the owner table of an
> arbitrary HTML element, I could query:
>    ancestor::table
> And this would give me the first ancestor element that was of
> type table. Similarly, if I wanted to find the next table row
> group in an HTML table, I could query:
>    following-sibling::tbody
> The XSLT spec defines this behaviour for it's xsl:value-of
> element <http://www.w3.org/TR/xslt#value-of>, but does not
> use the term 'evaluation order' specifically. It says:
>    The following [example] precedes each procedure element with
>    a paragraph containing the security level of the procedure.
>    It assumes that the security level that applies to a
>    procedure is determined by a security attribute on the
>    procedure element or on an ancestor element of the procedure.
>    It also assumes that if more than one such element has a
>    security attribute then the security level is determined by
>    the element that is closest to the procedure.
>    <xsl:template match="procedure">
>      <fo:block>
>        <xsl:value-of
>         select="ancestor-or-self::*[@security][1]/@security"/>
>      </fo:block>
>      <xsl:apply-templates/>
>    </xsl:template>
> Thanks, Dominic.
Received on Wednesday, 6 November 2002 15:06:23 UTC

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