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xsl:script and side-effects
- To: xsl-list at lists dot mulberrytech dot com
- Subject: [xsl] xsl:script and side-effects
- From: Joe English <jenglish at flightlab dot com>
- Date: Thu, 15 Feb 2001 08:50:55 -0800
- Reply-To: xsl-list at lists dot mulberrytech dot com
The introduction of <xsl:script> raises the possibility
that evaluating an XPATH expression may produce side-effects.
Since the XPATH and XSLT Recs are completely silent on
the issue of evaluation order -- as well they should be! --
this can lead to ambiguities.
For example, suppose you have:
<!-- I don't know ECMAScript, hopefully a Tcl example will be clear -->
<xsl:script implements-prefix="my" language="tcl">
set counter 0
proc incrementCounter {} { global counter; return [incr counter] }
</xsl:script>
<xsl:template match="SOME-ELEMENT">
<xsl:element name="COUNTER-VALUE"
><xsl:value-of select="my:incrementCounter()"
/></xsl:element>
</xsl:template>
It seems intuitive that this will "do the right thing" --
successive <COUNTER-VALUE> elements in the result tree
will contain successive integers -- but AFAICT nothing
in the specifications actually guarantees this.
Things get even more complicated in the presence of
(what used to be called) result tree fragments.
Suppose further you have:
<xsl:template match="/">
<xsl:variable name="x"
><xsl:apply-templates select="//SOME-ELEMENT"
/></xsl:variable>
<xsl:element name="RESULT">
<xsl:copy-of select="$x[1]"/>
<xsl:copy-of select="$x[4]"/>
</xsl:element>
</xsl:template>
and the source document is:
<ROOT>
<SOME-ELEMENT/>
<SOME-ELEMENT/>
<SOME-ELEMENT/>
<SOME-ELEMENT/>
<SOME-ELEMENT/>
</ROOT>
The output will look like:
<RESULT>
<COUNTER-VALUE>a</COUNTER-VALUE>
<COUNTER-VALUE>b</COUNTER-VALUE>
</RESULT>
but it's not at all clear what 'a' and 'b' will be.
I can think of plausible evaluation strategies which would
yield b=2, 3, 5, or 9, even in this simple case.
The XSLT Rec ought to at least mention this as an issue.
(There is a similar issue with the 'saxon:assign' extension
element, BTW.)
--Joe English
jenglish@flightlab.com
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