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Re: [Accumulators] Another stupid question

Subject: Re: [Accumulators] Another stupid question
From: "Michael Kay mike@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Tue, 6 Oct 2020 22:33:43 -0000
Re:  [Accumulators] Another stupid question
It's fundamental to accumulators that the value of an accumulator at a
particular node in a document is a pure function of the preceding nodes in
that document (or in the case of an "after" value, the descendant nodes), and
nothing else (other than constant data such as global variables). You can't
inject values into this calculation in the way you are trying to do.
Accumulators were invented, of course, for streaming, and represent "things
that are remembered from earlier in the stream".

Your private:block element is matched by a different "accumulator instance"
from the one that matches the page element; an accumulator instance
corresponds to one document, and accumulator instances for different documents
don't interact. Otherwise, there would be a terrible dependency on order of
processing.

Saxon evaluates unstreamed accumulators lazily: the first time you evaluate
accumulator-before, or accumulator-after, on any node in a particular
document, the accumulator values are computed for all nodes in that document.

Michael Kay
Saxonica


> On 6 Oct 2020, at 21:58, Christophe Marchand cmarchand@xxxxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello !
>
> I have another question with accumulators. I use accumulators to calculate
block height, and then to calculate block locations (y).
>
> I have this XSL for a repro...
>
> <?xml version="1.0" encoding="UTF-8"?>
> <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
>   xmlns:xs="http://www.w3.org/2001/XMLSchema"
>   xmlns:private="top:marchand"
>   exclude-result-prefixes="#all"
>   version="3.0">
>
>   <xsl:output method="xml" indent="true"/>
>
>   <xsl:mode on-no-match="shallow-copy" use-accumulators="#all"/>
>
>   <xsl:accumulator name="blockHeight" as="xs:double" initial-value="0.0">
>     <xsl:accumulator-rule match="page">
>       <xsl:message>Reset block height to 3.2</xsl:message>
>       <xsl:sequence select="3.2"/>
>     </xsl:accumulator-rule>
>     <xsl:accumulator-rule match="private:block">
>       <xsl:message>Adding block height {@id} : {@height}</xsl:message>
>       <xsl:sequence select="$value + xs:double(@height)"/>
>     </xsl:accumulator-rule>
>   </xsl:accumulator>
>
>   <xsl:template match="block">
>     <xsl:variable name="height" as="xs:double" select="3.5"/>
>     <!-- produce some output -->
>     <xsl:variable name="TempTree" as="element(private:block)">
>       <private:block height="{$height}"/>
>     </xsl:variable>
>     <xsl:apply-templates select="$TempTree"/>
>     <xsl:copy>
>       <xsl:attribute name="y" select="accumulator-after('blockHeight')"/>
>       <xsl:apply-templates select="node() | @*"/>
>     </xsl:copy>
>   </xsl:template>
>
>   <xsl:template match="private:block">
>     <xsl:message>private:block matched</xsl:message>
>   </xsl:template>
>
>   <xsl:template name="xsl:initial-template">
>     <xsl:variable name="content" as="document-node()">
>       <xsl:document>
>         <doc>
>           <page id="p1">
>             <block id="b1">
>               <!-- arbitrary content -->
>             </block>
>             <block id="b2">
>               <!-- another arbitrary content -->
>             </block>
>           </page>
>           <page id="p2">
>             <block id="b3">
>               <!-- arbitrary content -->
>             </block>
>             <block id="b4">
>               <!-- another arbitrary content -->
>             </block>
>           </page>
>         </doc>
>       </xsl:document>
>     </xsl:variable>
>     <xsl:apply-templates select="$content"/>
>   </xsl:template>
> </xsl:stylesheet>
>
> When I run this XSL on the sample XML, I have messages displayed : "Reset
block height to 3.2", "private:block matched", but never the "Adding block
height..."
>
> According to specification [1], I expect private:block matches accumulator
rule, and accumulator value being augmented by block/@height.
>
> What am I doing wrong ?
>
> Thanks in advance,
> Christophe
>
> [1] : 18.2.2 https://www.w3.org/TR/xslt-30/#applicability-of-accumulators
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