RE: [xsl] Limitation of <xsl:result-document> in <xsl:function> in XSLT2

["Michael Kay" <mhk@xxxxxxxxx>]

> Isn't this splitting hairs? To me, there is no real 
> distinction between
> 
> <xsl:template match="foo">
>      <xsl:call-template name="outputBar">
>             <xsl:with-param name="filename" select="'foobar.xml'"/>
>             <xsl:with-param name="data" select="."/>
>      </xsl:call-template>
> </xsl:template>
> 
> <xsl:template name="outputBar">
>             <xsl:param name="filename"/>
>             <xsl:param name="data"/>
>             <xsl:result-document href="{$filename}">
>                 <xsl:copy-of select="$data"/>
>             </xsl:result-document>
> </xsl:template>
> 
> and
> 
> <xsl:template match="foo">
>      <xsl:value-of select="outputBar('foobar.xml',.)"/>
> </xsl:template>
> 
> <xsl:function name="outputBar">
>             <xsl:param name="filename"/>
>             <xsl:param name="data"/>
>             <xsl:result-document href="{$filename}">
>                 <xsl:copy-of select="$data"/>
>             </xsl:result-document>
>             <xsl:result></xsl:result>
> </xsl:function>
> 
> Neither produces primary stream output, both induce 
> side-effects. 

The difference is that the first one is allowed, the second one isn't,
and the fact that the second one isn't allowed enables optimizations
that are not possible for the first.

Consider this:

<xsl:template match="x">
  <xsl:variable name="t" select="outputBar(...)"/>
</xsl:template>

Would you want to say that the variable must be evaluated, even though
it is never referenced, because the expression it is bound to might have
side-effects? Or would you want to say that it is implementation-defined
whether the secondary output file gets written or not? The working group
chose to keep things interoperable by banning this kind of side-effect.

Yes, it's splitting hairs. That's what you have to do when you specify
programming languages.

Perhaps one day we'll find a way of managing the side-effects in a
controlled way along the lines that Haskell uses. In the meantime, we
prefer to be strict. xsl:result-document is designed as a way of
producing multiple result documents, not as a way of breaking the
no-side-effects rule of XSLT.

Michael Kay
Software AG
home: Michael.H.Kay@xxxxxxxxxxxx
work: Michael.Kay@xxxxxxxxxxxxxx 



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