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changing final output while writing temporary tree

From: Andre Cusson <ac@hyperbase.com>
Date: Sun, 26 Jan 2003 15:47:06 -0500
To: public-qt-comments@w3.org
Cc: michael.h.kay@ntlworld.com, saxon-help@lists.sourceforge.net
Message-id: <5.2.0.9.0.20030126112616.032e3d48@pop.hyperbase.com>

Hi,

I would like to make a case for the removal of the restriction imposed on 
the result-document instruction when writing to a temporary result tree.

The proposal states:

[ERR146] It is a dynamic error to evaluate the xsl:result-document 
instruction when the current destination node is not a node belonging to a 
result tree (for example, when it is a node belonging to a temporary tree). 
The processor must signal the error. . This means, for example, that it is 
an error to use xsl:result-document when the tree containing the current 
destination node is a temporary tree created using xsl:variable, or a tree 
created using xsl:message, xsl:attribute, etc

and, as I understand, the issue that brought this restriction was related 
to limiting side-effects and a typical case was stated as:

But let's take a global variable

<xsl:variable name="x">
   <xsl:result-document ...>
     <a/>
   </
</

The variable is never referenced. Should the result document be output, or 
not?

There are a few alternatives to answer this question, but restricting 
result-document is not the right one because it is much to restrictive.
In fact it blocks recursion for serialization of documents which I would 
think is one of the last things to do in a functional environment.

XSLT is functional programming and functional programming is recursion for 
recursive structures. Blocking recursion is defeating the main purpose.

Here is a brief summary of the issue.

First a trivial example where a "Cannot switch to a final result 
destination while writing a temporary tree" error on the line marked with 
an x is generated when the processor (ex: Saxon v7.3.1 reaches the 
recursive call to 'publish' and executes the 'make-page' template from there.

<xsl:output name="outputf-format" .../>

<xsl:template name="make-page">
         <xsl:param name="src-nodeset"/>
         <xsl:param name="fname"/>
x       <xsl:result-document format="output-format" href="{$fname}">
                 ...
                 <xsl:copy-of select="$src-nodeset"/>
                 ...
         </xsl:result-document>
</xsl:template>

<xsl:template name="publish">
         <xsl:param name="src-nodeset"/>
         <xsl:for-each select="$src-nodeset/section">
                 ...
                 <xsl:call-template name="make-page">
                         <xsl:with-param name="fname" select="@id"/>
                         <xsl:with-param name="src-nodeset">
                                 ...
                                 <xsl:for-each select="sub-section-group">
                                         <xsl:call-template name="publish">
                                                 <xsl:with-param 
name="src-nodeset" select="."/>
                                         </xsl:call-template>
                                 </xsl:for-each>
                         </xsl:with-param>
                 </xsl:call-template>
         <xsl:for-each>
</xsl:template>

Writing a secondary document is not a side effect, it is simply dealing 
with (serializing to) the real world where information is mapped to finite 
flat files like web pages.
It seems to us like a major impediment if a style sheet is limited to 
outputting a single page at a time, without recursion.

We have here an application with 10 to 20K lines of XSLT, processing 
millions of lines of streaming and static XML into infinitely complex 
documents for printed and interactive access.  There are many issues 
involved and performance is one of them.  One of the keys is to stream and 
process graphs and structures efficiently.  Recursion and not having to 
reprocess the same (often very large) structures iteratively is crucial.

So as one traverses such structures, gathering data and building a page, 
and one finds information there that warrants a sub page (and so on 
recursively), one needs to also produce those pages.  With the current 
restriction, the only alternative is to iteratively process and reprocess 
the structure(s), building multiple temporary trees and variables, 
recalculating values and node sets until the full depth of the 
structure(s)I has been reached and all sub-pages at all levels of the 
recurring structure have been generated. This is using iteration to deal 
with recursion.  It is not only (very) inefficient and awkward (and not 
streamable), it is plain ugly.

The example code here was purposely simple to show where the error message 
occurred but we do have real world examples and they are far more 
intricate.  This feature is required by a few major modules in our 
application (ex: publishing, O-R mapping, invoicing, etc.).  We built this 
application with Saxon v6.5.2 that complied to the XSLT 1.1  draft which 
defined a "document" instruction that did not have this restriction on 
changing the final output document while writing to a temporary tree.

Our application works fine with Saxon v6.5.2 (XSLT v1.1) but cannot be 
ported to Saxon 7.3.1 (XSLT v2.0) for this single reason.
This is a major limitation to us.  But we also feel that it is a major 
limitation for XSLT.

Of course side-effects should be avoided but preventing recursion (for 
serialisation) is not the right way, because the cost is to high and out 
weights the benefits.

As for the unused global variable with a result-document in the initial 
example above, I tend to believe that if the variable is unused, the 
document should probably not be output and the processor should probably 
signal that the variable is not used (and (possibly) that the document will 
not be output).  Other solutions could also be acceptable to us, including 
outputting the document anyway or even, as was suggested, that the document 
be output only if the temporary tree gets copied to the final result tree.

We would be happy to contribute in any way on the resolution of this issue 
(changing final output while writing a temporary tree) so do not hesitate 
to contact us.

Thank you.
Regards,
Andre Cusson
ac@hyperbase.com
Tel: (1) 514 984 1601
01 Communications Inc.
11840 Norwood Avenue
Montreal, QC, H3L 3H7
Received on Sunday, 26 January 2003 15:38:42 GMT

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