Re: [xsl] Re: How to output open/close tags independently?

Subject: Re: [xsl] Re: How to output open/close tags independently?
From: Mitch Amiano <mitch.amiano@xxxxxxxxxxxxxxxxxxxx>
Date: Tue, 31 Dec 2002 19:18:56 -0500
Actually, I thought the recursive approach was entirely reasonable yet
found an obvious performance issue on my own system, only to have that small data point wiped out by someone else with a single counterexample.


Good, bad, or ugly, snobbish, purist, or whatever, your milage is going
to vary. Sometimes I do end up working around the behavior of a tool rather than working with it, but try to minimize that because it usually makes stuff break later, when I'm not looking, and in wildly unexpected ways.


- Mitch The suitability of a tool for a task is inversely proportional to the number of lies you have to tell it to get something done.

Edward L. Knoll wrote:
Not that I'm picking on you specifically Wendell, but your reply was the
most blatantly representative of a class of responses of a particularly
XSL purist/snobbish nature which I find extremely objectionable.  There
was a reply from a Mitch Amiano which actually supplied a suggested
approach which "appeared" entirely reasonable, so tried it out.  I've
included the core XSL for both approaches below: the "bad" code which
had the 'disable-output-escaping' clause and the "good" code which
generated the Page element directly.  Following are my performance
numbers on a test input file which had 22,004 Row elements and was
13,425,501 bytes large (the time output is from the Unix time(1)
command):

For the "good" (XSLT-correct) approach:
  real  2:41:32.6
  user  2:31:57.0
  sys         1.9

For the "bad" (d-o-e) approach:
  real     1:38.4
  user     1:31.8
  sys         1.0

The "good" approach took hours; the "bad" approach took minutes.  For
those that will care, the test environment was a Sun Solaris platform
using the interim release of the Xalan C++ 1.4 XSLT processor.

I'm just curious, do those of you with this hard-line "purist" attitude
actually use XSL to do real work or are you mostly academics and tool
developers/vendors? I understand staying true to a paradigm up to a
point, but sooner or later "the rubber has to hit the road".


Regards,
Ed Knoll

p.s. This is not all of the XSL, just the differences.


---- "Good" XSL ------------------------------


<xsl:variable name='PageFirstRows'
        select='/gnsl:Results/gnsl:Table/gnsl:Row[
                                        (position() mod $RowsPerPage) =
1]' />

<xsl:template match="gnsl:Table">
   <xsl:copy>
      <xsl:copy-of select="@*" />
      <xsl:apply-templates select="gnsl:Columns" />

      <xsl:choose>
         <xsl:when test="gnsl:Row">
            <xsl:apply-templates select='$PageFirstRows' />
         </xsl:when>
         <xsl:otherwise>
            <xsl:element name='Page' />
         </xsl:otherwise>
      </xsl:choose>
   </xsl:copy>
</xsl:template>

<xsl:template match="gnsl:Row">
   <xsl:element name="Page">
      <xsl:for-each
           select='.|following-sibling::gnsl:Row[$RowsPerPage >
position()]'>
         <xsl:call-template name='CopyAll' />
      </xsl:for-each>
   </xsl:element>


---- "Bad" XSL ------------------------------


<xsl:template match="gnsl:Table">
   <xsl:copy>
      <xsl:copy-of select="@*" />
      <xsl:apply-templates select="gnsl:Columns" />
      <xsl:choose>
         <xsl:when test="gnsl:Row">
            <xsl:apply-templates select="gnsl:Row" />
         </xsl:when>
         <xsl:otherwise>
            <xsl:value-of select="$LineBreak" />
            <xsl:text
disable-output-escaping="yes">&lt;Page/&gt;</xsl:text>
            <xsl:value-of select="$LineBreak" />
         </xsl:otherwise>
      </xsl:choose>
   </xsl:copy>
</xsl:template>

<xsl:template match="gnsl:Row">
   <xsl:if test="(position() mod $RowsPerPage) = 1">
      <xsl:if test="position() != 1">
         <xsl:value-of select="$LineBreak" />
         <xsl:text
disable-output-escaping="yes">&lt;/Page&gt;</xsl:text>
         <xsl:value-of select="$LineBreak" />
      </xsl:if>
      <xsl:value-of select="$LineBreak" />
      <xsl:text disable-output-escaping="yes">&lt;Page&gt;</xsl:text>
      <xsl:value-of select="$LineBreak" />
   </xsl:if>

<xsl:call-template name="CopyAll" />

   <xsl:if test="position() = last()">
      <xsl:value-of select="$LineBreak" />
      <xsl:text disable-output-escaping="yes">&lt;/Page&gt;</xsl:text>
      <xsl:value-of select="$LineBreak" />
   </xsl:if>
</xsl:template>


Hey Mitch,

The horribleness of disable-output-escaping is not (to my mind) really an issue of the well-formedness constraint either in the stylesheet or in the output -- that's something of a red herring (though it is a risk and a sign of the deeper problem). Rather, it's the violation of XSLT's processing model, in which the transformation of the node tree and the post-transformation serialization are clearly distinguished and kept separate by design. *Any* solution that works by writing markup to output using d-o-e creates a dependency on the serialization step. While this may be acceptable in certain circumstances (e.g. writing SGML entity references to output that are not otherwise provided for, when you *know* you're going to write a file), it's horrible at other times, if only because the designer has created this dependency unwittingly, and thus doesn't understand why the transform breaks in a conformant architecture, like Mozilla or transformation chains in Cocoon, where no file is getting serialized.

The relevance of grouping is only that the "write markup" approach is usually resorted to by newer XSLT programmers who don't know how else to do grouping, and who fall back on their Perl or Javascript experience (or just sheer ingenuity) to suppose that writing markup is the best or only solution to the problem (it is neither).

I doubt that any experienced XSLTer would have a problem with either of the solutions you offered (or Dimitre's, or Tom's), since none of them introduce the dependency on serialization that is the problem with d-o-e-based techniques for "outputting open/close tags independently". There the distinctions are much more of coding style and performance; but none of them use a technique that is prone to break the minute you move your stylesheet into a different environment.

Cheers,
Wendell



-- Edward L. Knoll Phone (work) : (719)484-2717 e-mail (work) : ed.knoll@xxxxxxxxxxxxxx e-mail (business): eknoll@xxxxxxxxxx e-mail (personal): edward@xxxxxxxxxxx XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list



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