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RE: Re: apply-templates and predicates

2005-04-28 06:49:31
If you want to process the descendants of an unknown element, then do so:

<xsl:template match="*">
  <xsl:message>unexpected element encountered: <xsl:value-of select="name()"
/></xsl:message>
  <xsl:apply-templates/>
</xsl:template>

Michael Kay
http://www.saxonica.com/
 

-----Original Message-----
From: cknell(_at_)onebox(_dot_)com [mailto:cknell(_at_)onebox(_dot_)com] 
Sent: 28 April 2005 14:45
To: xsl-list(_at_)lists(_dot_)mulberrytech(_dot_)com
Subject: RE: Re: [xsl] apply-templates and predicates

I like this idea and I'm thinking of incorporating it as a 
"best practice", but when trying it out it didn't do 
something I expected it to do. Specifically, with this stylesheet:

<?xml version="1.0" encoding="UTF-8" ?>
<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform";>
  <xsl:output method="xml" indent="yes" encoding="UTF-8" />

  <xsl:template match="*">
    <xsl:message>unexpected element encountered: 
<xsl:value-of select="name()" /></xsl:message>
  </xsl:template>

  <xsl:template match="/">
    <xsl:apply-templates />
  </xsl:template>

</xsl:stylesheet>

The only message output concerned the root element, and none 
of its descendants. I was expecting to see a message for each 
element in the document, but got only one. I tried changing 
the XPath expression from "*" to "//*", but to no effect.

Could you elaborate on this concept?
-- 
Charles Knell
cknell(_at_)onebox(_dot_)com - email



-----Original Message-----
From:     David Carlisle <davidc(_at_)nag(_dot_)co(_dot_)uk>
Sent:     Thu, 28 Apr 2005 10:45:05 +0100
To:       xsl-list(_at_)lists(_dot_)mulberrytech(_dot_)com
Subject:  Re: [xsl] apply-templates and predicates

  It also increases the chances that the stylesheet will do 
something unwanted 
  when the schema changes

Not necessarily.

As I've mentioned earlier in this thread I almost always 
start off with
a template
<xsl:template match="*">
  <xsl:message>unexpected element ...</xsl:message>
<xsl:template>

when I think I have finished and added sufficient templates that 
<xsl:apply-templates/>
generates no more warning messages, then if the input changes 
on me and
new elements are added the stylesheet doesn't just silently accept the
input, it starts warning again. This mechanism is flexible in that
depending on the context it can be modified  to make the 
warnings fatal
errors, or no warnings at all, depending on circumstances.

If on the other hand instead of <xsl:apply-templates/> I just have
<xsl:apply-templates select="A|b[(_at_)foo]|C/>
because that's the only type of element that I expect to see, 
and I only
have templates for those elements, then if the schema changes and the
input has new elements, they will be silently ignored.

Similarly if instead of <xsl:apply-templates/> I just have
<xsl:apply-templates select="*[not(self::F)]/>
Because I believed that the only element type that I didn't need to
handle was F, and I didn't want to write an empty template for F, then
if the schema changes and  a new element type appears this will be
handled by the default template (whatever that does) without warning.

It's not a clear cut choice: most stylesheets use both forms to some
extent, but I think that it is definitely the case that in the face of
under specified or changing input, keeping select attributes 
simple and
putting more logic into the match templates is usually preferable.

David

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