W3C home > Mailing lists > Public > public-qt-comments@w3.org > February 2004

[XSLT20] Backward Compatibility

From: David Carlisle <davidc@nag.co.uk>
Date: Tue, 17 Feb 2004 16:40:46 GMT
Message-Id: <200402171640.QAA00722@penguin.nag.co.uk>
To: public-qt-comments@w3.org


Mike Kay wrote over on xsl-list

  My own experience of migrating real stylesheets has been that there have
  been very few problems, and that they have generally related to use of
  extensions rather than to facilities in the W3C specs. If anyone does
  have any negative experiences of migration, I will be very interested to
  hear them (ideally, send them direct to the public-qt-comments@w3.org
  list).


I just tried running a (set of) XSLT transformation (around 13000 lines
of xslt 1.0) over a sample document using saxon 7.8, which implements
most of the current draft. I have not yet compared the results to see if
they are the same or equivalent (which is hard and would require visual
checking) I am here just concentrating on cases where errors are raised
where previously there were none.

XSLT2 "backward compatibility"  failed to produce a compatible result
(or in fact, any result at all) due to a dozen or so instances of the
same error.

It is now an error to pass parameters to named templates if they are not
declared in the template.

I strongly believe that this should _not_ be an error in BC mode.

Reviewing the cases that required change:

one or two were "dead" parameters that had been used in earlier
versions, but no longer appeared to be used. So their presence
did no harm, but did no good either. Being warned of this might be seen
as a good thing, but since it was a fatal error that stopped compliation
of the stylesheet this would have been rather an expensive thing to
test/debug and basically would mean that a production system using xsl1
will probably need to stay using xslt1 as long as possible. It might seem
that this is a trivial change to make run on xslt2 (just remove the
offending xsl:with-param) but in fact the hard part is distinguishing
this class (which is a minority in my case) from the other class of
constructs that triggered this error described below.

The majority case was due to the fact that we produce two sets of
documents (describing C and Fortran interfaces, as it happens) from the
same source. The set up is the usual one, two top level stylesheets each
importing a common one.

However there are many parts of the transformation that are different in
the two cases and need to be paramaterised diferently. The common code
calls a named template with any parameters that might be useful in
either interface. This now generates an error.

A typical case is the named template

<xsl:template name="optparam-test">
 <xsl:param name="id"/>
 ....

which does something in the C interface but this is a no-op in the
Fortran interface so that has (had)

<xsl:template name="optparam-test"/>

In XSLT2 this will have to be defined to take the same parameters (just
id in this case) as the C interface.

It is not clear to me that this requirement is necessarily a good thing
(even in native xslt 2 mode) however I don't strongly object to it in
2.0 mode. As I say above, it did catch a couple of cases of dubious
coding. However there is a real temptation to respond to the error
message by commenting out the xsl:with-param only to find later that you
have broken some other stylesheet that was expecting that parameter to
be passed.


This change is too much for backward compatibility mode.



Having resolved all these parameter passing problems the stylesheeet
then compiled and produced just one warning (twice).

Warning: at xsl:for-each on line 3403 of file:/c:/engine-cvs/doc/xsl/libdoc-c-xml.xsl:
  Less-than and greater-than comparisons between strings have changed since XPath 1.0

 <xsl:variable name="a" select="mn[1]/text()"/>
 <xsl:variable name="b" select="mn[2]/text()"/>
 <xsl:for-each select="$enum[@fname=$fpname]/v[$a &lt;= @f and @f &lt;= $b]">

which is checking that three "numbers" obtained from the source files
satisfy a constraint that one lies between the other two.

Is it really necessary for this to break in BC mode?
Is it not possible for the mapping of &lt;= to the underlying F&O
operators is changed in BC mode to more closely match the behaviour in
1.0? While this is annoying it is actually less trouble to fix than the
previous error, especially in this case, where the node sets such as @f
in the expression really are only going to return one node  so I would
just need to add a couple of instances of number() (I hope:-)
However if tehre are cases where the implicit existential quantification
are used, it will be tricky for an end user to get right (easier for the
system, I would have thought).

While I do appreciate the facility to switch between native and BC mode
on specific elements within the stylesheet,  I believe that verion="1.0"
on an xsl:stylesheet (or outer most element of a simplified stylesheet)
should be treated differently: the stylesheet should be processed by an
XSLT 1.0 processor, or an error raised if the processor does not support
1.0. 

David

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Received on Tuesday, 17 February 2004 11:41:06 UTC

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