RE: [xsl] Want to process XHTML in my XSL stylesheets.

["Michael Kay" <michael dot h dot kay at ntlworld dot com>]

> >I don't think an XSLT processor will ever be able to determine 
> >statically that the output of a stylesheet is guaranteed be valid 
> >XHTML. There may well be cases where it is able to determine 
> statically 
> >that it won't be valid, however.
> >
> Any specific reason for that (first) conclusion - for 
> instance, do you 
> have any feedback from XQuery implementors in this area?

XQuery doesn't have the rule-based approach to tree construction that
XSLT uses. They deliberately avoided it because it makes static typing
so difficult.

There are some things that happen in a stylesheet that make the analysis
impossible, e.g. use of <xsl:element name="{expr}">. There are many
other things that make the analysis impractical, for example if the
stylesheet does <xsl:apply-templates select="*[position()>2]", you might
have to infer that this can't select a <title> or <subtitle> element
because those can only occur in the first two positions; or you have to
infer that the template rule for match="para[1]" will always be fired
before the one for match="para". In general that requires some quite
powerful analysis.

It's still very much an open question how much is achievable form static
analysis of a stylesheet, but my suspicion is that in most cases the
only type you can deduce for the result tree is "document". One reason
for this is that I think most real stylesheets assume things about the
source documents that are not actually explicit in the schema, things
like "all rows will contain the same number of columns", and the
stylesheet will actually fail if these assumptions prove wrong.

> Moving even 
> further off-topic, any feedback on how suitable Haskell is for XML 
> processing? 

The Software AG XQuery prototype (QuiP)[1] is written in Haskell, and
its developers are very enthusiastic about the language.

[1] http://developer.softwareag.com/tamino/quip/

Michael Kay
Software AG
home: Michael.H.Kay@ntlworld.com
work: Michael.Kay@softwareag.com 


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