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   RE: XSLT processor internals (was RE: [xml-dev] Another XML parsing idea

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> I start an unrelated topic. How do you implement
> xsl:template name="something" and its required
> instruction xsl:call-template in the XSLT processor.
> i.e. what kind of data structures do you use in Java
> and what kind of control flow takes place. Am just
> curious since many days..

Depends greatly on the processor. Saxon compiles each template or function
into an abstract syntax tree, which is then heavily re-arranged by the
optimizer and type-checker, and is then executed interpretively at run-time.
A call-template instruction is a node in this tree, containing a pointer to
the tree representing the template to be called. Template names aren't used
at run-time, they are fully resolved at compile time.

You asked about control flow. In Saxon 6.x, and probably in most XSLT 1.0
processors, the XSLT and XPath engines were very separate. In 8.x they have
become much more integrated, probably reflecting the influence of XQuery.
However Saxon still has two modes of execution internally, pull and push:
expressions such as path expressions are usually evaluated by a stack of
iterators sucking data in a pipeline from the source document, while element
and attribute construction instructions are usually evaluated by means of a
push pipeline pushing SAX-like events out to the serializer (or the result
tree builder if the user requests one). Control instructions such as xsl:if
and xsl:call-template can be evaluated in either mode, depending on where
they appear. There's also a fairly experimental capability to execute
element constructors in pull mode, leading to lazy tree construction, so
parts of a temporary tree that aren't ever referenced don't actually get

If you're interested in understanding such things it would be instructive to
read the source code of a processor such as Saxon. It's probably a bit
daunting at first, but you'll find it interesting.

Michael Kay


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