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[xsl] XSLT streaming: the processor "remembers" things as it descends the XML tree?

2013-11-20 04:42:56
Hi Folks,

Consider an XSLT program that is at some node. The XSLT streaming rules say 
that the following nodes are accessible from that node: its ancestors and their 
attributes and namespaces, and its descendants and their attributes and 
namespaces.

If my XSLT program accesses ancestor nodes, that seems to require the XSLT 
processor to back up. And isn't that a violation of the fundamental law of 
streaming, "The XSLT processor shall not back up"?

Or, perhaps my XSLT program can access ancestor nodes because, as the XSLT 
processor descends the XML tree it keeps a record of each node through which it 
descends (the node's name, its attributes, its namespaces). Yes, that must be 
what the XSLT processor does. Suppose that my XML tree is very deep, then the 
XSLT processor will have to remember a lot of stuff, right? In the extreme 
case, every node in the XML document has no siblings, just one child. Thus, the 
XSLT processor would have to remember the entire XML document, right? 

Questions:

1. In XSLT streaming the processor "remembers" things as it descends the XML 
tree, right?

2. In XSLT streaming, allowing access to ancestors seems like a bad idea, for 
the reasons described above. What is the rationale for allowing access to 
ancestors?

/Roger

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