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   Re: [xml-dev] XPath and XPattern (was Re: [xml-dev] More on taming SAX)

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On Friday 24 December 2004 14:36, Uche Ogbuji wrote:
> Kevin Jones wrote:
> Of course XPath is always streamable, if you have no
> practical limitations (e.g. you're willing to store all
> node data and implement unbounded look-ahead).  When
> people speak of a "streamable subset", they mean a subset
> that works practically with most streaming host
> languages, meaning that these host languages should not
> have to compromise too much memory or state complexity in
> order to satisfy the scope of node requests from the
> XPath subset.


I was probably being a little brief. I was not intending 
just to comment that XPath was theoretically possible, but 
the assumption you need a subset for this case may be 
wrong. Taking the examples of storing all node data and 
unbounded look-ahead, clearly neither of these are 
requirements of streaming XPath, just features of some way 
of implementing it.

I would say that having written/supported one streamable 
subset for a few years they never implemement enough. Some 
of the harder features to implement in a streamable way are 
just the ones that make XPath better than any old pattern 
language, e.g. predicates as expressions.

> BTW, I don't know what you mean by "XPath execution from
> a context node".  XPath *always* has a context node. 

Loose language, I meant as apposed to the context node being 
the document root which is a special case that allows 
reverse axis removal via path re-writing. 



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