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   Re: [xml-dev] Annotations in XPath-NG? (Re: [xml-dev] XPath/XSLT 2.0 con

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> This is actually something similar to what I intend to implement over an XML
> Repository [probably Xindice].
> We have a requirement to be able to; given a node, find all annotations [of
> a particular type or matching certain criteria], on that node.
> While I want to be able to express this as an Axis in the XPath object
> model, I've got a few problems with that:
> 1.  How do I specify the Annotation context for the annotation axis.  Our
> annotations are stored as standoff annotations, and to complicate things
> further, may apply to either a single document or a collection of documents.
> 2.  It's a heck of a lot easier to create a new XPath function in existing
> public domain XPath implementations than it is to change the parsing and
> execution model.

This is very true, and another key reason with my uneasiness with extension 

> 3.  The association between a node and it's annotation isn't totally clear.
> Does a node have an annotation if the annotation directly specifies the
> node?  That's easy -- yes.  Does the node have an annotation if it's
> ancestor has an annotation?  That's not very clear, although I lean towards
> yes.  So, then I really want to axes, the annotation:: axis which returns
> the former, and the annotation-closure:: axis, which returns the latter.
> This became to complicated for what I required in the short run, so, I've
> made a design decision that we'll probably have to go the function route.
> What I came up with for an API was:
> node-set extension:annot(node-set source [, string tag])
> Returns the node-set consisting of all annotations on source.  If tag is
> supplied, then only those annotations whose element name matches it will be
> returned.  The value of tag is a list of tags to be included in the result,
> separated by white space or | symbols [NOTE: What I really wanted to pass in
> to this function here was a relative location path so that I could optimize
> the selection].

Looks good, and pretty much what I've proposed, but I still wonder what the 
form is of annotation nodes.  Are they the data model equivalent of

<annotation id="type">integer</annotation>

for example?

> So expr[extension:annot(.,"tag")] returns anly those nodes which have a
> <tag> annotation, and expr[extension:annot(./ancestor-or-self,"tag")]
> selects a similar set, save that the annotation can be on the nodes or its
> ancestors.  I didn't feel it necessary to have a function that automatically
> applied the ancestor-or-self notion simply because the number of ancestors
> is going to be small [3-7] for any given node I have to apply this to in our
> system, however, I may change my mind once I start implementing.
> Of course, I still haven't figured out how to set the context up yet for
> annotations, but still I have a few weeks to do some more design.

I think that once we figure ou the data mnodel for annotations, we can come up 
with a context analog.  For example, if we go with my annotation element 
above, then we might say, roughly that such elements are top-level elements in 
a separate, special annotations document which contains all the annotation 
nodes for a given XML source document.  The annotations() function could then 
be defined as shifting the context *as if* it were an axis which selected a 
set of annotation nodes as the new context node list.

Then again, as I think of it, this may also introduce implementation 
difficulties.  In 4Suite, this would work because extension functions receive 
a mutable instance of the context.  I expect this is not the general case.  Do 
other processors pass the context to extension functions by value rather than 

Uche Ogbuji                                    Fourthought, Inc.
http://uche.ogbuji.net    http://4Suite.org    http://fourthought.com
Apache 2.0 API - http://www-106.ibm.com/developerworks/linux/library/l-apache/
Python&XML column: Tour of Python/XML - http://www.xml.com/pub/a/2002/09/18/py.
Python/Web Services column: xmlrpclib - http://www-106.ibm.com/developerworks/w


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