Subject: [xsl] Fwd: Re: Comments on the XPath data model, from a DOM perspective. From: Dimitre Novatchev <dnovatchev@xxxxxxxxx> Date: Thu, 4 Apr 2002 07:52:01 -0800 (PST) |
--- Dimitre Novatchev <dnovatchev@xxxxxxxxx> wrote: > Date: Thu, 4 Apr 2002 07:48:36 -0800 (PST) > From: Dimitre Novatchev <dnovatchev@xxxxxxxxx> > Subject: Re: Comments on the XPath data model, from a DOM > perspective. > To: www-xml-query-comments@xxxxxx, Ray Whitmer <rayw@xxxxxxxxxxxx> > CC: Michael.Kay@xxxxxxxxxxxxxx > > Hi Ray, > > I am addressing only your first two comments. > > In a thread in the xsl-list (in January and February) I did raise the > problem of XPath 2.0 sequences allowing heterogeneously-typed > elements. > I also raised the problem why a sequence is not allowed to have > elements, which are themselves sequences. It seems that the latter > problem is just a consequence of the former problem. > > Although there was understanding and positive response from some good > XSLT specialists, it seems that these problems are not going to be > solved in XPath 2.0 (as far as I'm aware). > > The reply from Michael Kay was that all this can be modelled by > node-sets, but he also shared his expertise that building elements is > and will continue to be an expensive operation. > > The above recommendation to use node-sets actually admits that there > is > a special and very useful datatype (simulated, typed, logically > correct > sequences), that is not at all described in the XPath data model. > > The starting messages of the threads in which these problems were > raised and discussed are: > > "The hard cocktail of sequence and (node-)set " > > see in particular see: > http://sources.redhat.com/ml/xsl-list/2002-01/msg00192.html > > and > > "An issue with XPath 2.0 sequences " > > http://sources.redhat.com/ml/xsl-list/2002-01/msg01603.html > > I think your message and this response to it should be forwarded to > www-xpath-comments@xxxxxx > > > Cheers, > Dimitre Novatchev. > > > > Ray Whitmer wrote: > ------------------------------- > > * It seems clear that the XPath 2.0 specification has no type > comparable to > the node set or other built-in types of XPath 1.0. The concept of a > typeless > sequence does not seem to work as effectively. In many languages, > arrays of > objects are typed. Although some people use untyped languages, those > who > rely on a certain level of typing are likely to complain when they > lose > > that, > as is being lost in this case. There is certain distress in worrying > that > your array of matching nodes might have strings interspersed in it, > and > applications which in XPath 1.0 relied on receiving sets only > containing > nodes are not going to be able to deal compatibly with a model which > no > longer is able to return that type of guarantee. > > * XPath 1.0 was based on explicitly unordered sets of nodes that > could > be > accessed in order. XPath 2.0 claims that every sequence is ordered, > but > there is not sufficient discussion of what that means, which has > caused > significant confusion. The logical conclusion could be drawn that it > is > referring to document order, which is the only order it seems to > define > and was the order of XPath 1.0, but this makes no sense when > considering > non-node items now possible in the result sets. Also, the > incompatible > treatment of duplicates is confusing, if the sets are now ordered, > rather > than unordered, it seems pointless to not eliminate the duplicates, > but > there is probably something lost between the different versions of > the > specification. > > Based upon recent discussions, it seems that the XPath 2.0 > specification > may not be comparable or compatible with the XPath 1.0 specification > in > its > use of these terms, but the specification needs better treatment of > the > concepts, and explanation of the impact on backwards compatibility. > Elimination of duplicates also seems like a significant compatibility > problem since 1.0 implementations went to great lengths to accomplish > this. __________________________________________________ Do You Yahoo!? Yahoo! Tax Center - online filing with TurboTax http://taxes.yahoo.com/ XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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