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Re: Meaning of /descendant-or-self::node()

From: by way of <Steven_DeRose@brown.edu>
Date: Thu, 25 May 2000 14:47:02 -0400
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To: "IETF/W3C XML-DSig WG" <w3c-ietf-xmldsig@w3.org>
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At 6:55 PM -0400 5/24/00, John Boyer wrote:
>I was just reading through the XPath spec for the umpteenth time when I
>realized that although the node() function matches any time of node, it can
>only match any type of node available on the given axis.  Furthermore, the
>descendant and descendant-or-self axes of XPath exclude attribute and
>namespace nodes (for some completely unfathomable reason).

Hmm; I'm surprised that seems unfathomable:

a) Attributes and namespaces are unordered while children are ordered -- so
the means of accessing them is going to be quite different, and the order
in which they would be candidates in a merged list must be defined

b) Typical operations for one type do not generally want to look at the
other (note I do not say *all* operations; just typical ones).

c) Attributes and sub-elements in XML can have the same names (<p
z="a"><z>...), and so having operations that go through both of them would
pose some interesting ambiguities and potential user confusion.

>Although the whole idea of having special axes for namespaces and attributes
>is flawed and unnecessary (rather than simply leaving them as children of
>the element and letting the fact that they have a *TYPE* of namespace or
>attribute carry the information), we have to live with the recommendation.

It is not clearly "flawed", nor is there any consensus that is should be
"fixed". It is clearly true in XML that attributes and namespaces are
different animals from sub-elements; it would be far odder for us to try to
obscure that distinction in XPointer. Yes, the distinction is technically
"unnecessary", but so are lots of things that are awfully useful and that
intuitively model the way users think about their data: All programming
languages, for example (all you really need is 2 machine-language
instructions). Minimizing the number of primitives does not equal

You may want to read my paper from last year's ACM conference on Very Large
Data Bases for more on this, or the sections on attributes in my "The SGML
FAQ Book". Attributes are not going away, even if foreign to the RDB world.
The issue of order as information being one of several fundamental
differences. Attributes model things needed in XML; and given their
presence this definition of descendant makes considerable sense for typical

>Therefore, in places where I have in the past used the subexpression
>"/descendant-or-self::node()" to mean ALL of the nodes in the parse tree
>INCLUDING namespace and attribute nodes, it is necessary to substitute the
>following subexpression: (//. | //@* | //namespace::*)

Yup; that is because this is far less likely to be frequently used. I am
willing to believe that your application might require it -- though it
would be helpful to know just why; but this is pretty clearly not a common
need for XML in general.

>Do you think there is any possibility that this might get fixed at some
>point in the future?   It seems terribly tedious to have to go through such
>hoops to simply say "give me a set containing all nodes in the whole parse
>tree".  If not, I suppose we can be content that at least there is *some*
>way to indicate what we want.

It would be trivial to add *syntax* to do this (though I agree with James
it would be hugely disruptive to change any already-defined axes!) -- but
we would need a clear definition of things like relative ordering,
intercomparisons, name conflicts, etc; and a clear reason this would be
widely used.

Can you say a little about why this seems so crucial? It sounds like
arguing to pitch alists from LISP because they're just lists anyway (which
they are, but the conclusion to pitch them doesn't follow). And I've
honestly never heard any similar request before, despite having worked with
a huge variety of XML and SGML users for a long time.

Steven_DeRose@Brown.edu; http://www.stg.brown.edu/~sjd
Received on Thursday, 25 May 2000 14:48:33 UTC

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