[xquery-talk] Accessing empty namespaced elements in non-empty contexts

[Joe Wicentowski]

Hi Michael,

Thank you for your reply.  It's interesting that until XPath 3.0, we could
*construct* empty-namespace elements inside non-empty namespace elements
(via the xmlns="" declaration), but we couldn't *query* them without
resorting to the somewhat means of indirect wildcards (e.g., `/*:elem`) or
use of the local-name function (e.g., `/*[local-name() eq 'elem']`).  Is
that right?  If so, this drives home the importance to me of the
URI-qualified names feature (e.g., `/Q{}elem`) released with XPath 3.0.

I'd be interested to know more about the history of the development of
namespaces and efforts to bridge the world of namespaces (namespace
axis-land) with the world of documents without this axis (namespace
flat-land).  Is there any good published account of this?

Joe


On Mon, Apr 2, 2018 at 3:19 PM Michael Kay <mike at saxonica.com> wrote:

> XML doesn't allow you to bind a prefix to the
> thing-that-is-not-a-namespace.
>
> With XML namespaces 1.1, the declaration xmlns:x="" doesn't bind x to
> anything, it unbinds x. Within the scope of this (un)declaratiion, the
> prefix x cannot be used.
>
> It makes sense for XQuery to do the same thing. The rules are in §3.9.1.2:
>
> If the namespace URI is a zero-length string and the implementation
> supports [XML Names 1.1] <https://www.w3.org/TR/xquery-31/#XMLNAMES11>,
> any existing namespace binding for the given prefix is removed from the in-scope
> namespaces <https://www.w3.org/TR/xquery-31/#dt-in-scope-namespaces> of
> the constructed element and from thestatically known namespaces
> <https://www.w3.org/TR/xquery-31/#dt-static-namespaces> of the
> constructor expression. If the namespace URI is a zero-length string and
> the implementation does not support [XML Names 1.1]
> <https://www.w3.org/TR/xquery-31/#XMLNAMES11>, a static error is raised [
> err:XQST0085 <https://www.w3.org/TR/xquery-31/#ERRXQST0085>]. It is
> implementation-defined
> <https://www.w3.org/TR/xquery-31/#dt-implementation-defined> whether an
> implementation supports [XML Names]
> <https://www.w3.org/TR/xquery-31/#XMLNAMES> or [XML Names 1.1]
> <https://www.w3.org/TR/xquery-31/#XMLNAMES11>.
>
> Michael Kay
> Saxonica
>
> On 2 Apr 2018, at 20:00, Joe Wicentowski <joewiz at gmail.com> wrote:
>
> Hi all,
>
> A common question for beginners is how to access empty namespaced elements
> in non-empty contexts.  The easiest answer is to use a wildcard.  For
> example, we use one here inside the curly braces:
>
>   let $x := <x><y/></x>
>   return
>     <z xmlns="foo">{ $x/*:y }</z>
>
> As expected, this query returns:
>
>   <z xmlns="foo"><y xmlns=""/></z>
>
> The other method I know of is to use the URI-qualified name:
>
>   let $x := <x><y/></x>
>   return
>     <z xmlns="foo">{ $x/Q{}y }</z>
>
> A third method occurred to me, but it does not work, and I haven't been
> able to pin down the reason despite reading the spec.  The idea is to bind
> the empty namespace URI to a namespace prefix, e.g., "my":
>
>   declare namespace my="";
>
>   let $x := <x><y/></x>
>   return
>     <z xmlns="foo">{ $x/my:y }</z>
>
> In BaseX, eXist, and Saxon, this returns err:XPST0081 (
> https://www.w3.org/TR/xquery-31/#ERRXPST0081).  The location of the error
> points to the step, "my:y" on the final line.
>
> Can anyone enlighten me on the reason this 3rd method is invalid?  Bonus
> points for an explanation that includes why I cannot bind a namespace
> prefix to the empty namespace, but I can access it via *: and Q{}?
>
> Thanks,
> Joe
> _______________________________________________
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>
>
>
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