[xquery-talk] Q: computed element and namespace

[Michael Kay]

On 31 Jan 2014, at 08:12, Hermann Stamm-Wilbrandt <STAMMW at de.ibm.com> wrote:

> 
> From (1):
> In XPath Version 2.0, the namespace axis is deprecated and need not be
> supported by a host language. A host language that does not support the
> namespace axis need not represent namespace bindings in the form of nodes.
> 
> XQuery (eg. Zorba or DataPower) does not support the namespace axis.
> 
> 
> <a><pre:b xmlns:pre="urn:u"/><pre:c xmlns:pre="urn:u"/></a>
> 
> can be created by computed element constructors (2) by this:
> 
> element {"a"}{
>  (
>    element {fn:QName("urn:u", "pre:b")}{},
>    element {fn:QName("urn:u", "pre:c")}{}
>  )
> }
> 
> Is it true, that "equivalent" (not sure whether right term) XML
> <a xmlns:pre="urn:u"><pre:b/><pre:c/></a>
> 
> cannot be created by computed element constructors in XQuery?

If we're talking about XQuery 1.0, then yes, I believe that there is no way you can create an element with a namespace that is (a) unused on that element, and (b) not known statically.

(I might be wrong; I seem to recall David Carlisle coming up with a devious workaround, but it was so devious I don't recall how it works).

Query 3.0 introduces dynamic namespace node constructors to solve this problem. This leaves the language in the rather absurd situation that there is a construct to create namespace nodes, but no construct to retrieve them.

The XQuery WG was very reluctant to support the namespace axis because many vendors claimed that it was impossible to support it efficiently. Some of those vendors, when told to look at open source implementations like xt and Saxon that had efficient implementations, said that their company policy did not allow them to read open source code. Shame.

Michael Kay
Saxonica

> 
> 
> It can be created by XSLT 1.0+EXSLT stylesheet:
> 
> $ java -jar ~/Desktop/saxon6-5-5/saxon.jar dummy.xml compel.xsl ; echo
> <a xmlns:pre="urn:u"><pre:b/><pre:c/></a>
> $
> $ cat compel.xsl
> <xsl:stylesheet version="1.0"
>  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
>  xmlns:exslt = "http://exslt.org/common"
>> 
>  <xsl:output omit-xml-declaration="yes" />
> 
>  <xsl:template match="/">
>    <xsl:variable name="tmp">
>      <xsl:element name="a">
>        <xsl:attribute name="pre:ʎɯɯnp" xmlns:pre="urn:u"/>
>        <xsl:element name="pre:b" xmlns:pre="urn:u"/>
>        <xsl:element name="pre:c" xmlns:pre="urn:u"/>
>      </xsl:element>
>    </xsl:variable>
> 
>    <xsl:apply-templates select="exslt:node-set($tmp)/*"/>
>  </xsl:template>
> 
>  <xsl:template match="*">
>    <xsl:copy>
>      <xsl:copy-of select="namespace::*|attribute::*[local-name
> ()!='ʎɯɯnp']"/>
>      <xsl:apply-templates select="*"/>
>    </xsl:copy>
>  </xsl:template>
> 
> </xsl:stylesheet>
> $
> 
> 
> (1) http://www.w3.org/TR/xpath20/#dt-in-scope-namespaces
> (2) http://www.w3.org/TR/xquery/#id-computedElements
> 
> Mit besten Gruessen / Best wishes,
> 
> Hermann Stamm-Wilbrandt
> Level 3 support for XML Compiler team and Fixpack team lead
> WebSphere DataPower SOA Appliances
> https://www.ibm.com/developerworks/mydeveloperworks/blogs/HermannSW/
> https://twitter.com/HermannSW/
> http://stamm-wilbrandt.de/GraphvizFiddle/
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