Allowing an attribute-set to set a namespace required by an attribute - xslt

Question

I understand that xsl:attribute-set exists to allow a set of XML attributes to be grouped under a single name, which can then then easily be applied to several similar elements at a later date.
I understand that namespaces are not attributes and cannot be set using this.
However in Saxon9.8EE I note this works and I was wondering if this is safe to use:
<xsl:attribute-set name="swbml.ir" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<xsl:attribute name="version">4-2</xsl:attribute>
<xsl:attribute name="xsi:schemaLocation">http://www.fpml.org/2005/FpML-4-2 /path/to/swbml-ird-main-4-2.xsd</xsl:attribute>
</xsl:attribute-set>
By adding the xsi namespace to the xsl:attribute-set itself, it applies this namespace to any element using the swbml.ir attribute set.
(of course it has to because one of the attributes sits in the xsi namespace)
So this:
<SWBML xmlns="http://www.fpml.org/2005/FpML-4-2" xsl:use-attribute-sets="swbml.ir">
Results in:
<SWBML xmlns="http://www.fpml.org/2005/FpML-4-2"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
version="4-2"
xsi:schemaLocation="http://www.fpml.org/2005/FpML-4-2 /path/to/swbml-ird-main-4-2.xsd">
This is exactly what I want. But it feels like I might be stretching the intended use-case for attribute sets?
Specifically if I try to go one step further and add xmlns="http://www.fpml.org/2005/FpML-4-2" like so:
<xsl:attribute-set name="swbml.ir" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.fpml.org/2005/FpML-4-2">
The default xmlns is not applied to <SWBML> - which is kinda what I expect.
So - is the rule that attribute sets will add any namespace that is required in order to qualify any attribute the set contains, BUT will not add any other namespace? Or have I strayed into undefined territory?

Answer 1

Your understanding is basically correct, in that if there is content bound to a namespace, and you include it in your output, then the namespace will come along for the ride. However, the fact that you happen to have declared it on the attribute-set is not critical. It could be declared in other places in the stylesheet, such as on the xsl:stylesheet element, to be in-scope and referenced in the attribute-set.
Building upon the examples that you posed, you could move the declaration of the xsi namespace prefix out of the xsl:attribute-set and up to the xsl:stylesheet element, and it would still appear in your output if the attribute-set were applied to the element:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
exclude-result-prefixes="xsi">
<xsl:output method="xml"/>
<xsl:attribute-set name="swbml.ir">
<xsl:attribute name="version">4-2</xsl:attribute>
<xsl:attribute name="xsi:schemaLocation">http://www.fpml.org/2005/FpML-4-2 /path/to/swbml-ird-main-4-2.xsd</xsl:attribute>
</xsl:attribute-set>
<xsl:template match="/">
<SWBML xmlns="http://www.fpml.org/2005/FpML-4-2" xsl:use-attribute-sets="swbml.ir"/>
</xsl:template>
</xsl:stylesheet>
And it would not appear in the output if the attribute-set is not applied to the content:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
exclude-result-prefixes="xsi">
<xsl:output method="xml"/>
<xsl:attribute-set name="swbml.ir">
<xsl:attribute name="version">4-2</xsl:attribute>
<xsl:attribute name="xsi:schemaLocation">http://www.fpml.org/2005/FpML-4-2 /path/to/swbml-ird-main-4-2.xsd</xsl:attribute>
</xsl:attribute-set>
<xsl:template match="/">
<SWBML xmlns="http://www.fpml.org/2005/FpML-4-2" />
</xsl:template>
</xsl:stylesheet>
Note that I used exclude-result-prefixes in both examples to ensure that the xsi namepsace is pruned from the output if unused. Otherwise, the in-scope namespace might come along for the ride in the output, even if it were not applied to any content.

Answer 2

Yes, this will work: as #MadsHansen points out, when you use <xsl:attribute name="p:u"/> the only thing that really matters is that the prefix p is declared somewhere - on the xsl:attribute element itself, or on one of its ancestors. If it's convenient to declare it at the level of the xsl:attribute-set itself, then fine, do that.
A thing to watch out for here is that this doesn't apply to QName-valued attributes. If you want to do
<xsl:attribute name="xsi:type">xs:date</xsl:attribute>
then you can get the prefix xsi declared in the result document simply by having it in-scope for the xsl:attribute instruction, but for the xs prefix you need to work a bit harder (because the XSLT processor doesn't know that the attribute value xs:date is a QName). In this case you need to explicitly ensure that some containing element in the result tree declares the xs namespace.