Avoiding circular key definitions in XSLT - xslt-3.0

Question

I am writing some code which turns valid XML instances into presentations of code in a DITA codeblock.
I have some wrapper elmenents in my input that allow me to define some emphasis on the output. This is easy for node() items as the wrapper directly wraps the code to be emphasised. For attributes and so on, though, I need to specify some #select on my emphasis element.
Here is a brief code excerpt of how I'm trying to do this (for attributes; I've removed similar templates for other types of content):
<xsl:key name="emph" match="eg:emph[#select]">
<xsl:variable name="selected">
<xsl:evaluate xpath="#select" context-item="."/>
</xsl:variable>
<xsl:for-each select="$selected">
<xsl:sequence select="generate-id()"/>
</xsl:for-each>
</xsl:key>
<xsl:template match="#*[key('emph', generate-id(.))]" mode="eg">
<xsl:variable name="style" select="if (not(key('emph', generate-id(.))/#style)) then 'italic' else key('emph', generate-id())/#style"/>
<xsl:text> </xsl:text>
<ph outputclass="{$style}">
<xsl:next-match>
<xsl:with-param name="includeSpace" select="false()"/>
</xsl:next-match>
</ph>
</xsl:template>
<xsl:template match="#*" mode="eg">
<xsl:param name="includeSpace" as="xs:boolean" select="true()"/>
<xsl:if test="$includeSpace">
<xsl:text> </xsl:text>
</xsl:if>
<ph outputclass="AttributeName">
<xsl:value-of select="name()"/>
</ph>
<ph outputclass="equals">=</ph>
<ph outputclass="attributeQuotes">"</ph>
<ph outputclass="AttributeValue">
<xsl:value-of select="."/>
</ph>
<ph outputclass="attributeQuotes">"</ph>
</xsl:template>
With an input such as:
<eg:emph select="abbrev-journal-title/#abbrev-type">
<abbrev-journal-title abbrev-type="custom">JPhysA</abbrev-journal-title>
</eg:emph>
I would like to generate something like:
<ph outputclass="XmlFurniture"><</ph><ph outputclass="ElementName">abbrev-journal-title</ph> <ph outputclass="italic"><ph outputclass="AttributeName">abbrev-type</ph><ph outputclass="equals">=</ph><ph outputclass="attributeQuotes">"</ph><ph outputclass="AttributeValue">custom</ph><ph outputclass="attributeQuotes">"</ph></ph><ph outputclass="XmlFurniture">></ph>JPhysA<ph outputclass="XmlFurniture"></</ph><ph outputclass="ElementName">abbrev-journal-title</ph><ph outputclass="XmlFurniture">></ph>
Transforming with Saxon (PE 9.8.0.12) returns 'key definition is circular' errors - but as far as I can tell, that isn't actually the case.
Can anyone suggest a workaround, or at least explain why this approach isn't working?

Answer 1

Well, for the use of xsl:evaluate see https://www.w3.org/TR/xslt-30/#evaluate-dynamic-context explaining
The context item, position, and size depend on the result of
evaluating the expression in the context-item attribute. If this
attribute is absent, or if the result is an empty sequence, then the
context item, position, and size for evaluation of the target
expression are all absent.
As you don't set that attribute context-item on your xsl:evaluate at all your attempt with xpath="#select" doesn't make sense, I guess you want to use context-item="." to select the matched element.
As for selecting attribute nodes and storing them in a variable, I think you need to use
<xsl:variable name="selected" as="attribute()*">
<xsl:evaluate xpath="#select" context-item="."/>
</xsl:variable>
instead of
<xsl:variable name="selected">
<xsl:evaluate xpath="#select" context-item="."/>
</xsl:variable>
Then I would think that
<xsl:for-each select="$selected">
<xsl:sequence select="generate-id()"/>
</xsl:for-each>
can be shortened/simplified to
<xsl:sequence select="$selected!generate-id()"/>
in the context of XSLT 3.
I have now tried to build a minimal but complete example and to test that with Saxon, using Saxon 9.8.0.12 EE and Saxon 9.9.0.1 EE, I get no errors and it seems the approach work as far as the templates you have created are used.
Test XSLT is:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:eg="http://example.com/eg"
exclude-result-prefixes="#all"
default-mode="eg"
version="3.0">
<xsl:mode name="eg" on-no-match="shallow-copy"/>
<xsl:output indent="yes"/>
<xsl:key name="emph" match="eg:emph[#select]">
<xsl:variable name="selected" as="attribute()*">
<xsl:evaluate xpath="#select" context-item="."/>
</xsl:variable>
<xsl:sequence select="$selected ! generate-id()"/>
</xsl:key>
<xsl:template match="#*[key('emph', generate-id(.))]" mode="eg">
<xsl:variable name="style" select="if (not(key('emph', generate-id(.))/#style)) then 'italic' else key('emph', generate-id())/#style"/>
<xsl:text> </xsl:text>
<ph outputclass="{$style}">
<xsl:next-match>
<xsl:with-param name="includeSpace" select="false()"/>
</xsl:next-match>
</ph>
</xsl:template>
<xsl:template match="#*" mode="eg">
<xsl:param name="includeSpace" as="xs:boolean" select="true()"/>
<xsl:if test="$includeSpace">
<xsl:text> </xsl:text>
</xsl:if>
<ph outputclass="AttributeName">
<xsl:value-of select="name()"/>
</ph>
<ph outputclass="equals">=</ph>
<ph outputclass="attributeQuotes">"</ph>
<ph outputclass="AttributeValue">
<xsl:value-of select="."/>
</ph>
<ph outputclass="attributeQuotes">"</ph>
</xsl:template>
</xsl:stylesheet>
Sample input is
<root>
<eg:emph select="abbrev-journal-title/#abbrev-type" xmlns:eg="http://example.com/eg">
<abbrev-journal-title abbrev-type="custom">JPhysA</abbrev-journal-title>
</eg:emph>
<eg:emph select="abbrev-journal-title/#abbrev-type" xmlns:eg="http://example.com/eg" style="bold">
<abbrev-journal-title abbrev-type="custom">JPhysA</abbrev-journal-title>
</eg:emph>
</root>
Result shows the attributes are matched and transformed:
<root>
<eg:emph xmlns:eg="http://example.com/eg">
<ph outputclass="AttributeName">select</ph>
<ph outputclass="equals">=</ph>
<ph outputclass="attributeQuotes">"</ph>
<ph outputclass="AttributeValue">abbrev-journal-title/#abbrev-type</ph>
<ph outputclass="attributeQuotes">"</ph>
<abbrev-journal-title>
<ph outputclass="italic">
<ph outputclass="AttributeName">abbrev-type</ph>
<ph outputclass="equals">=</ph>
<ph outputclass="attributeQuotes">"</ph>
<ph outputclass="AttributeValue">custom</ph>
<ph outputclass="attributeQuotes">"</ph>
</ph>JPhysA</abbrev-journal-title>
</eg:emph>
<eg:emph xmlns:eg="http://example.com/eg">
<ph outputclass="AttributeName">select</ph>
<ph outputclass="equals">=</ph>
<ph outputclass="attributeQuotes">"</ph>
<ph outputclass="AttributeValue">abbrev-journal-title/#abbrev-type</ph>
<ph outputclass="attributeQuotes">"</ph>
<ph outputclass="AttributeName">style</ph>
<ph outputclass="equals">=</ph>
<ph outputclass="attributeQuotes">"</ph>
<ph outputclass="AttributeValue">bold</ph>
<ph outputclass="attributeQuotes">"</ph>
<abbrev-journal-title>
<ph outputclass="bold">
<ph outputclass="AttributeName">abbrev-type</ph>
<ph outputclass="equals">=</ph>
<ph outputclass="attributeQuotes">"</ph>
<ph outputclass="AttributeValue">custom</ph>
<ph outputclass="attributeQuotes">"</ph>
</ph>JPhysA</abbrev-journal-title>
</eg:emph>
</root>