Implement dynamic xpath in XSLT: for-each-group - xslt

Question

I am trying to use saxon:evaluate to reduce repeated code in an xsl function.
However anything I try returns an error.
This is a section of the repetitive code.
<!--Select by outputclass and group by attribute-->
<xsl:when test="$from='oclass-attribute'">
<xsl:for-each-group select="$compose//*[contains(#outputclass,$sel)]" group-by="#*[name()=$group]">
<xsl:sort select="current-grouping-key()" />
<element key="{current-grouping-key()}">
<xsl:if test="number(current-group()[1])=number(current-group()[1])">
<xsl:attribute name="max" select="max(current-group())"/>
<xsl:attribute name="sum" select="sum(current-group())"/>
</xsl:if>
<xsl:copy-of select="current-group()[1]"/>
</element>
</xsl:for-each-group>
</xsl:when>
<!--Select by node and group by child node-->
<xsl:when test="$from='node-childnode'">
<xsl:for-each-group select="$compose//*[name()=$sel]" group-by="child::*[name()=$group]">
<xsl:sort select="current-grouping-key()" />
<element key="{current-grouping-key()}">
<xsl:if test="number(current-group()[1])=number(current-group()[1])">
<xsl:attribute name="max" select="max(current-group())"/>
<xsl:attribute name="sum" select="sum(current-group())"/>
</xsl:if>
<xsl:copy-of select="current-group()[1]"/>
</element>
</xsl:for-each-group>
</xsl:when>
What I want is to pass a parameter dictating whether it is an element-name or outputclass-attribute that is selected.
Then another parameter dictating what to group by: attribute or parent, child, following or preceding node.
What I have tried is below:
<xsl:variable name="oSel">
<xsl:choose>
<xsl:when test="starts-with($from,'oclass-')">
<xsl:value-of select="$compose//*[contains(#outputclass,$sel)]"/>
</xsl:when>
<xsl:when test="starts-with($from,'node-')">
<xsl:value-of select="$compose//*[name()=$sel]"/>
</xsl:when>
<xsl:otherwise/>
</xsl:choose>
</xsl:variable>
<xsl:variable name="oGroup">
<xsl:choose>
<xsl:when test="ends-with($from,'-attribute')">
<xsl:value-of select="*/#*[local-name()=$group]"/>
</xsl:when>
<xsl:when test="ends-with($from,'-childnode')">
<xsl:value-of select="*/*[name()=$group]"/>
</xsl:when>
<xsl:when test="ends-with($from,'-parentnode')">
<xsl:value-of select="parent::*[name()=$group]"/>
</xsl:when>
<xsl:when test="ends-with($from,'-followingnode')">
<xsl:value-of select="following-sibling::*[name()=$group][1]"/>
</xsl:when>
<xsl:when test="ends-with($from,'-precedingnode')">
<xsl:value-of select="preceding-sibling::*[name()=$group][1]"/>
</xsl:when>
<xsl:otherwise/>
</xsl:choose>
</xsl:variable>
<xsl:variable name="test">
<!--<xsl:for-each-group select="saxon:evaluate($oSel)" group-by="saxon:evaluate($oGroup)">-->
<xsl:for-each-group select="saxon:evaluate($oSel)" group-by="saxon:evaluate($oGroup)">
<element key="{current-grouping-key()}">
<xsl:if test="number(current-group()[1])=number(current-group()[1])">
<xsl:attribute name="max" select="max(current-group())"/>
<xsl:attribute name="sum" select="sum(current-group())"/>
</xsl:if>
<xsl:copy-of select="current-group()[1]"/>
</element>
</xsl:for-each-group>
</xsl:variable>
I have looked into the saxon documentation and tried all sorts of solutions but none of them are working. Is it possible to do this?
Should have added that I am using Saxon 9.1.0.8 - Sorry, still new to XSLT

Answer 1

Firstly, saxon:evaluate() isn't the right tool for the job.
Now, why isn't it working? To declare the value of $oSel, you've done something like this:
<xsl:value-of select="$compose//*[contains(#outputclass,$sel)]"/>
which evaluates the expression in the select attribute and returns its result. But you're then passing $oSel to saxon:evaluate(), which expects a string containing an XPath expression. I think you're trying to bind the variable to the expression "$compose//*[contains(#outputclass,$sel)]", not to the result of evaluating this expression. To do that you would have to write
<xsl:value-of select="'$compose//*[contains(#outputclass,$sel)]'"/>
Note the extra quotes; but that would now fail because the expression passed to saxon:evaluate() can't explicitly use variables such as $compose (there's a mechanism to pass parameters, but you don't really want to go there).
In XSLT 3.0 saxon:evaluate is superseded by the standard instruction xsl:evaluate; but you don't want that one either.
The right mechanism to be using here is higher order functions.
In XSLT 3.0 you can write
<xsl:for-each-group select="$compose//*[$predicate(.)]" group-by="$grouping-key(.)">
Where $predicate and $grouping-key are variables bound to user-defined functions. You can bind these variables with logic like this:
<xsl:variable name="predicate" as="function(element()) as xs:boolean">
<xsl:choose>
<xsl:when test="starts-with($from,'oclass-')">
<xsl:sequence select="function($n){contains($n/#outputclass,$sel)}"/>
</xsl:when>
<xsl:when test="starts-with($from,'node-')">
<xsl:sequence select="function($n){name($n)=$sel}"/>
</xsl:when>
</xsl:choose>
</xsl:variable>

Answer 2

Change the xpath as below.
$compose//*[name()=$sel or contains(#outputclass,$sel)]
Here is the final code looks like
<xsl:when test="$from='oclass-attribute'">
<xsl:for-each-group select="$compose//*[name()=$sel or contains(#outputclass,$sel)]" group-by="#*[name()=$group]">
<xsl:sort select="current-grouping-key()" />
<element key="{current-grouping-key()}">
<xsl:if test="number(current-group()[1])=number(current-group()[1])">
<xsl:attribute name="max" select="max(current-group())"/>
<xsl:attribute name="sum" select="sum(current-group())"/>
</xsl:if>
<xsl:copy-of select="current-group()[1]"/>
</element>
</xsl:for-each-group>

Answer 3

Try
select="if ($from='oclass-attribute') then $compose//*[contains(#outputclass,$sel)] else $compose//*[name()=$sel]"
and use the same approach for the group-by attribute:
group-by="if ($from='oclass-attribute') then #*[name()=$group] else child::*[name()=$group]"