Select part of XML Document by dynamic evaluated XPath test expression-CodePudding

I am failing with a seemingly simple task.

I have a configuration file which defines different scenarios. Each scenario has a test expression. The idea is, that instructions within the scenario shall be applied to input documents which match the test expression. For example:

<config>
<scenario test="/input/@id eq 'X'">
    ...
</scenario>
<scenario test="/input/@id eq 'Y'">
    ...
</scenario>
</config>

My Problem / Question is: With a given input File, how to identify the matching scenario?

Let $d be a variable with some document node. I can check if it matches the pattern P by writing $d[P]. So i tried with something like

let $p:=doc("config.xml")/config/scenario/@test,
$d:=doc("input.xml")
return $d[$p]

I expected an non-empty sequence if and only if the input Document $i matches the test Pattern $p. But the result of the expression is never empty, no matter what the @test Attribute is. Even if there is just one scenario with a test expression that definitive does not match.

Thanks in advance, Frank

CodePudding user response：

In your example the variable $p is a sequence of attributes, and when you use that $p in your predicate, it's converted to a boolean value, as if by the boolean() function. The XPath spec says:

If its operand is a sequence whose first item is a node, fn:boolean returns true.

So if your config file has any test scenarios, then the predicate will return true.

What you need to do is evaluate that expression.

CodePudding user response：

XPath evaluation can be done in XSLT 3.0 using xsl:evaluate:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:mf="http://example.com/mf"
    exclude-result-prefixes="#all"
    version="3.0">

  <xsl:function name="mf:evaluate-predicate" as="item()*">
    <xsl:param name="context-item" as="item()"/>
    <xsl:param name="predicate" as="xs:string"/>
    <xsl:evaluate context-item="$context-item" xpath="'.[' || $predicate || ']'"/>
  </xsl:function>
  
  <xsl:template match="/input[some $pred in $config/config/scenario/@test satisfies mf:evaluate-predicate(., $pred)]">
    <xsl:copy-of select="."/>
  </xsl:template>
  
  <xsl:param name="config" select="doc('config.xml')"/>

</xsl:stylesheet>

xsl:evaluate is an optional feature, it was not supported in early XSLT 3 Saxon HE version (i.e. 9.8 and 9.9) but is supported in Saxon HE 10 and 11 (and of course in PE/EE since 9.8). It is also supported in SaxonJS 2.