Help implementing `last()`, `position()` functions for XSLT

[egh]

Hi,

For my XSLT 2 processor, https://github.com/egh/xjslt, I would like to fix the implementation of last() and position(). The behavior is a little tricky, though.

For instance, this stylesheet:

<?xml version="1.0" encoding="UTF-8" ?>
<t:transform xmlns:t="http://www.w3.org/1999/XSL/Transform" version="2.0">
  <t:template match="/">
    <out>
      <t:for-each select="/doc/foo">
        <t:value-of select="position()"/>
      </t:for-each>
    </out>
  </t:template>
</t:transform>

when applied to:

<?xml version="1.0" encoding="UTF-8"?>
<doc>
  <foo/><bar/>
  <foo/><bar/>
  <foo/><bar/>
</doc>

should output:

<?xml version="1.0" encoding="UTF-8"?><out>123</out>

In contrast, for instance, the stylesheet:

...
      <t:for-each select="/doc/element()">
        <t:if test="local-name() = 'foo'">
          <t:value-of select="position()"/>
        </t:if>
      </t:for-each>

would output 135. In other words, the position() (and last()) depend on the for-each xpath and what was selected there.

In order to do this with my implementation, I need to either be able:

1. to pass in something to evaluateXPath* functions to override the dynamicContext.contextSequence, or
2. to override the position() and last() functions in some cases, i.e. when the contextItem is the same as the contextNode I passed in (and so I can calculate these values based on the node set being processed). But this means both:
   a. I need access to the contextItem and:
   b. Ideally, I need to pass computation to the built in implementation if the contextItem is not the contextNode I passed in.
3. something else that I haven't thought of?

(1) is probably the easiest solution from my point of view, but I don't know if this is feasible.

I am happy to implement a solution to this, but I wanted to get a sense of what the best solution might be.

Thank you for your help! Let me know what other info might help you.

[DrRataplan]

this is tricky. One one side you can override those functions easily: by overriding the function resolver you can plug in your own version. But then you cannot make the normal version work anymore: no access to context sequence...

If you feel adventurous, you can use parseScript (parse an XPath to an XQueryX document) to find any root-level calls to position() (ie, those not in a PathExpr) and change the namespaceURI attributes of those function calls to your own, and provide your own implementation. A bit of work, and a bit finnicky because you will need to change only the wanted ones. If only there was a nice XSLT implementation in JavaScript 😉

[egh]

Thanks for the advice, Martin! That's basically exactly what I ended up doing, except I just did a preprocess step on the xpath itself. Any root level calls to position or last get rewritten to my own function. https://github.com/egh/xjslt/blob/main/src/compile.ts#L100. I think it's probably the best solution to the problem at the moment. Thanks again!