Question

I have an XSLT script which I would like to number things sequentially each time I call a template. So a very abbreviated version of it looks a bit like:

<xsl:call-template name="insertHeader" />
<xsl:for-each ...>  
    <xsl:call-template name="insertHeader" />
    ...
</xsl:for-each>
<xsl:call-template name="insertHeader" />

<xsl:template name="insertHeader>
    This is item number <xsl:value-of select="$numberOfInvocations />
</xsl:template>

So obviously that $numberOfInvocations thing doesn’t work, and in XSLT you can’t increment a global counter variable which would seem like an obvious approach in a procedural language. I would like it to print out 1 the first time that template is called, 2 the second, etc. How should I go about doing this? Is this even remotely possible in XSLT?

Thanks 🙂

Edit: So there have been a couple of comments that this isn’t clearly defined enough. What I want to do is label a series of tables in the (HTML) output. The most obvious way I see of doing this is to call a function (you can probably tell that I am not an XSLT wizard here) that will automatically increment the number each time. I think the reason this seems so hard is because it’s the XSLT itself that defines where these tables appear rather than the input.

This extra info may not be of that much use though since Dimitre’s answer makes it sound rather like this is never going to work. Thanks anyway 🙂

Answer 1

In a functional language as XSLT there is no “order of computation” defined.

Therefore, trying to number the “computations” by their ordering “in time” isn’t meaningful and if attempted will often produce surprising results.

For example, nothing restricts <xsl:apply-templates> to apply templates in the same order in time as the document order of the nodes in the selected node-list. These could be done in parallel, meaning in any order.

Many XSLT processors perform lazy evaluation which means that a certain XSLT instruction will only be evaluated when it is really needed and not according to its textual order in the XSLT stylesheet. Often some instructions are not executed at all.

Sometimes the optimizer will execute a given XSLT instruction twice because it decided to discard the first result in order to optimize space utilization.

The requested numbering can be produced using recursion (generally) and continuation-passing style CPS or Monads (more specifically).

The FXSL library (both version 1 — for XSLT 1.0 and version 2 — for XSLT 2.0) contains templates that can be used to organize such numbering: foldl, foldr, iter, iterUntil, scanl, scanr, …, etc.

Whenever the problem is precisely defined (which is not the current case), such numbering can be produced but be warned about the results.