XSLT: When using apply-templates should I get more specific in the select or the match?

Question

I'm working through my first XSL transformation using the apply-templates element. When developing XSLT, what is the best practice? Should I be very specific in the select of the apply-templates or in the match of the template or something I haven't considered?

Example A

<xsl:template match="/">
    <xsl:apply-templates select="A/B/C/D"/>
    <xsl:apply-templates select="J/K/L/M"/>
    <xsl:apply-templates select="W/X/Y/Z"/>
</xsl:template>

<xsl:template match="Q"/>

or

Example B

<xsl:template match="/">
    <xsl:apply-templates/>
</xsl:template>

<xsl:template match="A/B/C/D/Q or J/K/L/M/Q or W/X/Y/Z/Q"/>

As with all languages, there are trade-offs to be made between performance, extensibility and design simplicity. You need to say which of these has the highest priority. — Sean B. Durkin
@Sean Good point. I was asking from a design simplicity standpoint. I was not aware that this would affect performance, so I'd be interested in how this affects performance as well. — Dale

David Carlisle David Carlisle · Accepted Answer · 2013-01-24T00:08:08

It's hard to say which is better as the two suggested fragments do very different things so it depends what the intended result is.

<xsl:template match="A/B/C/D/Q or J/K/L/M/Q or W/X/Y/Z/Q"/>

is a syntax error it should be

<xsl:template match="A/B/C/D/Q  | J/K/L/M/Q | W/X/Y/Z/Q"/>

Or probably more efficiently (but not equivalent in general)

<xsl:template match="Q"/>

So once the error is fixed, sample B will do a walk over the tree in depth first order applying the default template to every node so in particular copying all text to the output until it gets to Q element nodes where it will prune the traversal and not copy Q or its descendants to the result tree.

Sample A in complete contrast does not traverse the tree in depth first document order at all, and first processes A/B/C/D nodes, in particular it does not process any other children of A or B or C at this stage so any text node descendants of those children are not copied to the output. Then it will start a depth first traversal starting at D, pruning Q as before. The process then starts again with J/K/L/M" so any output from M will be output after the output from D even if M occurs earlier in the source. This is in contrast to sample B where the result order reflects the order in the input.

It's probably an artefact of the minimal example but if the input is a well formed document only one of the three apply templates in sample A can produce any result as / can only have a single element child child so at least two of A J W must select nothing.

XSLT: When using apply-templates should I get more specific in the select or the match?

2 Answers