Re: [xsl] Restrictions on XPath expressions in <xsl:key>

["G. Ken Holman" <gkholman@xxxxxxxxxxxxxxxxxxxx>]

At 2007-12-19 16:39 -0500, Nassar, Anthony wrote:
> Does someone know a reason why this won't compile:
>
> <xsl:key name="templates"
> match="/onm:NetworkModel/onm:Graph/(onm:Nodes|onm:Links)/(onm:Link|onm:N
> ode)/onm:Templates/onm:Template" use="@Name" />

Because match= attributes are restricted to be XPath patterns, not 
all XPath expressions.

> ...but this will:
>
> <xsl:template match="onm:Graph">
>     <xsl:copy>
>         <xsl:if
> test="(onm:Nodes/onm:Node|onm:Links/onm:Link)/onm:Attributes/onm:Attribu
> te">

Because test= is not restricted to only being a pattern.

> In other words, why can't an XPath expression in an <xsl:key> element
> include parentheses and union operators?

Because patterns are restricted expressions that promote compile-time 
algorithms for detecting patterns of nodes in the source tree.  Not 
all expressions are like this, so the XPath designers categorized a 
subset of all XPath expressions, called them patterns, and they are 
the only ones allowed in a match= attribute.

> By the way, what is the order
> of precedence between '/' and '|'? I'd understand '\' to have higher
> precedence, in which case the latter expression above is actually wrong,
> though it's been working for me.

A step separator "/" has higher precedence in the syntax than the 
expression separator "|", which is why the parentheses are needed to 
tell a processor that the step is made up of two separate expressions.

Note that in XPath 1.0 the expression separator and parentheses are 
only allowed in the first step of a multiple-step location path.

> I'm using .NET 2.0, so let me know if this is a platform question, in
> which case I'll post elsewhere.

It is an XSLT 2.0 question, not a platform-dependent question.

I hope this helps.

. . . . . . . . . . . . . Ken

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