Re: [xsl] Why can a template rule match on a child of multiple elements but cannot match on multiple children of an element?

["Michael Kay mike@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

XPath 1.0 expressions (and therefore XSLT 1.0 patterns) only allow a
RelativeLocationPath on the RHS of the "/" operator. A RelativeLocationPath is
essentially a sequence of one or more Steps separated by "/", and a Step
(unless abbreviated) takes the form AxisSpecifier NodeTest Predicate*.

XPath 2.0 generalized the syntax (and semantics) so any expression could
appear on the RHS of the "/" operator (written in parentheses if necessary).
XSLT 2.0 however kept the syntax much more restricted for patterns, to ensure
efficient matching: patterns in 2.0 allowed basically only the Step syntax of
1.0 (plus the ability to call the id() function).

XSLT 3.0 then removed these restrictions to allow a parenthesized expression
on the RHS of "/". It's worth noting that this opened up a very wide range of
possibilities which weren't immediately covered by test cases or by the Saxon
implementation; we're still occasionally finding cases that aren't implemented
or are implemented incorrectly, see for example

https://saxonica.plan.io/issues/5368
https://saxonica.plan.io/issues/4755

As to the "why" question, I don't know why XPath 1.0 imposed the restriction,
though it's fairly obvious why it was subsequently relaxed. One of the design
debates you can have here is paternalism versus orthogolnality. Paternalism
says there's no point writing an expression on the RHS that doesn't depend on
the context item (for example, a literal or a variable reference), so it
should be flagged as an error. Orthogonality says you don't ban things just
because they appear to be useless: for example, you don't make it an error to
write $x + 0, so why make it an error to write $x / 0 ? And then paternalism
says, anyone who writes $x / 0 has probably made a mistake. There are
arguments both ways, which is why the spec isn't always consistent.

Michael Kay
Saxonica


> On 19 May 2022, at 14:22, Roger L Costello costello@xxxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hi Folks,
>
> Why can I have a template rule like this:
>
> <xsl:template
match="(sidRunwayTransition|sidCommonRoute|sidEnrouteTransition)/supplemental
Data">
>
> But I cannot have a template rule like this:
>
> <xsl:template
match="sid/(sidRunwayTransition|sidCommonRoute|sidEnrouteTransition)">
>
> /Roger