Re: [xsl] Numeric top-level predicates in patterns

[Michael Kay <mike@xxxxxxxxxxxx>]

I think this is all true. Some comments attached.

On 2 Apr 2014, at 15:51, Abel Braaksma (Exselt) <abel@xxxxxxxxxx> wrote:

>
> Apart from positional predicates, I am not sure whether any of these
> have any official bearing, as in whether they are elswehere defined as
such.

We might have used some of these terms in the streamability analysis, but they
certainly aren't official terms in 2.0.
>
> Now the question. After the discussion on position(), I started
> wondering about patterns and position. I believe the following is all
> true (assume they the patterns are placed inside <xsl:template
match="XXX">.
>
> Selects nothing, ever (not even in a select-expression):
> foo[2][3]
> foo[0]
> bar[last()][2]
>
> Redundant [1], always redundant:
> foo[2][1][1]
> foo[3][1]/bar[4][1]
>
> The rules I am after is, in this order:
> 1) numerical predicates lower than zero never select anything

More precisely, the only numerical predicates that can select anything are
positive integers.

> 2) a sibling numerical predicate that is one is always redundant in a
> pattern

"Sibling numerical predicate" isn't one of your defined terms. I assume you
mean a predicate of [1] preceded by another numerical predicate.

> 3) a sibling predicate that is a sibling of a numeric predicate that is
> higher than one never selects anything
>
> Then, to make it tricky, what happens here, if the applied set is //foo?
> (foo[2])[3]

This never selects anything, but (.//foo[2])[3] does. The difference is that
foo[2] always selects 0 or 1 elements, whereas .//foo[2] may select more than
one.
>
>
> Bottom line is, I think, that creating multiple sibling numerical
> predicates is (usually) counter-productive and won't match anything when
> used in a pattern (unless the number is "1", in which case it is also
> redundant).
>

More generally, an expression using a numeric predicate returns a value with
cardinality 0:1, and applying a numeric predicate to a value with cardinality
0:1 is pointless.

Michael Kay
Saxonica