[Xpath-ng] Positional predicates (WAS: Minimal FIXPath (with a bonus)?)

[Joe English]

Jeni Tennison wrote:

> [...] I'd also suggest limiting
> predicates to positional predicates in the core version, or perhaps
> omitting them altogether (XML Schema doesn't use them).

I'm strongly in favor of omitting positional predicates,
and using a different syntax for selection by position
(perhaps an infix operator "list # n", or a function
"nth(list, n)").

This would be a significant simplification.  The dynamic
parts of the evaluation context -- the context size and
context position -- would no longer be needed, leaving
only the static parts.  It would remove one of the major
obstacles to optimization (you can't optimize "expr[$n]" without
knowing if $n is a number or something else).  In the absense
of "last()", fully lazy evaluation becomes possible.  It would
make predicates associative -- (expr[p1])[p2] = (expr[p1][p2]) --
which is not the case in XPath 1.0.  Some of the most
obscure and confusing parts of the XPath spec could be
removed.

The "foo[n]" syntax in XPath is a DWIM gone bad.
I think we should drop it, if we can get away with it.


--Joe English

  jenglish at flightlab.com