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[XPath] Focus for evaluating E1/E2

From: Kay, Michael <Michael.Kay@softwareag.com>
Date: Tue, 18 Nov 2003 23:49:37 +0100
Message-ID: <DFF2AC9E3583D511A21F0008C7E62106073DD2BC@daemsg02.software-ag.de>
To: public-qt-comments@w3.org
I would like to make a proposal to change the rules affecting the focus when
evaluating a path expression in the form E1/E2. Specifically, I propose that
when evaluating E2, the context position and context size should both be set
to 1 (one), not to the position of the context node in the list of nodes
selected by E1 and the size of this list.

There are very few expressions that will be affected by this difference.
Because E2 must return a sequence of nodes, you need to find an expression
that uses position() or last() but still returns nodes. It's not enough to
use position() or last() within a predicate, because a predicate changes the
context. You need to find expressions such as

  a/b/remove($c, position())

or

  a/b/subsequence($c, position())

or 

  a/b/(for $i in 1 to last() return $c[$i])

I have not found any expression in this category that is actually useful, or
that cannot be written in a more natural way.

But why change the rules? The reason is that the current rule makes the "/"
operator non-associative. The result of (a/b)/remove($c, position()) is not
the same as a/(b/remove($c, position()). Rewriting a/b/c as a/(b/c) can be
an important rewrite for optimizers as it often reduces the need to perform
a sort.

To be fair, I should point out that

(a) it's not difficult to detect path expressions that use position() or
last() on the right hand side of "/", and

(b) there are other situations that can make "/" non-associative:
specifically, when one of the operands creates new nodes with distinct
identity by means of a function call (or in XQuery, a node constructor) 

However, I think it's not reasonable to ask implementations to do a lot of
extra work for the sake of constructs that will only ever be found in
conformance tests. The rule that position() and last() should both be set to
1 is just as good as the present rule from a user perspective (there are
plently of cases in XSLT where these values are always set to 1 already,
e.g. when evaluating keys) and a lot easier from an implementor's
perspective.

There are no backwards compatibility implications, because XPath 1.0
restricted the rhs of "/" to be a step, which could not contain calls on
position() or last() except in a predicate.

Michael Kay
Received on Tuesday, 18 November 2003 17:50:38 GMT

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