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[Bug 25160] [XSLT 3.0] a//c with striding CP is roaming by the rules, though we strove to make it streamable

From: <bugzilla@jessica.w3.org>
Date: Fri, 28 Mar 2014 08:59:28 +0000
To: public-qt-comments@w3.org
Message-ID: <bug-25160-523-IfslHHY1th@http.www.w3.org/Bugs/Public/>

--- Comment #5 from Michael Kay <mike@saxonica.com> ---
Unfortunately the above proposal has a rather serious bug. The expression
child::x gets rewritten to something like //*[self::x] which not only gives the
wrong answer, it also has the wrong posture.

There's also (at least one) further bug. The expression //x is not streamable
unless the context item happens to be the document node, but it is equivalent
to a motionless pattern and gets rewritten under the proposed rules.

I'm therefore wondering about a different formulation. Delete section 19.1, and
instead of preprocessing the expression tree, we do the streamability analysis
as described and then have a second attempt if it fails. More specifically, in
section Streamability of Path Expressions, we recast the rules as

1. Compute the provisional posture and sweep, as follows:

  (insert the existing rules)

2. If the provisional posture is roaming or the provisional sweep is
free-ranging (or both), and if the path expression is a *scanning expression*
(defined below), then the posture is crawling and the sweep is consuming.
Otherwise, the posture and sweep of the path expression are the provisional
posture and sweep.

Definition: A *scanning expression* is a relative path expression in the form
L/R that satisfies the following conditions:

(a) each of the operands L and R is either (i) a scanning expression, or (ii)
an AxisStep whose axis is child, descendant, descendant-or-self, or self.

(b) the expression is the *equivalent expression* of some *motionless pattern*.
(That is, when the expression is considered as a pattern, the rules for
classifying patterns (19.8.9) deem the pattern to be motionless.)

Rationale: note that we are no longer doing anything to change the semantics of
the expression, we are only defining its streamability properties. The
responsibility for devising a correct streaming implementation of such
expressions is thus left entirely to the implementor, as it should be. In
addition, we only invoke this rule as a fallback if the expression would
otherwise be non-streamable, which reduces the risk of introducing bugs.

As an alternative to (b) we could inline the rules for motionless patterns:
basically, the top-level predicates must be motionless and non-positional.

We will obviously need to add explanations for the reader; much of the required
material already exists.

The new rules no longer apply to union expressions. But I think we've relaxed
the rules for unions in sufficiently that they no longer need special
treatment. Under the above rules, section//(author|publisher) is not streamable
because the expression is not a "scanning expression". I think we can live with

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Received on Friday, 28 March 2014 08:59:30 UTC

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