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[Bug 29482] [XSLT30] Focus-changing constructs

From: <bugzilla@jessica.w3.org>
Date: Thu, 03 Nov 2016 17:40:20 +0000
To: public-qt-comments@w3.org
Message-ID: <bug-29482-523-6jRZk7CgXL@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=29482

--- Comment #5 from Michael Kay <mike@saxonica.com> ---
The definition of higher-order operand is effectively:

[An operand O of a construct C is higher-order if the semantics of C
potentially require O to be evaluated more than once during a single evaluation
of C.] More specifically, O is a higher-order operand of C if any of the
following conditions is true:

(1) The context item for evaluation of O is different from the context item for
evaluation of C.

(2) C is an instruction and O is a pattern (as with the from and count
attributes of xsl:number, and the group-starting-with and group-ending-with
attributes of xsl:for-each-group).

(3) C is an XPath for, some, or every expression and O is the expression in its
return or satisfies clause.

(4) C is an inline function declaration and O is the expression in its body.

I think the "]" here is misplaced (we used to have markup restrictions that
prevented multi-paragraph definitions). The real test as to whether something
is higher order is the conditions (1) to (4).

In this example, the body of xsl:copy satisfies condition (1) even though it is
only evaluated once.

So if we want to make this example streamable then we have to change the rules,
and the question is how we should change them.

I think my preference is NOT to make this guaranteed streamable: (a) it
complicates the spec; (b) complicates my implementation; (c) I think it's not
easy to convince ourselves that we aren't opening up lots of unintended
consequences if we make a change here.

As regards the examples in comment #4, my reading of the spec is that the "more
specifically" rules take precedence; the rule "C is an XPath for, some, or
every expression and O is the expression in its return or satisfies clause."
applies whether or not the controlling expression can actually deliver multiple
items.

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Received on Thursday, 3 November 2016 17:40:27 UTC

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