Re: [xsl] W3C Specification of fn:filter() -- is this a bug in the document or in Saxon?

Subject: Re: [xsl] W3C Specification of fn:filter() -- is this a bug in the document or in Saxon?
From: "Dimitre Novatchev dnovatchev@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 9 Sep 2019 22:43:15 -0000
> The alternative formulation wouldn't change anything. It would still have
the same theoretical weakness that the rewritten
> expression might use different resources and therefore fail under
different circumstances. It might make it less likely that the
> two formulations would differ in practice, but this is a specification,
not a suggested implementation.
>
> The convention in specifications is to ignore efficiency considerations
when specifying functionality. Saying that A is equivalent
> to B carries implicit caveats, like, "provided you have enough memory and
no-one turns the power switch off while waiting for it
> to finish".

Sounds like a convenient excuse for providing code that may be far from
good :(

Dimitre

On Mon, Sep 9, 2019 at 7:12 AM Michael Kay mike@xxxxxxxxxxxx <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> The alternative formulation wouldn't change anything. It would still have
the same theoretical weakness that the rewritten expression might use
different resources and therefore fail under different circumstances. It
might make it less likely that the two formulations would differ in
practice, but this is a specification, not a suggested implementation.
>
> The convention in specifications is to ignore efficiency considerations
when specifying functionality. Saying that A is equivalent to B carries
implicit caveats, like, "provided you have enough memory and no-one turns
the power switch off while waiting for it to finish".
>
> Michael Kay
> Saxonica
>
> On 9 Sep 2019, at 14:20, Dimitre Novatchev dnovatchev@xxxxxxxxx <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> > I'm aware that some languages have attempted to formulate rules in the
language semantics making tail call optimization mandatory. The XSL and
>  > XQuery WGs considered several times whether to try and make the whole
"errors and optimization" rules more formal and rigorous, and we decided we
> > didn't have the skills and resources to do it, for the same reason that
work on the XQuery formal semantics was abandoned.
> >
> > Michael Kay
> > Saxonica
>
> The original problem can be eliminated (and the same solution may be
applicable in similar cases), if the "equivalent implementations" were
replaced with non-recursive code, As in this case -- just use:
>
> function($f as function(item()) as xs:boolean, $list as item()*) as
item()*
> {
>   $list ! .[$f(.)]
> }
>
> Thanks,
> Dimitre
>
>
> On Sun, Sep 8, 2019 at 10:22 PM Michael Kay mike@xxxxxxxxxxxx <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> The "errors and optimization" rule in XPath says that processors can
quite legitimately rewrite one expression with another that has different
resource requirements and that therefore has different failure
characteristics. This is by design. It means that either of these
formulations could fail with a stack overflow, and in that sense they are
indeed equivalent.
>>
>> I'm aware that some languages have attempted to formulate rules in the
language semantics making tail call optimization mandatory. The XSL and
XQuery WGs considered several times whether to try and make the whole
"errors and optimization" rules more formal and rigorous, and we decided we
didn't have the skills and resources to do it, for the same reason that
work on the XQuery formal semantics was abandoned.
>>
>> Michael Kay
>> Saxonica
>>
>> On 9 Sep 2019, at 02:44, Dimitre Novatchev dnovatchev@xxxxxxxxx <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> >  You can never guarantee that two expressions are equivalent in your
>> > sense, because of "errors and optimization". Any construct might raise
>> > an error - in the case of this example, stack overflow if the recursion
>> > gets too deep.
>>
>> What about tail-recursion?
>>
>> For years we have known recursive expressions whose tail-recursiveness
is correctly recognized in BaseX and it provides correct evaluation
regardless of the input size (recursion depth) but other processors fail
miserably...
>>
>> How much value for the developers would have been provided by the
specification if it mandated proper handling of tail-recursion!!!
>>
>> The value provided in a document is rather debatable when specifying
"equivalent implementations" that blow up for reasonably long inputs
(several thousand items isn't too high!) when other implementations could
have been provided that demonstrate equivalence with much longer inputs
(millions of items)
>>
>> Also, why in an XPath specification give "equivalent implementations" in
two different languages neither of which is XPath?
>>
>> Cheers,
>> Dimitre
>>
>> On Sun, Sep 8, 2019 at 5:54 PM Liam R. E. Quin liam@xxxxxxxxxxxxxxxx <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>> On Mon, 2019-09-09 at 00:18 +0000, Dimitre Novatchev
>>> dnovatchev@xxxxxxxxx wrote:
>>> > The W3C F&O 3.1 spec (at
>>> > https://www.w3.org/TR/xpath-functions-31/#func-filter ) says:
>>> >
>>> > Rules
>>> >
>>> > The effect of the function is equivalent to the following
>>> [...]
>>> >
>>> > Because "equivalent" means the two functions must produce the same
>>> > result
>>> > for for all possible values in the same set of arguments,
>>>
>>> That is one possible definition of "equivalent" but it is not the one
>>> used in the Functions and Operators document...
>>>
>>> You can never guarantee that two expressions are equivalent in your
>>> sense, because of "errors and optimization". Any construct might raise
>>> an error - in the case of this example, stack overflow if the recursion
>>> gets too deep.
>>>
>>> Liam
>>>
>>> --
>>> Liam Quin, https://www.delightfulcomputing.com/
>>> Available for XML/Document/Information Architecture/XSLT/
>>> XSL/XQuery/Web/Text Processing/A11Y training, work & consulting.
>>> Carefoot Web-slave for historical images http://www.fromoldbooks.org/
>>>
>
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-- 
Cheers,
Dimitre Novatchev
---------------------------------------
Truly great madness cannot be achieved without significant intelligence.
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Never fight an inanimate object
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To avoid situations in which you might make mistakes may be the
biggest mistake of all
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Facts do not cease to exist because they are ignored.
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Sanity is madness put to good use.
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I finally figured out the only reason to be alive is to enjoy it.

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