To my mind the solution is to add to the literature, not try to repeal
the past. Of course, you are doing that (as will your forthcoming
papers on the topic). Which is the part I find interesting and
instructive, not the fault-finding.
Wendell -- Absolutely!
I was just trying to use the suggested "equivalent implementation" as a
base for my own tests -- then the question naturally had to arise -- Why on
earth did they provide **this** implementation and not something better
(shorter, non-recursive, easier to read and hopefully in pure XPath 3.1
only)
even if we were to concede the truth of everything you are
saying, no standards committee can operate forever can it?
Yes, and we have the good example of Satoshi Nakamoto who published just a
single white paper that is revolutionizing technology today.
Most good results we all are aware of were produced outside of any
committee :)
Cheers,
Dimitre
On Wed, Sep 11, 2019 at 2:17 PM Wendell Piez wapiez(_at_)wendellpiez(_dot_)com <
xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:
Dimitre -- even if we were to concede the truth of everything you are
saying, no standards committee can operate forever can it?
To my mind the solution is to add to the literature, not try to repeal
the past. Of course, you are doing that (as will your forthcoming
papers on the topic). Which is the part I find interesting and
instructive, not the fault-finding.
Cheers, Wendell
On Mon, Sep 9, 2019 at 6:43 PM Dimitre Novatchev
dnovatchev(_at_)gmail(_dot_)com
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> 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".
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(_at_)saxonica(_dot_)com <
xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> 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(_at_)gmail(_dot_)com <
xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> 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(_at_)saxonica(_dot_)com <
xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> 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(_at_)gmail(_dot_)com <
xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> 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(_at_)fromoldbooks(_dot_)org
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:
On Mon, 2019-09-09 at 00:18 +0000, Dimitre Novatchev
dnovatchev(_at_)gmail(_dot_)com 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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