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Re: formal objection to advancing to candidate recommendation status

From: Irene Polikoff <irene@topquadrant.com>
Date: Tue, 28 Mar 2017 18:43:13 -0400
Message-Id: <09834F23-B8FB-44B0-94F2-8C16D88D5F6E@topquadrant.com>
To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, "<public-rdf-shapes@w3.org>" <public-rdf-shapes@w3.org>
Peter,

When will you be able to answer questions in this e-mail?

Irene
> On Mar 24, 2017, at 9:18 AM, Irene Polikoff <irene@topquadrant.com> wrote:
> 
> Peter,
> 
> In order to make sure the WG clearly understands your objection, let me ask you the following questions.
> 
> Your objection is based on the following 5 problems you have identified for SPARQL EXISTS. Correct?
>> 
>> 1. The proposed definition of EXISTS depends on a mapping that replaces local
>>> variables in a SPARQL algebra expression.  This mapping is defined for all
>>> SPARQL algebra expressions except for projections.  Pre-binding is thus
>>> undefined on any query of the form SELECT $this ?value WHERE { ... }  and
>>> indeed on any SELECT query with no DISTINCT or REDUCED keyword.  This
>>> problem was reported to the working group in
>>> https://lists.w3.org/Archives/Public/public-rdf-shapes/2017Mar/0022.html <https://lists.w3.org/Archives/Public/public-rdf-shapes/2017Mar/0022.html>
>> 
> 
> 
> Your example above appears to be wrong as per point 2 - as it binds $this. Nevertheless, issues with mapping have been resolved based on your own proposal.
> 
> Please clarify what problems remain.
> 
>> 
>> 2. Another problem affects SPARQL queries using AS.  Here the problem involves
>>> the Extend operator in the SPARQL algebra.  For example, the query
>>>  SELECT DISTINCT ?this WHERE { BIND ( ex:a AS ?this ) }
>>> with the variable this pre-bound results in an undefined use of Extend.
>>> This problem was reported to the SPARQL EXISTS Community Group in
>>> https://lists.w3.org/Archives/Public/public-sparql-exists/2017Mar/0000.html <https://lists.w3.org/Archives/Public/public-sparql-exists/2017Mar/0000.html>
> 
> 
> This issue has been responded to on the EXISTS mailing list https://lists.w3.org/Archives/Public/public-sparql-exists/2017Mar/0009.html <https://lists.w3.org/Archives/Public/public-sparql-exists/2017Mar/0009.html>
> 
> It is an illegal query. And you said in the referenced e-mail that “perhaps, there is no problem”. Do you have another example of a problem?
> 
>> 
>> 3. Another problem is that sometimes the proposal for pre-binding doesn't
>>> actually use the bindings.  For example,
>>>  SELECT DISTINCT $this WHERE { VALUES $this { ex:a ex:b ex:c } }
>>> with the variable this pre-bound to ex:d returns three solution bindings,
>>> one each with the variable this mapped to ex:a, ex:b, and ex:c, ignoring the
>>> pre-binding of the variable this.  This problem was reported to the SPARQL
>>> EXISTS Community Group in
>>> https://lists.w3.org/Archives/Public/public-sparql-exists/2017Mar/0001.html <https://lists.w3.org/Archives/Public/public-sparql-exists/2017Mar/0001.html>
> 
> We do not see this as a problem. Nevertheless, it has been addressed.
> 
> What is remaining?
>> 
>> 
>> 4. Yet another problem with the definition of pre-binding involves an
>>> unintuitive result.  One might expect that
>>>  SELECT DISTINCT ?this WHERE { ex:a ex:b ex:c . MINUS { ex:a ex:b ex:c . } }
>>> with ?this pre-bound would produce some results because there is no variable
>>> shared between the left-hand and right-hand sides of the MINUS.  However,
>>> pre-binding introduces a binding for the variable this on both sides so
>>> no results are produced.  This problem was reported to the SPARQL EXISTS
>>> Community Group quite some time ago.
>> 
> 
> 
> The SPARQL EXISTS Community Group has already noted that MINUS is a problem in the current SPARQL 1.1 specification : see Issue 4. Some changes in the evaluation of MINUS are therefore to be expected. Use cases are used to understand the impact of such changes. In case of SHACL, pre-binding is for patterns that involve the use of ?this. Thus, we see this example as somewhat artificial and without practical relevance for SHACL.
> 
> Issue 4:
> https://w3c.github.io/sparql-exists/docs/sparql-exists.html#issue-4-substitution-can-flip-minus-to-its-disjoint-domain-case <https://w3c.github.io/sparql-exists/docs/sparql-exists.html#issue-4-substitution-can-flip-minus-to-its-disjoint-domain-case>
> 
>> 
>> 
>> 5. There is also a potential problem with the definition of pre-binding as it
>>> depends on replacing variables in SPARQL queries with arbitrary new (fresh)
>>> variables.  As variable names do show up in SPARQL results, it should be
>>> demonstrated that this replacement does not affect the result of a SPARQL
>>> query.  I expect that this is indeed the case, but there is no demonstration
>>> of it in the SHACL document.  This problem was reported to the working group
>>> in https://lists.w3.org/Archives/Public/public-rdf-shapes/2017Mar/0009.html <https://lists.w3.org/Archives/Public/public-rdf-shapes/2017Mar/0009.html>
>> 
> 
> As we understand it, the problem has not been identified. There is no example or any other evidence of the problem. it is just that there is no mathematical proof that the problem does not exist. Correct?
> 
> The EXISTS document has some explanatory verbiage. It is not clear to us how to identify the basis on which the satisfactory proof could be built.  Since you are the person who needs to be satisfied, the WG is ready to work with you on this after the CR transition - if you are willing to do so. In the absence of your willingness, we do not see a practical way in which a proof you'd find satisfactory could be produced.
>> 
>> 
>> 
>>> Begin forwarded message:
>>> 
>>> From: "Peter F. Patel-Schneider" <pfpschneider@gmail.com <mailto:pfpschneider@gmail.com>>
>>> Subject: formal objection to advancing to candidate recommendation status
>>> Date: March 22, 2017 at 8:13:42 PM EDT
>>> To: "public-rdf-shapes@w3.org <mailto:public-rdf-shapes@w3.org>" <public-rdf-shapes@w3.org <mailto:public-rdf-shapes@w3.org>>
>>> Resent-From: public-rdf-shapes@w3.org <mailto:public-rdf-shapes@w3.org>
>>> 
>>> This is a formal objection to the resolution of the RDF Data Shapes Working
>>> Group to advance the maturity level of https://www.w3.org/TR/shacl/ <https://www.w3.org/TR/shacl/> to
>>> Candidate Recommendation using the editors' draft at
>>> http://w3c.github.io/data-shapes/shacl/ <http://w3c.github.io/data-shapes/shacl/> as of 22 March 2017.  The reason for
>>> this objection is that a fundamental part of SHACL, pre-binding, still has
>>> significant known problems.  Problems with pre-binding have been known to
>>> the working group since at least June of 2015.  A summary of some current
>>> known problems with pre-binding is given at the end of this message.
>>> 
>>> This objection is being filed at this time because pre-binding has been
>>> undergoing recent change and its known problems might have been fixed before
>>> requesting advancement for the document.
>>> 
>>> Pre-binding is so central to the SPARQL extension mechanism for SHACL,
>>> SHACL-SPARQL, that any significant problem with pre-binding undermines the
>>> entirety of SHACL-SPARQL.  Pre-binding is also a central part of
>>> non-normative descriptions of targets and constraints in SHACL Core so
>>> problems with pre-binding also affect the rest of the SHACL document.
>>> 
>>> The working group could have removed pre-binding from the document by
>>> splitting it into two pieces, one for SHACL Core and one for SHACL-SPARQL,
>>> and only advancing the SHACL Core piece.  This possibility had been
>>> discussed several times in the working group when there was more than
>>> adequate time to perform the split.
>>> 
>>> So, in summary, as of 22 March 2017 pre-binding underpins a large part of
>>> SHACL and has several known problems which affect the definition of SHACL.
>>> The SHACL document should not advance to candidate recommendation until
>>> these problems are fixed or pre-binding is eliminated from the document.
>>> 
>>> Peter F. Patel-Schneider
>>> Nuance Communications
>>> 
>>> 
>>> 
>>> A summary of some known problems with pre-binding in SHACL:
>>> 
>>> The current definition of pre-binding in SHACL is adapted from a proposed
>>> fix for SPARQL EXISTS, first appearing in
>>> https://lists.w3.org/Archives/Public/public-sparql-exists/2016Sep/0024.html <https://lists.w3.org/Archives/Public/public-sparql-exists/2016Sep/0024.html>
>>> Its current version is in a proposed editor's draft at
>>> https://w3c.github.io/sparql-exists/docs/sparql-exists.html <https://w3c.github.io/sparql-exists/docs/sparql-exists.html>
>>> The current adaptation of this proposal for pre-binding is at
>>> http://w3c.github.io/data-shapes/shacl/#pre-binding.
>>> 
>>> The proposal for EXISTS itself has several problems, and its adaptation for
>>> defining pre-binding has several more.  Some of these problems are minor and
>>> technical but some of them are quite major or affect a large number of
>>> SPARQL queries.
>>> 
>>> The proposed definition of EXISTS depends on a mapping that replaces local
>>> variables in a SPARQL algebra expression.  This mapping is defined for all
>>> SPARQL algebra expressions except for projections.  Pre-binding is thus
>>> undefined on any query of the form SELECT $this ?value WHERE { ... }  and
>>> indeed on any SELECT query with no DISTINCT or REDUCED keyword.  This
>>> problem was reported to the working group in
>>> https://lists.w3.org/Archives/Public/public-rdf-shapes/2017Mar/0022.html
>>> 
>>> Another problem affects SPARQL queries using AS.  Here the problem involves
>>> the Extend operator in the SPARQL algebra.  For example, the query
>>>  SELECT DISTINCT ?this WHERE { BIND ( ex:a AS ?this ) }
>>> with the variable this pre-bound results in an undefined use of Extend.
>>> This problem was reported to the SPARQL EXISTS Community Group in
>>> https://lists.w3.org/Archives/Public/public-sparql-exists/2017Mar/0000.html
>>> 
>>> Another problem is that sometimes the proposal for pre-binding doesn't
>>> actually use the bindings.  For example,
>>>  SELECT DISTINCT $this WHERE { VALUES $this { ex:a ex:b ex:c } }
>>> with the variable this pre-bound to ex:d returns three solution bindings,
>>> one each with the variable this mapped to ex:a, ex:b, and ex:c, ignoring the
>>> pre-binding of the variable this.  This problem was reported to the SPARQL
>>> EXISTS Community Group in
>>> https://lists.w3.org/Archives/Public/public-sparql-exists/2017Mar/0001.html
>>> 
>>> Yet another problem with the definition of pre-binding involves an
>>> unintuitive result.  One might expect that
>>>  SELECT DISTINCT ?this WHERE { ex:a ex:b ex:c . MINUS { ex:a ex:b ex:c . } }
>>> with ?this pre-bound would produce some results because there is no variable
>>> shared between the left-hand and right-hand sides of the MINUS.  However,
>>> pre-binding introduces a binding for the variable this on both sides so
>>> no results are produced.  This problem was reported to the SPARQL EXISTS
>>> Community Group quite some time ago.
>>> 
>>> There is also a potential problem with the definition of pre-binding as it
>>> depends on replacing variables in SPARQL queries with arbitrary new (fresh)
>>> variables.  As variable names do show up in SPARQL results, it should be
>>> demonstrated that this replacement does not affect the result of a SPARQL
>>> query.  I expect that this is indeed the case, but there is no demonstration
>>> of it in the SHACL document.  This problem was reported to the working group
>>> in https://lists.w3.org/Archives/Public/public-rdf-shapes/2017Mar/0009.html
>>> 
>>> The above are mostly problems about how pre-binding is defined.  Only when
>>> these problems are eliminated can the definition of pre-binding be examined
>>> to see whether it does some right thing as far as SHACL is concerned.
>>> Indeed, a previous definition of pre-binding in SHACL suffered from this
>>> sort of problem.
>>> 
>> 
> 


Received on Tuesday, 28 March 2017 22:43:50 UTC

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