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formal objection to advancing to candidate recommendation status

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Wed, 22 Mar 2017 17:13:42 -0700
To: "public-rdf-shapes@w3.org" <public-rdf-shapes@w3.org>
Message-ID: <73464b3a-3f14-4cb9-a9ab-89b3876eb5d8@gmail.com>
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/ to
Candidate Recommendation using the editors' draft at
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
Its current version is in a proposed editor's draft at
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 Thursday, 23 March 2017 00:14:20 UTC

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