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formal objection on pre-binding

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Tue, 2 May 2017 08:52:40 -0700
To: "public-rdf-shapes@w3.org" <public-rdf-shapes@w3.org>
Message-ID: <3330e0af-0420-1830-efb7-f98c4609a224@gmail.com>
This is a formal objection to the definition of pre-binding approved by the
working group in its meeting of 26 April 2017 and in the SHACL Editor's
Draft current as of 28 April 2017 (but with date indicated as 11 April
2017).

Pre-binding has never had a suitable definition in SHACL.  Problems with
pre-binding have been known to the working group since at least June of
2015.  The current definition of pre-binding has multiple documented
problems.  The only change to pre-binding from Candidate Recommendation has
been to paper over some of these problems by excluding large numbers of
SPARQL queries from SHACL-SPARQL.

These exclusions introduce new interoperability problems for SHACL.  These
exclusions remove many useful SPARQL queries from SHACL-SPARQL, including an
example SPARQL query in the SHACL document.  These exclusions also do not
exclude all SPARQL queries whose meaning is not as expected with
pre-binding.

The solution to the continuing problems with pre-binding is not to exclude
more and more of SPARQL but to either fix pre-binding so that it works
correctly or eliminate pre-binding from SHACL.



Although this is nowhere stated in the SHACL document, it appears that the
indent of the current definition of pre-binding is to make the binding of a
pre-bound variable available everywhere in the query except for in
sub-queries that do not project the variable.  The current definition of
pre-binding tries to achieve this by two main mechanisms.  The first
mechanism is renaming of variables in sub-queries that are not projected to
fresh variables.  The second mechanism joins pre-bound bindings to all basic
graph patterns, property path expressions, and named graph patterns in the
query.

However, the current definition of pre-binding fails to achieve the goal
stated above.  The bindings of pre-bound variables are sometimes not
available when needed.  This causes problems in a large number of queries,
many of which have been already pointed out.  Also the pre-bound bindings
are available even in places where none of the variables are in scope.  This
causes problems for MINUS.

The recent changes to the pre-binding section of the SHACL document do not
actually change the definition of pre-binding to fix these problems but
instead just exclude many SPARQL queries from SHACL, presumably to keep only
non-problematic queries.  The exclusions are very broad, and eliminate many
queries that do not have any problems for the current definition of
pre-binding.

The exclusion of so many SPARQL queries, particularly as many of them are
non-problematic, is a new source of interoperability problems for SHACL.  It
is easy for users to write SHACL-SPARQL shapes that are excluded.  The
behaviour of SHACL-SPARQL implementations on these excluded shapes is
undefined but SHACL-SPARQL implementations are not required to produce any
sort of signal when they encounter these excluded shapes.  Different
completely conforming SHACL-SPARQL implementations are thus allowed to
silently produce different results for these shapes.

The exclusions are so broad that they even exclude the example potential
definitional query for sh:EqualsConstraintComponent in the current SHACL
document itself.

The exclusions do not even exclude all problematic queries.  For example, a
SPARQL-SHACL shape based on the non-excluded query
  SELECT DISTINCT $this WHERE {
    { SELECT $this ?class WHERE { VALUES ?class { ex:C ex:D } } }
    BIND ( $this AS ?that )
    ?that a ?class . }
will produce violations for all target nodes whenever any node in the graph
has ex:C or ex:D as a value for rdf:type counter to the expectation that it
should produce violations only for target nodes that have ex:C or ex:D as a
value for rdf:type.

The use of a problematic definition of pre-binding needs to be eliminated
from SHACL.  Papering over these problems, even if possible, is not a
suitable solution.


Peter F. Patel-Schneider
Nuance Communications
Received on Tuesday, 2 May 2017 15:53:16 UTC

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