Re: what is inferencing?

Peter,
If by inferencing, you mean the the validation results are inferred from 
information in the associated graph, then I see your point.

Is the discussion about inferencing for SHACL addressing the case where 
there is the possibility of using RDFS or OWL schema information to infer 
additional triples in the graph itself that must be included in the SHACL 
evaluations? If so, that might require any conforming implementation to 
support inferencing in order to support SHACL, and that may not only be 
limiting, but it might also be undesirable in some usage scenarios such as 
those typically handled by OSLC Resource Shapes. And it raises the 
question of which inferencing.

A similar argument would apply to inferencing on the SHACL graph itself 
assuming SHACL provides an RDFS schema.



Jim Amsden, Senior Technical Staff Member
OSLC and Linked Lifecycle Data
919-525-6575




From:   "Peter F. Patel-Schneider" <pfpschneider@gmail.com>
To:     RDF Data Shapes Working Group <public-data-shapes-wg@w3.org>
Date:   05/09/2016 09:26 PM
Subject:        what is inferencing?



There has been quite a bit of discussion of inferencing and SHACL 
recently.
However, there has not been much discussion on the actual relationship 
between
SHACL and inferencing.

It turns out that SHACL depends entirely on inferencing.  The definition 
of
sh:nodeKind, probably the simplest construct in SHACL, says "A validation
result must be produced for each value node that does not match the given 
node
kind."  But this is inferencing!  True, a very degenerate form of 
inferencing
but inferencing none the less.

OK, let's rule out degenerate forms of inferencing, i.e., where all that 
is
required is looking at the form of an RDF term.

But then there is sh:hasValue.  Its definition says "A validation result 
must
be produced if the sh:hasValue is not among the value nodes."  This is 
again
inferencing and not so degenerate as the inferencing required for 
sh:nodeKind.
 This construct succeeds or fails depending on whether a specfic triple is
present in an RDF graph.   This is inferencing.  True, trivial inferencing 
but
inferencing none the less.

OK, let's rule out trivial forms of inferencing , i.e., where all that is
required is determining the presence of a single triple in an RDF graph.

But then there is sh:minCount.  Its definition says "A validation result 
must
be produced if the number of value nodes is less than the value of
sh:minCount."  This again inferencing and not inferencing that can be done 
by
only looking for a single RDF triple.  Here the number of triples that 
satisfy
a particular criterion has to be determined.  This is actually quite
sophisticated inferencing indeed.

It thus seems that SHACL needs sophisticated inferencing.  Nonetheless 
let's
press on and rule out even the sort of inteferencing where triples have to 
be
retrieved from an RDF graph until a set number of matching triples are 
obtained.

But then there is sh:class.  The definition of sh:class says "A validation
result must be produced for each value node that is either a literal or a
non-literal without a matching rdf:type. A non-literal matches a type if 
it
has an rdf:type value that is the given type or one of its (transitive)
subclasses, via rdfs:subClassOf."   So sh:class depends on the 
determination
of transitive closure, again a quite sophisticated kind of inference.

So what does it mean that SHACL doesn't use inferencing?  As far as I can 
see
the only true statement about SHACL and (non-)inferencing is that 
everything
in SHACL can be implemented by a simple translation from a SHACL shape to 
a
SPARQL query.  (This dividing line isn't even supported by the SHACL 
document
as the translation there is not to SPARQL but some very significantly 
extended
SPARQL.)   And even this isn't true if recursion is added to SHACL.

It certainly isn't the case that the dividing line is the need to reason 
by
cases (or, equivalently, to entertain multiple models).   RDFS inferencing
does not need reasoning by cases and SHACL does not need all of RDFS 
inferencing.

If there is a dividing line, it may be that SHACL inferencing is not
inherently serial.  However, I'm not at all sure that SHACL is indeed not
inherently serial, even with numbers written in unary and without 
recursion.


The net result is that SHACL depends on inferencing, and even quite
sophisticated inferencing at that.

peter