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introducing CONSTRAINTS

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Fri, 23 Jan 2015 14:55:45 -0800
Message-ID: <54C2D171.1040103@gmail.com>
To: RDF Data Shapes Working Group <public-data-shapes-wg@w3.org>
Hash: SHA1

The LDOM proposal has a particular view of what sort of thing the working
group shoud be producing.  Here is an opposing view.


		Introducing CONSTRAINTS

CONSTRAINTS (Class-based Or Neutral SpecificaTion of Rdf And lINked daTa
Shapes) defines shapes and controls how shapes (local and global) are
evaluated.  In CONSTRAINTS, a node in an RDF graph satisfies or violates a
local shape over the RDF graph.  In CONSTRAINTS, an RDF graph satisfies or
violates a global shape.  There are the only two possibilities that can
result from such evaluations---either the shape is satisfied or it is
violated.  It is not possible for a particular shape evaluation to be
neither satisfied nor violated or both satisfied and violated.

A CONSTRAINTS document contains global shapes and local shapes as well as
control information.  Control information takes the form of instance-based
CONSTRAINTS links that associate IRIs with local shapes, class-based
CONSTRAINTS links that associate non-datatype RDFS classes with local
shapes, and inter-shape CONSTRAINTS links that associate two shapes---a
scope local shape and a validation local shape.  Control information can
also specify what information is reported when a shape evaluation violation

A CONSTRAINTS document is satisfied by an RDF graph precisely when
1/ all the global shapes in the document are satisfied by the RDF graph,
2/ for all instance-based CONSTRAINTS links the IRI is a node in the
   graph and that node satisfies the local shape over the graph,
3/ for all class-based CONSTRAINTS links all nodes that are RDFS instances
   of the class (technically, all nodes for which their membership in the
   class is an RDFS entailment of the graph) also satisfy the local
   shape over the graph, and
4/ for all inter-shape links all nodes that satisfy the scope shape over the
   graph also satisfy the validation shape over the graph.

NOTE: If there is a way of constructing a shape that is precisely satisfied
by a given node then instance-based control can be transformed into
inter-shape control.  If there is way of constructing a shape that is
satisfied precisely by the nodes that are instances of a class then
class-based control can be transformed into inter-shape control.

There are no other organizational facilities in CONSTRAINTS, except saying
that the RDF graph over which shape evaluations is performed is the
combination of several other RDF graphs, e.g., to have one RDF graph
providing data and another an RDFS ontology combined into the RDF graph over
which shapes are evaluated.  All ontological information in CONSTRAINTS is
provided by the RDF graph and is interpreted as in RDFS.

NOTE: Although RDFS is used as the semantic basis of CONSTRAINTS, evaluating
shapes over RDF graphs that don't use RDFS vocabulary doesn't need to use
any aspect of RDFS.  RDFS class instance then reduces to explicit rdf:type
links in this case, with the sole exceptions that all properties are
implicitly members of rdf:Property and all literal values are implicitly
members of the datatypes to which they belong.  It is not even necessary to
give rdf:type any special syntactic status, as in reasonable versions of
CONSTRAINTS class-based links can be replaced with inter-shape links where
the scope shape uses rdf:type as a regular property.

Details of the Shapes in CONSTRAINTS

What remains to be determined is just what facilities are provided by
shapes, including whether and how shapes can be related to other shapes.
One option is that global shapes are SPARQL queries, local shapes are SPARQL
queries with a special variable, and that the core operation is running the
query on the RDFS consequences of the RDF graph.  Another option is that
local shapes are ShExC shape expressions and interpreted as in ShExC over
the RDFS consequences of an RDF graph and that global shapes are not used.
A third option is that local shapes are OWL 2 class expressions, global
shapes are OWL 2 axioms, and that the core operation is evaluating OWL
axioms on the Herbrand model of the RDFS consequences of the RDF graph.
Combinations or variations of these options are also possible.
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Received on Friday, 23 January 2015 22:56:19 UTC

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