Like Resource Shapes 2.0, the Shape Expressions provides a way to constrain the properties, types and cardinality of nodes in RDF graphs. Also like Resource Shapes 2.0, Shape Expressions provides a recursive definition for a shape where the value of some property is defined by another shape (see ValueReferencevalueShape). Shape Expressions provides a domain-specific language and a semantics for describing/validating RDF graphs, and expands the expressivity to include disjunction and groups. The domain specific language is reminiscent of Relax NG Compact Syntax, though it lacks that language's features to combine, extend and override schemas. The Shape Expressions Demo compiles to validating SPARQL queries, though the limitations of that mapping are yet to be explored.
The Submission contrasts the constraints-based definition of RDF graph shapes with the inference-based methods available with RDFS and OWL. While the Shape Expressions Submission cites no use cases or requirements, the high-level use cases in the Resource Shapes clearly motivate Shape Expressions. Furthermore, it is evident from the text that significant implementation experience exists, notably the examples, thus providing evidence of usefulness of the technology. Combined with Resource Shapes, Shape Expressions defines an infrastructure that is taken for granted in formats like XML and relational databases, the absence of which from RDF infrastructures is (anecdotally) sometimes greeted with distaste."
The Shape Expressions language includes rules to:
Importantly, the language also includes semantic actions to perform extensible tests on validated data or other behavior defined by the particular extension.
The 2013 W3C RDF Validation Workshop assembled requirements and proposals around RDF validation and interface specification. The agenda for that workshop lists several approaches to meeting similar use cases. Dublin Core's Description Set Profiles (DSP) identifies similar cardinality constraint restrictions. The European Commission's DCAT Application Profile is a visible example of a non-machine readable set of rules that could benefit from automation using Resource Shapes/Shape Expressions.
This document provides a useful constraints language as well as laying the groundwork for future work on validation and shape definition languages and protocols.
Continuing discussion of this topic, including whether or not this area is ready for W3C standards-track work, is welcome via public-rdf-shapes@w3.org [achive].