Recursion in RDF Data Shape Languages

TL;DR

This paper defines recursion in RDF data shape languages like SHACL and Resource Shape 2.0, providing a language-independent formalization that could influence the development of future standards.

Contribution

It offers a precise, language-agnostic definition of recursion in RDF shape languages, applicable to current and future standards like SHACL and ShEx.

Findings

Recursion is formally defined in RDF data shape languages.

The definition is largely independent of language-specific details.

Recursion could be incorporated into SHACL-SPARQL using this formalization.

Abstract

An RDF data shape is a description of the expected contents of an RDF document (aka graph) or dataset. A major part of this description is the set of constraints that the document or dataset is required to satisfy. W3C recently (2014) chartered the RDF Data Shapes Working Group to define SHACL, a standard RDF data shape language. We refer to the ability to name and reference shape language elements as recursion. This article provides a precise definition of the meaning of recursion as used in Resource Shape 2.0. The definition of recursion presented in this article is largely independent of language-specific details. We speculate that it also applies to ShEx and to all three of the current proposals for SHACL. In particular, recursion is not permitted in the SHACL-SPARQL proposal, but we conjecture that recursion could be added by using the definition proposed here as a top-level control structure.