  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.
