Computing Recursive SPARQL Queries

We present a simple approach to handle recursive SPARQL queries, that is, nested queries that may contain references to the query itself. This powerful feature is obtained by implementing a custom SPARQL function that takes a SPARQL query as a parameter and executes it over a specified endpoint. The behaviour is similar to the SPARQL 1.1 SERVICE clause, with a few fundamental differences: (1) the query passed as argument can be arbitrarily complex, (2) being a string, the query can be created at runtime in the calling (outer) query, and (3) it can reference to itself, enabling recursion. These features transform the SPARQL language into a Turing-equivalent one without introducing special constructs or needing another interpreter implemented on the endpoint server engine. The feature is implemented using the standard Estensible Value Testing described in the recommendations since 1.0, therefore, our proposal is standard compliant and also compatible with older endpoints not supporting 1.1 Specifications, where it can be also a replacement for the missing SERVICE clause.