The SPARQL query language for RDF is the standard recommended by the W3C to query the Semantic Web. It enables to query and search RDF data by using query graph patterns and filters restricting the solutions produced by graph pattern matching according to the constraints they express. Constraints are either expressions defined according to the expression language specified in the W3C recommendation or function calls to SPARQL builtin functions or to extension functions. The use of extension functions in a SPARQL query, although compliant to the recommendation, limits the interoperability of the query. In this report we propose a lightweight extension of SPARQL to enable the \textit{definition} of extension functions in the SPARQL filter language. We call it FunSPARQL, standing for Functional SPARQL, as it consists in providing SPARQL with a functional language for expressing constraints.