  In constraint languages for RDF graphs, such as ShEx and SHACL, constraints
on nodes and their properties in RDF graphs are known as "shapes". Schemas in
these languages list the various shapes that certain targeted nodes must
satisfy for the graph to conform to the schema. Using SHACL, we propose in this
paper a novel use of shapes, by which a set of shapes is used to extract a
subgraph from an RDF graph, the so-called shape fragment. Our proposed
mechanism fits in the framework of Linked Data Fragments. In this paper, (i) we
define our extraction mechanism formally, building on recently proposed SHACL
formalizations; (ii) we establish correctness properties, which relate shape
fragments to notions of provenance for database queries; (iii) we compare shape
fragments with SPARQL queries; (iv) we discuss implementation options; and (v)
we present initial experiments demonstrating that shape fragments are a
feasible new idea.
