Support all SPARQL functions as Node Expressions

[HolgerKnublauch]

As discussed in #367 there is an RDF/Turtle file at https://github.com/w3c/sparql-query/blob/main/sparql-ns.ttl that enumerates all SPARQL functions and establishes URIs for them. Those should all become List Parameter Functions in SHACL.

For example:

[ sparql:plus ( 38 4 ) ]

would be produce the result of 38 + 4 as a list with one RDF node.

The Node Expressions document could contain an auto-populated HTML table listing all these functions with their description and URI, and then a general description of how this table is used by SHACL engines.

[HolgerKnublauch]

@afs do you have a mapping from these sparql: URIs to executable Java code implemented in Jena already, so that I don't need to reinvent the wheel for the node expressions engine?

[afs]

There is an earlier partial one in package org.apache.jena.sparql.expr.urifunctions but there is a complete replacements which includes a class ExprNodeExpression extends ExprNode implements Expr in my "rules" project.

What oprrations in API are you expecting?

[HolgerKnublauch]

Thank you. I believe all I really need is the function SPARQLDispatch.exec, which looks good from a cursory view.

[afs]

Naming will change (to something not SPARQL specific), and it will be extended to general node expression evaluation.

[VladimirAlexiev]

@HolgerKnublauch will the same mechanism work for any extension functions? There are many libraries, eg:

• https://jena.apache.org/documentation/query/library-function.html
• https://www.daml.org/rules/proposal/builtins.html
• https://dotnetrdf.org/docs/3.2.x/developer_guide/sparql/leviathan_functions.html
• http://www.w3.org/2005/xpath-functions/math
• https://graphdb.ontotext.com/documentation/11.0/sparql-ext-functions-reference.html
• https://docs.stardog.com/query-stardog/query-functions

[HolgerKnublauch]

Yes, implementations can install their own node expression functions, based on SPARQL functions. The syntax would be like with the sparql: functions discussed here - a blank node with a single predicate and then an rdf:List of arguments. The examples you mention are outside of the standardization process of this WG.

[afs]

Support all SPARQL functions as Node Expressions

Those should all become List Parameter Functions in SHACL.

By "support", do you mean that a compliant SHACL processor that claims support for node expressions must implement all the functions?

An alternative is to "recommend" (== SHOULD) using the corresponding SPARQL function rather than another library (c.f. RDF Concepts recommending the use of XSD datatypes ) on the grounds of interoperability.

Some (a small number - I found ~16 in a quick scan) of entries in sparql-ns are not appropriate for SHACL.