ARQ - JavaScript SPARQL Functions

ARQ supports (Jena v3.7.0 onwards) writing custom SPARQL functions in JavaScript. These functions can be used in FILTERs and for calculating values to assign with AS in BIND and SELECT expressions.

XSD datatypes for strings, numbers and booleans are converted to the native JavaScript datatypes. RDFterms that do not fit easily into JavaScript datatypes are handled with a object class NV.

Loading JavaScript functions

JavaScript is loaded from an external file using the context setting “http://jena.apache.org/ARQ#js-library". This can be written as arq:js-library for commands and Fuseki configuration files.

Example:

sparql --set arq:js-library=SomeFile.js --data ... --query ...

will execute on the data with the JavaScript functions from file “SomeFile.js” available.

JavScript functions can also be set from a string directly from within Java using constant ARQ.symJavaScriptFunctions (“http://jena.apache.org/ARQ#js-functions").

Using JavaScript functions

SPARQL functions implemented in JavaScript are automatically called when a URI starting “http://jena.apache.org/ARQ/jsFunction#" used.

This can conveniently be abbreviated by:

PREFIX js: <http://jena.apache.org/ARQ/jsFunction#>

Arguments and Function Results

xsd:string (a string with no language tag), any XSD numbers (integer, decimal, float, double and all the derived types) and xsd:boolean are converted to JavaScript string, number and boolean respectively.

SPARQL functions must return a value. When a function returns a value, it can be one of these JavaScript native datatypes, in which case the reverse conversion is applied back to XSD datatypes. For numbers, the conversion is back to xsd:integer (if it has no fractional part) or xsd:double.

The JavaScript function can also create NodeValue (or NV) objects for other datatypes by calling Java from within the JavaScript script engine of the Java runtime.

URIs are passed as NV object and are available in JavaScript as a string.

The class NV is used for all other RDF terms.

Returning JavaScript null is the error indicator and a SPARQL expression error (ExprEvalException) is raised, like any other expression error in SPARQL. That, in turn, will cause the whole expression the function is part of to evaluate to an error (unless a special form like COALESCE is used). In a FILTER that typically makes the filter evaluate to “false”.

Example

Suppose “functions.js” contains code to camel case words in a string. For example, “some words to process " becomes “someWordsToProcess”.

// CamelCase a string
// Words to be combined are separated by a space in the string.

function toCamelCase(str) {
    return str.split(' ')
    .map(cc)
    .join('');
}

function ucFirst(word)    {
    return word.charAt(0).toUpperCase() + word.slice(1).toLowerCase();
}

function lcFirst(word)    {
    return word.toLowerCase();
}

function cc(word,index)   {
    return (index == 0) ? lcFirst(word) : ucFirst(word);
}

and the query Q.rq

PREFIX js: <http://jena.apache.org/ARQ/jsFunction#>

SELECT ?input (js:toCamelCase(?input) AS ?X)
{
    VALUES ?input { "some woRDs to PROCESS" }
}

which can be executed with:

sparql --set arq:js-library=functions.js --query Q.rq

and it results in:

--------------------------------------------------
| input                   | X                    |
==================================================
| "some woRDs to PROCESS" | "someWordsToProcess" |
--------------------------------------------------

Use with Fuseki

The context setting can be provided on the command line starting the server, for example:

fuseki --set arq:js-library=functions.js --mem /ds

or it can be specifiid in the server configuration file config.ttl:

PREFIX :        <#>
PREFIX fuseki:  <http://jena.apache.org/fuseki#>
PREFIX rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs:    <http://www.w3.org/2000/01/rdf-schema#>
PREFIX ja:      <http://jena.hpl.hp.com/2005/11/Assembler#>

[] rdf:type fuseki:Server ;
    # Set the server-wide context
    ja:context [
         ja:cxtName "arq:js-library" ;
         ja:cxtValue "/filepath/functions.js"
    ] ;
.

<#service> rdf:type fuseki:Service;
    rdfs:label                   "Dataset";
    fuseki:name                  "ds";
    fuseki:serviceQuery          "sparql";
    fuseki:dataset <#dataset> ;
    .

<#dataset> rdf:type ja:DatasetTxnMem;
    ja:data <file:D.trig>;
.

and used as:

fuseki --conf config.ttl