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             <h1 class="title">Extensions in ARQ</h1>

 	<p>There are several ways to extend the ARQ query engine within the
 SPARQL syntax.</p>
 <ul>
 <li><a href="#value-functions">Expression Functions</a> - additional operations in
 <code>FILTERS</code>, <code>BIND</code> and <code>SELECT</code> expressions.</li>
 <li><a href="#property-functions">Property functions</a> - adding predicates
 that introduce custom query stages</li>
 <li><a href="#describe-handlers">DESCRIBE handlers</a></li>
 <li>Support for
 <a href="#match-blank-node-labels">finding blank nodes by label</a></li>
 <li><a href="arq-query-eval.html">Extending query evaluation</a> for querying
 different storage and inference systems</li>
 </ul>
 <p>Functions are standard part of SPARQL. ARQ provides
 application-written functions and provides a
 <a href="library-function.html">function library</a>. Applications can
 <a href="writing_functions.html">write and register their own functions</a>.</p>
 <p>Property functions provide a way to provide custom matching of
 particular predicates. They enable triple matches to be calculated,
 rather than just looked up in a RDF graph and they are a way to add
 functionality and remain within SPARQL. ARQ has a
 <a href="library-propfunc.html">property function library</a>. Applications
 can
 <a href="writing_propfuncs.html">write and register their own property functions</a>.</p>
 <p>The <a href="/documentation/query/text-query.html/">free text support in ARQ</a> is provided by
 <a href="http://lucene.apache.org/java/docs/index.html">Lucene</a>, using
 property functions.</p>
 <h2 id="filter-functions">Filter Functions</h2>
 <p>A SPARQL custom function is implementation dependent. Most details
 of the ARQ query engine do not have to be understood to write a
 function; it is a matter of implementing one interface.  This is
 made simpler for many cases by a number of base classes that
 provide much of the machinery needed.</p>
 <h3 id="function-registry">Function Registry</h3>
 <p>Functions can be installed into the function registry by the
 application. The function registry is a mapping from URI to a
 factory class for functions (each time a function is mentioned in a
 query, a new instance is created) and there is an auto-factory
 class so just registering a Java class will work. A function can
 access the queried dataset.</p>
 <h3 id="dynamically-loaded-functions">Dynamically Loaded Functions</h3>
 <p>The ARQ function library uses this mechanism.  The namespace of the
 ARQ function library is <code>&lt;http://jena.apache.org/ARQ/function#&gt;</code>.</p>
 <pre><code>PREFIX afn: &lt;http://jena.apache.org/ARQ/function#&gt;
 PREFIX dc: &lt;http://purl.org/dc/elements/1.1/&gt;
 SELECT ?v {
     ?x dc:date ?date .
     FILTER (?date &lt; afn:now() )
 }
 </code></pre>
 <p>The afn:now returns the time the query started.</p>
 <p>The expression functions in the ARQ distribution are described on the
 <a href="library-function.html">expression function library page</a>.</p>
 <p>URIs for functions in the (fake)
 <a href="java-uri.html">URI scheme <code>java:</code></a> are dynamically loaded. The
 class name forms the scheme specific part of the URI.</p>
 <h2 id="property-functions">Property functions</h2>
 <p>Property functions, sometimes called &ldquo;magic properties&rdquo;, are
 properties that cause triple matching to happen by executing some
 piece of code, determined by the property URI, and not by the usual
 graph matching. They can be used to give certain kinds of inference
 and rule processing. Some calculated properties have additional,
 non-declarative requirements such as needing one of other of the
 subject or object to be a query constant or a bound value, and not
 able to generate all possibilities for that slot.</p>
 <p>Property functions must have fixed URI for the predicate (it can&rsquo;t
 be query variable). They may take a list for subject or object.</p>
 <p>One common case is for access to collections (RDF lists) or
 containers (rdf:Bag, rdf:Seq, rdf:Alt).</p>
 <pre><code>PREFIX list: &lt;http://jena.apache.org/ARQ/list#&gt;
 SELECT ?member {
     ?x :p ?list .     # Some way to find the list
     ?list list:member ?member .
 }
 </code></pre>
 <p>which can also be written:</p>
 <pre><code>PREFIX list: &lt;http://jena.apache.org/ARQ/list#&gt;
 SELECT ?member {
     ?x :p [ list:member ?member ]
 }
 </code></pre>
 <p>Likewise, RDF containers:</p>
 <pre><code>PREFIX rdfs: &lt;http://www.w3.org/2000/01/rdf-schema#&gt;
 SELECT ?member {
     ?x :p ?bag .     # Some way to find the bag
     ?bag rdfs:member ?member .
 }
 </code></pre>
 <p>Property functions can also take lists in the subject or object
 slot.</p>
 <p>Code for properties can be dynamically loaded or pre-registered.
 For example, <code>splitIRI</code> will take an IRI and assign the namespace
 ad localname parts to variables (if the variables are already
 bound, not constants are used, <code>splitIRI</code> will check the values).</p>
 <pre><code>PREFIX xsd: &lt;http://www.w3.org/2001/XMLSchema#&gt;
 PREFIX apf: &lt;java:org.apache.jena.query.pfunction.library.&gt;
 SELECT ?namespace ?localname {
     xsd:string apf:splitIRI (?namespace ?localname)
 }
 </code></pre>
 <p>Property functions might conflict with inference rules and it can
 be turned off by the Java code:</p>
 <pre><code> ARQ.setFalse(ARQ.enablePropertyFunctions) ;
 </code></pre>
 <p>or on a per instance basis:</p>
 <pre><code> try(QueryExecution qExec = ... ) {
      qExec.getContext().setFalse(ARQ.enablePropertyFunctions) ;
      ...
  }
 </code></pre>
 <p>The property functions in the ARQ distribution are described on the
 <a href="library-propfunc.html">property function library page</a>.</p>
 <p>URIs for functions in the (fake)
 <a href="java-uri.html">URI scheme <code>java:</code></a> are dynamically loaded. The
 class name forms the scheme specific part of the URI.</p>
 <h2 id="describe-handlers">DESCRIBE handlers</h2>
 <p>The <code>DESCRIBE</code> result form in SPARQL does not define an exact form
 of RDF to return. Instead, it allows the server or query processor
 to return what it considers to be an appropriate description of the
 resources located. This description will be specific to the domain,
 data modelling or application.</p>
 <p>ARQ comes with one built-in handler which calculates the blank node
 closure of resources found. While suitable for many situations, it
 is not general (for example, a <a href="http://www.foaf-project.org/">FOAF</a>
 file usually consists of all blank nodes). ARQ allows the
 application to replace or add handlers for producing <code>DESCRIBE</code>
 result forms.</p>
 <p>Application-specific handlers can be added to the
 <code>DescribeHandlerRegistry</code>. The handler will be called for each
 resource (not literals) identified by the <code>DESCRIBE</code> query.</p>
 <h2 id="blank-node-labels">Blank Node Labels</h2>
 <p>URIs from with scheme name &ldquo;_&rdquo; (which is illegal) are created as
 blank node labels for directly accessing a blank node in the
 queried graph or dataset. These are constant terms in the query -
 not unnamed variables. Do not confuse these with the standard
 qname-like notation for blank nodes in queries. This is not
 portable - use with care.</p>
 <pre><code>&lt;_:1234-5678-90&gt; # A blank node in the data

 _:b0 # A blank node in the query - a variable
 </code></pre>


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	</div>
	<h1 class="title">Extensions in ARQ</h1>

	<p>There are several ways to extend the ARQ query engine within the
	SPARQL syntax.</p>
	<ul>
	<li><a href="#value-functions">Expression Functions</a> - additional operations in
	<code>FILTERS</code>, <code>BIND</code> and <code>SELECT</code> expressions.</li>
	<li><a href="#property-functions">Property functions</a> - adding predicates
	that introduce custom query stages</li>
	<li><a href="#describe-handlers">DESCRIBE handlers</a></li>
	<li>Support for
	<a href="#match-blank-node-labels">finding blank nodes by label</a></li>
	<li><a href="arq-query-eval.html">Extending query evaluation</a> for querying
	different storage and inference systems</li>
	</ul>
	<p>Functions are standard part of SPARQL. ARQ provides
	application-written functions and provides a
	<a href="library-function.html">function library</a>. Applications can
	<a href="writing_functions.html">write and register their own functions</a>.</p>
	<p>Property functions provide a way to provide custom matching of
	particular predicates. They enable triple matches to be calculated,
	rather than just looked up in a RDF graph and they are a way to add
	functionality and remain within SPARQL. ARQ has a
	<a href="library-propfunc.html">property function library</a>. Applications
	can
	<a href="writing_propfuncs.html">write and register their own property functions</a>.</p>
	<p>The <a href="/documentation/query/text-query.html/">free text support in ARQ</a> is provided by
	<a href="http://lucene.apache.org/java/docs/index.html">Lucene</a>, using
	property functions.</p>
	<h2 id="filter-functions">Filter Functions</h2>
	<p>A SPARQL custom function is implementation dependent. Most details
	of the ARQ query engine do not have to be understood to write a
	function; it is a matter of implementing one interface. This is
	made simpler for many cases by a number of base classes that
	provide much of the machinery needed.</p>
	<h3 id="function-registry">Function Registry</h3>
	<p>Functions can be installed into the function registry by the
	application. The function registry is a mapping from URI to a
	factory class for functions (each time a function is mentioned in a
	query, a new instance is created) and there is an auto-factory
	class so just registering a Java class will work. A function can
	access the queried dataset.</p>
	<h3 id="dynamically-loaded-functions">Dynamically Loaded Functions</h3>
	<p>The ARQ function library uses this mechanism. The namespace of the
	ARQ function library is <code><http://jena.apache.org/ARQ/function#></code>.</p>
	<pre><code>PREFIX afn: <http://jena.apache.org/ARQ/function#>
	PREFIX dc: <http://purl.org/dc/elements/1.1/>
	SELECT ?v {
	?x dc:date ?date .
	FILTER (?date < afn:now() )
	}
	</code></pre>
	<p>The afn:now returns the time the query started.</p>
	<p>The expression functions in the ARQ distribution are described on the
	<a href="library-function.html">expression function library page</a>.</p>
	<p>URIs for functions in the (fake)
	<a href="java-uri.html">URI scheme <code>java:</code></a> are dynamically loaded. The
	class name forms the scheme specific part of the URI.</p>
	<h2 id="property-functions">Property functions</h2>
	<p>Property functions, sometimes called “magic properties”, are
	properties that cause triple matching to happen by executing some
	piece of code, determined by the property URI, and not by the usual
	graph matching. They can be used to give certain kinds of inference
	and rule processing. Some calculated properties have additional,
	non-declarative requirements such as needing one of other of the
	subject or object to be a query constant or a bound value, and not
	able to generate all possibilities for that slot.</p>
	<p>Property functions must have fixed URI for the predicate (it can’t
	be query variable). They may take a list for subject or object.</p>
	<p>One common case is for access to collections (RDF lists) or
	containers (rdf:Bag, rdf:Seq, rdf:Alt).</p>
	<pre><code>PREFIX list: <http://jena.apache.org/ARQ/list#>
	SELECT ?member {
	?x :p ?list . # Some way to find the list
	?list list:member ?member .
	}
	</code></pre>
	<p>which can also be written:</p>
	<pre><code>PREFIX list: <http://jena.apache.org/ARQ/list#>
	SELECT ?member {
	?x :p [ list:member ?member ]
	}
	</code></pre>
	<p>Likewise, RDF containers:</p>
	<pre><code>PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
	SELECT ?member {
	?x :p ?bag . # Some way to find the bag
	?bag rdfs:member ?member .
	}
	</code></pre>
	<p>Property functions can also take lists in the subject or object
	slot.</p>
	<p>Code for properties can be dynamically loaded or pre-registered.
	For example, <code>splitIRI</code> will take an IRI and assign the namespace
	ad localname parts to variables (if the variables are already
	bound, not constants are used, <code>splitIRI</code> will check the values).</p>
	<pre><code>PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
	PREFIX apf: <java:org.apache.jena.query.pfunction.library.>
	SELECT ?namespace ?localname {
	xsd:string apf:splitIRI (?namespace ?localname)
	}
	</code></pre>
	<p>Property functions might conflict with inference rules and it can
	be turned off by the Java code:</p>
	<pre><code> ARQ.setFalse(ARQ.enablePropertyFunctions) ;
	</code></pre>
	<p>or on a per instance basis:</p>
	<pre><code> try(QueryExecution qExec = ... ) {
	qExec.getContext().setFalse(ARQ.enablePropertyFunctions) ;
	...
	}
	</code></pre>
	<p>The property functions in the ARQ distribution are described on the
	<a href="library-propfunc.html">property function library page</a>.</p>
	<p>URIs for functions in the (fake)
	<a href="java-uri.html">URI scheme <code>java:</code></a> are dynamically loaded. The
	class name forms the scheme specific part of the URI.</p>
	<h2 id="describe-handlers">DESCRIBE handlers</h2>
	<p>The <code>DESCRIBE</code> result form in SPARQL does not define an exact form
	of RDF to return. Instead, it allows the server or query processor
	to return what it considers to be an appropriate description of the
	resources located. This description will be specific to the domain,
	data modelling or application.</p>
	<p>ARQ comes with one built-in handler which calculates the blank node
	closure of resources found. While suitable for many situations, it
	is not general (for example, a <a href="http://www.foaf-project.org/">FOAF</a>
	file usually consists of all blank nodes). ARQ allows the
	application to replace or add handlers for producing <code>DESCRIBE</code>
	result forms.</p>
	<p>Application-specific handlers can be added to the
	<code>DescribeHandlerRegistry</code>. The handler will be called for each
	resource (not literals) identified by the <code>DESCRIBE</code> query.</p>
	<h2 id="blank-node-labels">Blank Node Labels</h2>
	<p>URIs from with scheme name “_” (which is illegal) are created as
	blank node labels for directly accessing a blank node in the
	queried graph or dataset. These are constant terms in the query -
	not unnamed variables. Do not confuse these with the standard
	qname-like notation for blank nodes in queries. This is not
	portable - use with care.</p>
	<pre><code><_:1234-5678-90> # A blank node in the data

	_:b0 # A blank node in the query - a variable
	</code></pre>


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