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| <h1 class="title">SPARQL S-Expressions (or "SPARQL Syntax Expressions")</h1> |
| |
| |
| <main class="d-flex flex-xl-row flex-column"> |
| |
| <aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#need">Need</a></li> |
| <li><a href="#design-intent">Design Intent</a></li> |
| <li><a href="#other-approaches">Other Approaches</a> |
| <ul> |
| <li><a href="#rdf">RDF</a></li> |
| <li><a href="#lisp">Lisp</a></li> |
| <li><a href="#xml">XML</a></li> |
| <li><a href="#json">JSON</a></li> |
| </ul> |
| </li> |
| <li><a href="#design">Design</a> |
| <ul> |
| <li><a href="#tokens">Tokens</a></li> |
| <li><a href="#sse-comments">SSE Comments</a></li> |
| <li><a href="#sse-escapes">SSE Escapes</a></li> |
| <li><a href="#structures">Structures</a></li> |
| </ul> |
| </li> |
| <li><a href="#tagged-structures">Tagged Structures</a> |
| <ul> |
| <li><a href="#iri-resolution">IRI resolution</a> |
| <ul> |
| <li><a href="#base"><code>base</code></a></li> |
| <li><a href="#prefix"><code>prefix</code></a></li> |
| <li><a href="#nesting">Nesting</a></li> |
| </ul> |
| </li> |
| <li><a href="#links">Links</a></li> |
| </ul> |
| </li> |
| <li><a href="#building-java-objects">Building Java Objects</a> |
| <ul> |
| <li><a href="#sse-factory">SSE Factory</a></li> |
| </ul> |
| </li> |
| <li><a href="#sse-files">SSE Files</a></li> |
| <li><a href="#longer-examples">Longer Examples</a> |
| <ul> |
| <li><a href="#query-1">Query 1</a></li> |
| <li><a href="#complete-sparql-execution">Complete SPARQL Execution</a></li> |
| </ul> |
| </li> |
| <li><a href="#sse-grammar">SSE Grammar</a></li> |
| </ul> |
| </nav> |
| </aside> |
| <article class="flex-column me-lg-4"> |
| <p>A way to write down data structures in an RDF-centric syntax.</p> |
| <p>But not an idea for another RDF serialization format.</p> |
| <h2 id="need">Need</h2> |
| <p>The <a href="http://www.w3.org/TR/sparql11-query/#sparqlAlgebra">SPARQL algebra</a> |
| defines the semantics of a SPARQL graph pattern. Every SPARQL query |
| string (the syntax) is mapped to a SPARQL algebra expression.</p> |
| <p>It is convenient to be able to print out such algebra expressions for |
| discussion between people and for debugging. Further, if algebra |
| expressions can be read back in as well, testing of specific parts of an |
| implementation is also easier.</p> |
| <p>This is an example of a general problem : how to express data structures |
| where the basic elements of RDF are based on RDF nodes.</p> |
| <p>RDF itself is often the most appropriate way to do this, but sometimes |
| it isn’t so convenient. An algebra expression is a tree, and order |
| matters.</p> |
| <p>When expressing a data structure, there are certain key structure that |
| need to be expressible: arrays and maps, then sets and bags, but |
| expression of a data structure is not the same as the high-level |
| semantics of the data structure.</p> |
| <p>A stack can be expressed as a list. And because we want to express the |
| structure, and not express the operations on the structures, data |
| structures with operational meaning don’t enter the picture. There are |
| no operations, no <em>push</em>, <em>pop</em> or <em>peek</em>.</p> |
| <p>Note that this is to <em>express</em> a data structure, not <em>encode</em> or |
| <em>represent</em> it. By <em>express</em> we mean communicate it, between people or |
| between cooperating machines. The structures are not completely |
| self-representing. But we do discuss a way to express in RDF that does |
| give a self-describing nature through the use of tagged structures.</p> |
| <h2 id="design-intent">Design Intent</h2> |
| <ul> |
| <li>Concise (=> for people to write conveniently) format for data |
| structures</li> |
| <li>RDF-centric</li> |
| </ul> |
| <p>Non-goals:</p> |
| <ul> |
| <li>to directly represent any data structure.</li> |
| <li>to be another RDF syntax.</li> |
| </ul> |
| <p>So desirable features are:</p> |
| <ul> |
| <li>Concise syntax for RDF terms</li> |
| <li>Datastructures</li> |
| </ul> |
| <h2 id="other-approaches">Other Approaches</h2> |
| <h3 id="rdf">RDF</h3> |
| <p>RDF is “map-centric” but not all data structures are conveniently |
| expressible in maps. RDF has lists, and these lists have convenient |
| syntax in Turtle or N3.</p> |
| <p>If your data structure fits the RDF paradigm, then RDF is a better |
| choice that SSE. <a href="#mapping-to-rdf">Below</a> is a possible mapping from SSE |
| to RDF as Turtle.</p> |
| <h3 id="lisp">Lisp</h3> |
| <p>Lacks convenient syntax for the RDF terms themselves.</p> |
| <p>SSE syntax is almost valid |
| <a href="http://www.schemers.org" title="http://www.schemers.org">Scheme</a>; literal |
| language tags and datatypes get split a separate list symbols but the |
| information is recoverable. Scheme doesn’t use <code>[]</code> lists or |
| single-quoted strings.</p> |
| <h3 id="xml">XML</h3> |
| <p>Too verbose.</p> |
| <h3 id="json">JSON</h3> |
| <p><a href="http://json.org/" title="http://json.org/">JSON</a> provides values (strings, |
| numbers, booleans, null), arrays and object (which are maps). <a href="http://www.w3.org/TR/rdf-sparql-json-res/" title="http://www.w3.org/TR/rdf-sparql-json-res/">SPARQL |
| Query Results in |
| JSON</a> |
| shows how JSON might be used. It describes how RDF terms are encoded |
| into further substructures. Alternatively, we could put encoded terms in |
| strings like “<a href="http://w3.org/%5C">http://w3.org/\</a>” and have a parser-within-a-parser. But |
| both these approaches do not make the writing of RDF terms as easy as it |
| could be.</p> |
| <h2 id="design">Design</h2> |
| <p><a href="http://en.wikipedia.org/wiki/S-expression" title="http://en.wikipedia.org/wiki/S-expression">S-expressions</a> |
| using RDF terms.</p> |
| <p>The command <code>arq.qparse --print=op --file queryFile</code> will print the |
| SPARQL algebra for the query in SSE format.</p> |
| <h3 id="tokens">Tokens</h3> |
| <p>Tokens are the atomic elements of the syntax.</p> |
| <table> |
| <thead> |
| <tr> |
| <th>Example</th> |
| <th>Explanation</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><code>"abc"</code></td> |
| <td>string</td> |
| </tr> |
| <tr> |
| <td><code>"abc"@en</code></td> |
| <td>string with language tag.</td> |
| </tr> |
| <tr> |
| <td><code>123</code></td> |
| <td>number, specifically an xsd;integer.</td> |
| </tr> |
| <tr> |
| <td><code><http://example.org/></code></td> |
| <td>IRI (or URI).</td> |
| </tr> |
| <tr> |
| <td><code>_:abc</code></td> |
| <td>blank node.</td> |
| </tr> |
| <tr> |
| <td><code>?x</code></td> |
| <td>variable</td> |
| </tr> |
| <tr> |
| <td><code>?</code></td> |
| <td>variable</td> |
| </tr> |
| <tr> |
| <td><code>ex:thing</code></td> |
| <td>prefixed name</td> |
| </tr> |
| <tr> |
| <td><code>ex:123</code></td> |
| <td>prefixed name</td> |
| </tr> |
| <tr> |
| <td><code>SELECT</code></td> |
| <td>symbol</td> |
| </tr> |
| <tr> |
| <td><code>+</code></td> |
| <td>symbol</td> |
| </tr> |
| <tr> |
| <td><code>@xyz</code></td> |
| <td>symbol</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>For <code>?</code> (no name), a unique, internal name for a fresh variable will be |
| allocated; every use of <code>?</code> is a different variable.</p> |
| <p><code>??x</code> creates a non-distinguished variable. <code>??</code> creates a fresh |
| non-distinguished variable.</p> |
| <p><code>_:</code> creates a fresh blank node.</p> |
| <p><code>@xyz</code> - this is a symbol because a language tags only follow a lexical |
| form.</p> |
| <p>Almost any sequence of characters which is not an RDF term or variable |
| is a symbol that can be given special meaning by processing software.</p> |
| <h3 id="sse-comments">SSE Comments</h3> |
| <p><code>#</code> or <code>;</code> introduce comments, which run to the end of line, including |
| the end-of-line characters.</p> |
| <h3 id="sse-escapes">SSE Escapes</h3> |
| <p><code>\u</code> and <code>\U</code> escape sequences for arbitrary Unicode codepoints. These |
| apply to the input character stream before parsing. They don’t, for |
| example, permit a space in a symbol.</p> |
| <p>Strings provide <code>\n</code>, <code>\t</code>, <code>\r</code>, <code>\b</code>, <code>\b</code>, <code>\f</code>, <code>\"</code>, <code>\'</code> and <code>\\</code> |
| escape sequences as in SPARQL.</p> |
| <h3 id="structures">Structures</h3> |
| <p><code>(?x ns:p "abc")</code> - list of 3 elements: a variable, a prefixed name and |
| a string</p> |
| <pre><code>(bgp |
| [?x ns:p "abc"]) |
| </code></pre> |
| <p>A list of 2 elements: a symbol (<code>bgp</code>) and a list of 3 elements. Both |
| <code>()</code> and <code>[]</code> delimit lists; they must match but otherwise it’s a free |
| choice. Convention is that compact lists use <code>[]</code>; large lists use <code>()</code>.</p> |
| <h2 id="tagged-structures">Tagged Structures</h2> |
| <p>The basic syntax defines tokens and lists. Higher level processing |
| happens on this basic syntax and can be extended by interpreting the |
| structure.</p> |
| <p>Layers on top of the basic abstract syntax produce specialised data |
| structures. This can be a transformation into a new SSE structure or the |
| production of programming language objects.</p> |
| <p>This is driven by tagged (data) objects in an SSE expression. The tag is |
| a symbol and the elements of the data object are the rest of the list.</p> |
| <pre><code>(+ 1 2) |
| </code></pre> |
| <p>is tagged with symbol <code>+</code></p> |
| <pre><code>(triple ?s ?p "text"@en) |
| </code></pre> |
| <p>is tagged with symbol <code>triple</code></p> |
| <h3 id="iri-resolution">IRI resolution</h3> |
| <p>One such layer is IRI and prefix name resolution, using tags <code>base</code> and |
| <code>prefix</code>.</p> |
| <p>Basic syntax includes unresolved IRIs, (example <code><abc></code>) and prefixed |
| names (example <code>foaf:name</code>). These are turned into absolute IRIs and the |
| <code>base</code> and <code>prefix</code> tagged object wrappers are removed.</p> |
| <p>This is sufficiently important that the SSE library handles this in an |
| optimized fashion where the IRI processing directly rewrites the |
| streamed output of the parser.</p> |
| <h4 id="base"><code>base</code></h4> |
| <pre><code>(base <http://example/> |
| (triple <xyz> ?p "lex"^^<thing>)) |
| </code></pre> |
| <p>becomes</p> |
| <pre><code>(triple <http://example/xyz> ?p "lex"^^<http://example/thing>) |
| </code></pre> |
| <h4 id="prefix"><code>prefix</code></h4> |
| <pre><code>(prefix ((: <http://example/>) |
| (ns: <http://example/ns#>)) |
| (triple :x ns:p "lex"^^ns:type)) |
| </code></pre> |
| <p>becomes</p> |
| <pre><code> (triple <http://example/x> <http://example/ns#p> "lex"^^<http://example/ns#type>) |
| </code></pre> |
| <h4 id="nesting">Nesting</h4> |
| <p>The tagged structures can be combined and nested. The base or prefixes |
| declared only apply to the body of the data object.</p> |
| <pre><code> (prefix ((: <http://jena.hpl.hp.com/2007/>) |
| (foaf: <http://xmlns.com/foaf/0.1/>)) |
| (triple (base <http://jena.hpl.hp.com/> <afs> foaf:name "Andy"))) |
| </code></pre> |
| <p>Combined with the triple builder, this will produce a triple:</p> |
| <pre><code> <http://jena.hpl.hp.com/afs> <http://xmlns.com/foaf/0.1/name> "Andy" . |
| </code></pre> |
| <h3 id="links">Links</h3> |
| <p><em>Not implemented</em></p> |
| <p>Not all data structures can be conveniently expressed as nested lists. |
| Sub-element sharing matters. A structure with shared elements can’t be |
| serialized as a strict tree and some form of reference is needed.</p> |
| <p>Name a place in the structure: <code>(name@ symbol X)</code></p> |
| <p>Link to it: <code>(@link symbol)</code></p> |
| <p>The link layer will produce an SSE structure without these tags, having |
| replaced all <code>name@</code> and <code>@link</code> with the shared structure <em>X</em>.</p> |
| <p><em><code>@</code> is a convention for referencing.</em></p> |
| <h2 id="building-java-objects">Building Java Objects</h2> |
| <p>Builders are code classes that process the structure into Java objects. |
| Writing builders is straight-forward because low-level parsing details |
| have been taken care of in the basic syntax. A typical builder is a |
| recursive-decent parser over the abstract syntax tree, coding one is |
| primarily walking the structure, with a tagged object to Java instance |
| mapping being applied.</p> |
| <p>Some tagged objects with builders are:</p> |
| <ul> |
| <li><code>(triple S P O)</code> where <em>X</em> is an RDF node (RDF term or variable).</li> |
| <li><code>(quad G S P O)</code></li> |
| <li><code>(graph triple*)</code></li> |
| <li><code>(graph@ URL)</code> — Read a URL.</li> |
| </ul> |
| <p><em>@@ Need to write the abstract syntax for each tagged object</em></p> |
| <p>Many builders have convenience syntax. Triples can be abbreviated by |
| omitting the tag <code>triple</code> because usually the fact it is a triple is |
| clear.</p> |
| <pre><code>(bgp (triple ?s ?p ?o)) |
| (bgp (?s ?p ?o)) |
| </code></pre> |
| <p>Quads have a similar abbreviation as 4-lists. In addition, <code>_</code> is a quad |
| on the default graph.</p> |
| <p>Elements for executing SPARQL:</p> |
| <ul> |
| <li>SPARQL algebra operators and basic graph patterns</li> |
| <li>Filter expressions (in prefix notation <code>(+ 1 2)</code>)</li> |
| <li>Query solutions (Bindings) and tables.</li> |
| </ul> |
| <h3 id="sse-factory">SSE Factory</h3> |
| <p>The class <code>SSE</code> in package <code>org.apache.jena.sparql.sse</code> provides many |
| convenience functions to call builders for RDF and SPARQL structures.</p> |
| <pre><code>Node n = SSE.parseNode("<http://example/node>") ; |
| Triple t = SSE.parseTriple("(?s ?p ?o)") ; |
| Op op = SSE.parseOp("(filter (> ?v 123) (bgp (?s ?p ?v)))") ; |
| </code></pre> |
| <p>Most of the operations have forms that allow a <code>PrefixMapping</code> to be |
| specified - this is wrapped around the parser run so prefixed names can |
| be used without explicit prefix declarations.</p> |
| <p>There is a default prefix mapping with a few common prefixes: <code>rdf</code>, |
| <code>rdfs</code>, <code>owl</code>, <code>xsd</code> and <code>fn</code> (the XPath/XQuery functions and operators |
| namespace).</p> |
| <h2 id="sse-files">SSE Files</h2> |
| <p>The file extension is <code>.sse</code> and all files are UTF-8.</p> |
| <p>A quick and pragmatic Emacs mode is given by:</p> |
| <pre><code>;; ==== SSE mode |
| (define-derived-mode sse-mode lisp-mode "SSE" nil |
| (make-local-variable 'lisp-indent-function) |
| (setq lisp-indent-function 'sse-indent-function) |
| ) |
| |
| ;; Everything in SSE is "def" like |
| (defun sse-indent-function (indent-point state) |
| (lisp-indent-defform state indent-point)) |
| |
| (setq auto-mode-alist |
| (cons '("\\.sse" . sse-mode) auto-mode-alist)) |
| </code></pre> |
| <h2 id="longer-examples">Longer Examples</h2> |
| <h3 id="query-1">Query 1</h3> |
| <pre><code>PREFIX foaf: <http://xmlns.com/foaf/0.1/> |
| SELECT DISTINCT ?name ?nick |
| { |
| ?x foaf:mbox <mailt:person@server> . |
| ?x foaf:name ?name |
| OPTIONAL { ?x foaf:nick ?nick } |
| } |
| |
| (prefix ((foaf: <http://xmlns.com/foaf/0.1/>)) |
| (distinct |
| (project (?name ?nick) |
| (leftjoin |
| (BGP |
| [triple ?x foaf:mbox <mailto:person@server>] |
| [triple ?x foaf:name ?name] |
| ) |
| (BGP [triple ?x foaf:nick ?nick]) |
| )))) |
| </code></pre> |
| <h3 id="complete-sparql-execution">Complete SPARQL Execution</h3> |
| <p>The following is a complete query execution, data and query. There is an |
| inline dataset and a query of</p> |
| <pre><code> PREFIX : <http://example/> |
| SELECT * |
| { |
| GRAPH :g1 { ?x ?p ?v } |
| } |
| </code></pre> |
| <p>The tag <code>graph</code> is used twice, with different meanings. First, for an |
| RDF graph, and second in <code>GRAPH</code> SPARQL pattern. In a data structure, |
| context sorts out the different usages.</p> |
| <pre><code>(prefix ((: <http://example/>)) |
| (exec |
| (dataset |
| (default (graph |
| (:x :p 1) |
| (:x :p 2))) |
| (namedgraph :g1 |
| (graph |
| (:x :gp 1) |
| (:x :gp 2))) |
| (namedgraph :g2 |
| (graph |
| (:y :gp 1) |
| (:y :gp 2))) |
| ) |
| |
| (graph :g1 |
| (bgp (?x ?p ?v))) |
| )) |
| </code></pre> |
| <h2 id="sse-grammar">SSE Grammar</h2> |
| <p><em>@@ insert grammar here</em></p> |
| |
| </article> |
| |
| <aside class="text-muted align-self-start mb-3 mb-xl-5 p-0 d-none d-xl-flex flex-column sticky-top"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#need">Need</a></li> |
| <li><a href="#design-intent">Design Intent</a></li> |
| <li><a href="#other-approaches">Other Approaches</a> |
| <ul> |
| <li><a href="#rdf">RDF</a></li> |
| <li><a href="#lisp">Lisp</a></li> |
| <li><a href="#xml">XML</a></li> |
| <li><a href="#json">JSON</a></li> |
| </ul> |
| </li> |
| <li><a href="#design">Design</a> |
| <ul> |
| <li><a href="#tokens">Tokens</a></li> |
| <li><a href="#sse-comments">SSE Comments</a></li> |
| <li><a href="#sse-escapes">SSE Escapes</a></li> |
| <li><a href="#structures">Structures</a></li> |
| </ul> |
| </li> |
| <li><a href="#tagged-structures">Tagged Structures</a> |
| <ul> |
| <li><a href="#iri-resolution">IRI resolution</a> |
| <ul> |
| <li><a href="#base"><code>base</code></a></li> |
| <li><a href="#prefix"><code>prefix</code></a></li> |
| <li><a href="#nesting">Nesting</a></li> |
| </ul> |
| </li> |
| <li><a href="#links">Links</a></li> |
| </ul> |
| </li> |
| <li><a href="#building-java-objects">Building Java Objects</a> |
| <ul> |
| <li><a href="#sse-factory">SSE Factory</a></li> |
| </ul> |
| </li> |
| <li><a href="#sse-files">SSE Files</a></li> |
| <li><a href="#longer-examples">Longer Examples</a> |
| <ul> |
| <li><a href="#query-1">Query 1</a></li> |
| <li><a href="#complete-sparql-execution">Complete SPARQL Execution</a></li> |
| </ul> |
| </li> |
| <li><a href="#sse-grammar">SSE Grammar</a></li> |
| </ul> |
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