--- a/rdf-spaces/index.html Wed May 09 12:06:34 2012 -0400
+++ b/rdf-spaces/index.html Wed May 09 14:19:50 2012 -0400
@@ -391,14 +391,22 @@
<p>@@@ grammar, etc, based of Turtle spec</p>
</section>
-
-
</section>
<section>
<h2>Semantics</h2>
- <p>formalize what's been said above.</p>
+ <p>@@@ formalize what's been said above.</p>
+
+ <p class="note">
+ We say nothing here about the fact that the truth value of a quad
+ is likely to change over time. Time is orthogonal to RDF
+ semantics, and quads present no fundamentally different challenge
+ here than vcard, since people change addresses just like triples
+ are added to and removed some spaces. Some approaches to handling
+ change-over-time are discussed in @@@example? @@@temporal-rdf
+ note?
+ </p>
<p>We now give <a>dataset</a>s declarative semantics, allowing
dataset to be used as logical statements (like RDF Graphs). We
@@ -755,35 +763,169 @@
</section>
</section>
-<section>
+
+
+ <section class="appendix">
+ <h2>Reflection Vocabulary</h2>
+
+ <p class="note">This section is entirely novel/experimental at
+ this time.</p>
+
+ <p>This section specifies an RDF vocabulary for conveying datasets
+ in RDF Graphs. This is done by describing—in RDF
+ triples—the specific structures occuring in a given dataset.
+ This technique is sometimes called <dfn>reflection</dfn> or
+ <dfn>reification</dfn>. This vocabulary is distinct from the
+ 1999/2004 RDF reification vocabulary; we prefer to use the term
+ "reflection" to help maintain the separation.</p>
+
+ <p>This vocabulary allows systems to convey datasets using normal
+ triple-based RDF machinery, including RDF/XML, Turtle, and RDFa,
+ when necessary. In general, in systems where languages and APIs
+ are available which directly support datasets, this vocabulary is
+ not useful.</p>
+
+ <p>As an example, the quad [ eg:subject eg:predicate eg:object1
+ eg:space1 ] can be expressed in Turtle using this vocabulary:</p>
+
+ <pre class="example">@prefix : <http://example.org/>.
+[] a rdf:Triple;
+ rdf:subjectIRI "http://example.org/subject";
+ rdf:predicateIRI "http://example.org/predicate";
+ rdf:objectIRI "http://example.org/object1";
+ rdf:inSpace :space1.</pre>
+
+ <p>The terms in the triple are encoded (turned into literal
+ strings, in this example), to provide referential opacity. In the
+ semantics of quads, it does not follow from [a b c d] and a=aa that
+ [aa b c d]. Without this term-encoding, that conclusion <em>would</em>
+ follow from the reflection.</p>
+
+ <p>Terms in this vocabulary:</p>
+
+ <dl>
+
+ <dt>rdf:Triple</dt>
+ <dd>The class of RDF Triples, each of which is just a triple
+ (3-tuple) of a three RDF terms, called its "subject",
+ "predicate", and "object". Triples have no identity apart from
+ their three components.</dd>
+
+ <dt>rdf:subjectIRI</dt>
+ <dd>A predicate expressing the relationship to the triple's subject term,
+ when the subject term is an IRI. The value is the IRI (a string)
+ which is the subject-term part of the triple.</dd>
+
+ <dt>rdf:subjectNode</dt>
+ <dd>A predicate expressing the relationship to the triple's
+ subject term, when the subject term is a blank node. The value
+ is any RDF Resource; it simply serves as a placeholder,
+ representing the blank node which serves as the subject-term part
+ of the triple.</dd>
+
+ <dt>rdf:predicateIRI</dt>
+ <dd>A predicate expressing the relationship to the triple's
+ predicate term. The value is the IRI (a string) which serves as
+ the predicate-term part of the triples.</dd>
+
+ <dt>rdf:objectIRI</dt>
+ <dd>A predicate expressing the relationship to the triple's
+ object term, when the object term is an IRI. The value is the
+ IRI (a string) which is the object-term part of the triple.</dd>
+
+ <dt>rdf:objectNode</dt>
+ <dd>A predicate expressing the relationship to the triple's
+ object term, when the object term is a blank node. The value
+ is any RDF Resource; it simply serves as a placeholder,
+ representing the blank node which serves as the object-term part
+ of the triple.</dd>
+
+ <dt>rdf:objectValue</dt>
+ <dd>A predicate expressing the relationship to the triple's
+ object term, when the object term is literal. The value is the
+ value which serves as the object-term part of the triple.</dd>
+
+ <dt>rdf:inSpace</dt>
+ <dd>A predicate expressing the relationship between an a triple
+ and an RDF space which contains it.</dd>
+
+ </dl>
+
+ <p>This vocabulary is used in a specific template form, always
+ matching this SPARQL graph pattern: </p>
+
+ <pre>[] a rdf:Triple;
+ rdf:subjectIRI|rdf:subjectNode ?s;
+ rdf:predicateIRI ?p;
+ rdf:objectIRI|rdf:objectNode|rdf:objectValue ?o;
+ rdf:inSpace ?sp</pre>
+
+ <p>This one template uses SPARQL 1.1 property paths, with
+ alternation using the "|" character. It could also be expressed as
+ six different SPARQL 1.0 graph patterns.</p>
+
+ <p>The terms in this vocabulary only have fully-defined meaning
+ when they occur in the template pattern. When they do, the set of
+ triples matching the template has the same meaning as the
+ <a>quad</a> [ ?s ?p ?o ?sp ].</p>
+
+ <p><dfn>Reflecting a dataset</dfn> is the act of completely
+ conveying the facts in a dataset in RDF triples, using this
+ reflection vocabulary. The procedure is: (1) check for occurances
+ of the reflection template in the default graph -- if they occur,
+ abort, since reflection is not defined for this dataset; (2) copy
+ the triples in the default graph of the input to the output; (3)
+ for each quad in the input, generate a matching instance of the
+ reflection template and put the resulting five triples in the
+ output.</p>
+
+ <p><dfn>Unreflecting a dataset</dfn> is the act of turning an RDF
+ graph into a dataset, using this reflection vocabulary. The
+ procedure is: (1) make a mutable copy of the input graph, (2) for
+ each match of the reflection template, add the resulting quad to the
+ output dataset and delete the five triples which matched the
+ template, (3) copy the remaining triples to the output as the
+ default graph of the dataset.</p>
+
+ <p>The reflect and unreflect functions are inverses of each other.
+ That is, for all datasets D on which reflect is defined, D =
+ unreflect(reflect(D)) and for all graphs G, G =
+ (reflect(unreflect(G)). The functions are also essentially
+ idempotent. If we align their domains and ranges by considering a
+ graph to be dataset which has only its default graph, then
+ reflect(D) = reflect(reflect(D)) and unreflect(G) =
+ unreflect(unreflect(G))</p>
+
+</section>
+
+
+
+<section class="Appendix">
<h2>Issues</h2>
<p class="issue">Do the named graphs in a dataset include all the
triples in the spaces with those names, or only some of them? Aka
partial-graph or complete-graph semantics.</p>
- <p class="issue">What do we do about RDF reification. Should we try
- to fix it to be talking about spaces?</p>
-
<p class="issue">How should we talk about change-over-time? The
archiving example gets into it right now.</p>
-
</section>
+<section id="references">
+</section>
+
<section class="appendix informative" id="changes">
<h2>Changes</h2>
<ul>
- <li>2012-05-09: Renamed "layers" as "spaces"; some word-smithing in Concepts and the Abstract; removed "Turtle in HTML" as a dataset syntax; added some text about trig and nquads.</li>
+ <li>2012-05-09: Renamed "layers" as "spaces"; some word-smithing in Concepts and the Abstract; removed "Turtle in HTML" as a dataset syntax; added some text about trig and nquads; added a note about change-over-time; added an appendix with a reflection vocabulary</li>
<li>2012-05-02: Removed obsolete text from the introduction, removed the section on datasets borrowed from RDF Concepts, and added many entries to Concepts (and renamed it from Terminology).</li>
<li>2012-05-01: Starting with a little text from RDF Concepts, a few ideas, and the text from <a href="http://www.w3.org/2011/rdf-wg/wiki/Layers">Layers</a></li>
</ul>
</section>
-<section id="references">
-</section>
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