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- <title>RDF 1.1 Concepts and Abstract Syntax</title>
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- <body>
-
-<section id="abstract">
- <p>The Resource Description Framework (RDF) is a framework for
- representing information in the Web.</p>
- <p>RDF Concepts and Abstract Syntax defines an abstract syntax
- on which RDF is based, and which serves to link its concrete
- syntax to its formal semantics. It also includes discussion of
- key concepts, datatyping, character normalization
- and handling of IRIs.</p>
-</section>
-
-
-<section id="sotd">
- <p>This document is work in progress towards a revision of the
- <a href="http://www.w3.org/TR/rdf-concepts/"><em>RDF Concepts
- and Abstract Syntax</em></a> Recommendation,
- and is intended to eventually replace that document.
- It is part of a larger effort to revise the
- <a href="http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-Introduction">RDF specifications as published in 2004</a>.
- The most significant changes from the 2004 edition are:
- modified <a href="#section-Graph-Literal">string literals</a>,
- a <a href="#section-skolemization">section on skolemization
- of blank nodes</a>, and many updated
- <a href="#references">references</a> to other specifications
- (including a change in terminology from
- “URI references” to “IRIs”). A fuller list of changes that
- have been made to date is provided in <a href="#changes">Appendix A</a>.
- Various areas of work to be tackled in upcoming
- working drafts are highlighted throughout the document, but
- should not yet be understood as an exhaustive list.</p>
-</section>
-
-
-<section id="section-Introduction">
- <h2>Introduction</h2>
-
- <p class="issue">This document reflects current progress of the RDF Working
- Group towards updating the
- <a href="http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/">2004
- version of <em>RDF Concepts and Abstract Syntax</em></a>. The
- editors expect to work on a number of issues, some of which are
- listed in boxes like this throughout the document.</p>
-
- <p>The Resource Description Framework (RDF) is a framework for
- representing information in the Web.</p>
-
- <p>This document defines an abstract syntax (a data model)
- on which RDF is based,
- and which serves to link concrete syntaxes to its formal
- semantics. It also includes discussion of
- key concepts, datatyping, character normalization
- and handling of IRIs.</p>
-
- <p>Normative documentation of RDF falls into the following
- areas:</p>
-
- <ul>
- <li>Serialization syntaxes (Turtle [[TURTLE-TR]], RDFa [[RDFA-PRIMER]], RDF/XML [[RDF-SYNTAX-GRAMMAR]], N-Triples [[N-TRIPLES]]),</li>
-
- <li>the RDF Vocabulary Description Language ([[RDF-SCHEMA]]),</li>
-
- <li>a formal model-theoretic semantics [[!RDF-MT]], and</li>
-
- <li>this document.</li>
- </ul>
-
- <p>The framework is designed so that vocabularies can be layered.
- The terms defined in [[RDF-SCHEMA]] are the first such vocabulary.
- Several other vocabularies for RDF are
- mentioned in the Primer [[RDF-PRIMER]].</p>
-</section>
-
-
-<section id="conformance"></section>
-
-
-<section id="section-Concepts" class="informative">
- <h2>RDF Concepts</h2>
-
- <p class="issue">This section is quite redundant with later
- normative sections and the RDF Primer. Its removal has been
- proposed. This is
- <a href="http://www.w3.org/2011/rdf-wg/track/issues/68">ISSUE-68</a>.</p>
-
- <p>RDF uses the following key concepts:</p>
-
- <ul>
- <li>Graph data model</li>
-
- <li>IRI-based vocabulary</li>
-
- <li>Datatypes</li>
-
- <li>Literals</li>
-
- <li>Entailment</li>
- </ul>
-
-
-<section id="section-data-model">
- <h3>Graph Data Model</h3>
-
- <p>The underlying structure of any expression in RDF is a
- collection of triples, each consisting of a subject, a
- predicate and an object. A set of such triples is called an RDF
- graph (defined more formally in
-<a href="#section-Graph-syntax">section 6</a>). This can be
- illustrated by a node and directed-arc diagram, in which each
- triple is represented as a node-arc-node link (hence the term
- “graph”).</p>
-
- <div class="figure">
- <img src="Graph-ex.gif" alt="image of the RDF triple comprising (subject, predicate, object)" />
- </div>
-
- <p>Each triple represents a statement of a relationship between
- the things denoted by the nodes that it links. Each triple has
- three parts:</p>
- <ol>
- <li>a <a>subject</a>,</li>
- <li>an <a>object</a>, and</li>
- <li>a <a>predicate</a> (also called a
- <a>property</a>) that denotes a
- relationship.</li>
- </ol>
- <p>The direction of the arc is significant: it always points
- toward the object.</p>
- <p>The <a title="node">nodes</a> of an RDF graph
- are its subjects and objects.</p>
- <p>The assertion of an RDF triple says that some relationship,
- indicated by the predicate, holds between the things denoted by
- subject and object of the triple. The assertion of an RDF graph
- amounts to asserting all the triples in it, so the meaning of
- an RDF graph is the conjunction (logical AND) of the statements
- corresponding to all the triples it contains. A formal account
- of the meaning of RDF graphs is given in [[!RDF-MT]].</p>
-</section>
-
-
-<section id="section-IRI-Vocabulary">
- <h3>IRI-based Vocabulary and Node Identification</h3>
-
- <p>A <a>node</a> may be an <a>IRI</a>, a <a>literal</a>,
- or <a title="blank node">blank</a> (having no separate form of identification).
- Properties are <a title="IRI">IRIs</a>.</p>
- <p>An <a>IRI</a> or <a>literal</a> used as a node identifies what
- that node represents. An IRI used as a predicate
- identifies a relationship between the things represented by the nodes it connects. A
- predicate IRI may also be a node in the graph.</p>
- <p>A <a>blank node</a> is a node that is
- not an IRI or a literal. In the RDF abstract syntax, a
- blank node is just a unique node that can be used in one or
- more RDF statements.</p>
- <p>A convention used by some linear representations of an RDF
- graph to allow several statements to use the same
- blank node is to use a <dfn>blank node
- identifier</dfn>, which is a local identifier that can be
- distinguished from all IRIs and literals. When graphs are
- merged, their blank nodes must be kept distinct if meaning is
- to be preserved; this may call for re-allocation of blank node
- identifiers. Note that such blank node identifiers are not part
- of the RDF abstract syntax, and the representation of triples
- containing blank nodes is entirely dependent on the particular
- concrete syntax used.</p>
-</section>
-
-
-<section id="section-Datatypes-intro">
- <h3>Datatypes</h3>
-
- <p>Datatypes are used by RDF in the representation of values such
- as integers, floating point numbers and dates.</p>
-
- <p>
-A datatype consists of a lexical space, a value space and a lexical-to-value
-mapping, see <a href="#section-Datatypes">section 5</a>.
-</p>
-
- <p>For example, the lexical-to-value mapping for the XML Schema datatype
- <var>xsd:boolean</var>, where each member of the value space
- (represented here as 'T' and 'F') has two lexical representations,
- is as follows:</p>
-
- <table border="1" cellpadding="5" summary=
- "A table detailing the xsd:boolean datatype.">
- <tr>
- <th align="left">Value Space</th>
-
- <td>{T, F}</td>
- </tr>
-
- <tr>
- <th align="left">Lexical Space</th>
-
- <td>{"0", "1", "true", "false"}</td>
- </tr>
-
- <tr>
- <th align="left">Lexical-to-Value Mapping</th>
-
- <td>{<"true", T>, <"1", T>, <"0", F>,
- <"false", F>}</td>
- </tr>
- </table>
-
- <p>RDF predefines just one datatype <code><a>rdf:XMLLiteral</a></code>, used for
- embedding XML in RDF (see <a href="#section-XMLLiteral">section
- 5.1</a>).</p>
-
- <p>There is no built-in concept of numbers or dates or other common
- values. Rather, RDF defers to datatypes that are defined
- separately, and identified with <a title="IRI">IRIs</a>.
- The predefined XML Schema
- datatypes [[!XMLSCHEMA-2]] are expected
- to be widely used for this purpose.</p>
-
-
- <p>RDF provides no mechanism for defining new datatypes. XML Schema
- Datatypes [[!XMLSCHEMA-2]] provides an
- extensibility framework suitable for defining new datatypes for use
- in RDF.</p>
-</section>
-
-
-<section id="section-Literals">
- <h3>Literals</h3>
-
- <p><a title="literal">Literals</a> are used to identify values such as numbers and dates
- by means of a lexical representation. Anything represented by a
- literal could also be represented by an <a>IRI</a>, but it is often more
- convenient or intuitive to use literals.</p>
-
- <p>A literal may be the object of an RDF statement, but not the
- subject or the predicate.</p>
-
- <p>Literals may be <cite>typed</cite> or <cite>language-tagged</cite>:</p>
-
- <ul>
- <li>A <a>typed literal</a> is a string combined with a
- <a>datatype IRI</a>. It denotes the
- member of the identified datatype's value space obtained by
- applying the lexical-to-value mapping to the literal string.</li>
-
- <li>A <a>language-tagged literal</a> is a string combined
- with a language tag. This may be used for
- plain text in a natural language. Language-tagged literals
- are self-denoting.</li>
- </ul>
-
- <p>Continuing the example from <a href="#section-Datatypes-intro">section
- 3.3</a>, the typed literals that can be defined using the XML
- Schema datatype <var>xsd:boolean</var> are:</p>
-
- <table border="1" cellpadding="5" summary=
- "This table lists the literals of type xsd:boolean.">
- <tr>
- <th>Typed Literal</th>
-
- <th>Lexical-to-Value Mapping</th>
-
- <th>Value</th>
- </tr>
-
- <tr>
- <td align="center"><xsd:boolean, "true"></td>
-
- <td align="center"><"true", T></td>
-
- <td align="center">T</td>
- </tr>
-
- <tr>
- <td align="center"><xsd:boolean, "1"></td>
-
- <td align="center"><"1", T></td>
-
- <td align="center">T</td>
- </tr>
-
- <tr>
- <td align="center"><xsd:boolean, "false"></td>
-
- <td align="center"><"false", F></td>
-
- <td align="center">F</td>
- </tr>
-
- <tr>
- <td align="center"><xsd:boolean, "0"></td>
-
- <td align="center"><"0", F></td>
-
- <td align="center">F</td>
- </tr>
- </table>
-
- <p>For text that may contain
- markup, use typed literals
-with type <a href="#section-XMLLiteral">rdf:XMLLiteral</a>.
-If language annotation is required,
-it must be explicitly included as markup, usually by means of an
-<code>xml:lang</code> attribute.
-XHTML [[XHTML10]] may be included within RDF
-in this way. Sometimes, in this latter case,
- an additional <code>span</code> or <code>div</code>
- element is needed to carry an
-<code>xml:lang</code> or <code>lang</code> attribute.
- </p>
-
-<p class="issue">Update the XHTML 1.0 reference to something more recent?</p>
-
-<p>
-The string in both plain and typed literals is recommended to
-be in Unicode Normal Form C [[!NFC]]. This is motivated
-by [[CHARMOD]] particularly
-<a href="http://www.w3.org/TR/2003/WD-charmod-20030822/#sec-Normalization">section 4
-Early Uniform Normalization</a>.
-</p>
-
-</section>
-
-
-<section id="section-Entailment">
- <h3>Entailment</h3>
-
- <p>The ideas on meaning and inference in RDF are underpinned by the
- formal concept of <a href="http://www.w3.org/TR/2004/REC-rdf-mt-20040210/#entail">
-<cite>entailment</cite></a>, as
- discussed in the RDF
- semantics document [[!RDF-MT]].
-In brief, an RDF expression A is said to
-<dfn title="entailment">entail</dfn> another RDF expression B
-if every possible
-arrangement of things in the world that makes A true also makes B
-true. On this basis, if the truth of A is presumed or demonstrated
-then the truth of B can be inferred .
-</p>
-</section>
-
-</section>
-
-
-<section id="section-URIspaces">
- <h2>RDF Vocabulary IRI and Namespace</h2>
-
- <p>RDF uses <a title="IRI">IRIs</a> to identify resources
- and properties. Certain
- IRIs with the following leading substring are defined by the
- RDF specifications to denote specific concepts:</p>
-
- <ul>
- <li><code>http://www.w3.org/1999/02/22-rdf-syntax-ns#</code>
- (conventionally associated with namespace prefix <code>rdf:</code>)</li>
- </ul>
-
- <p>Vocabulary terms in the <code>rdf:</code>
- namespace are listed and described in detail in the
- RDF Schema specification [[!RDF-SCHEMA]].</p>
-
- <p class="note">The RDF namespace is also used as an
- XML namespace [[XML-NAMES]] to define a number of additional
- element and attribute names for purely syntactic purposes within
- the RDF/XML syntax ([[RDF-SYNTAX-GRAMMAR]],
- <a href="http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/#section-Namespace">section 5.1</a>).
- These terms (e.g., <code>rdf:about</code> and <code>rdf:ID</code>)
- do not denote concepts.</p>
-</section>
-
-
-<section id="section-Datatypes">
- <h2>Datatypes</h2>
-
- <p class="issue">This section perhaps should discuss
- <a href="http://www.w3.org/TR/rdf-mt/#dtype_interp">the XSD datatype map</a>
- and <code><a href="http://www.w3.org/TR/rdf-plain-literal/">rdf:PlainLiteral</a></code>.
- This is <a href="http://www.w3.org/2011/rdf-wg/track/issues/70">ISSUE-70</a>.</p>
-
-<p>
-The datatype abstraction used in RDF is compatible with
-the abstraction used in
-XML Schema Part 2:
- Datatypes [[!XMLSCHEMA-2]].</p>
-<p>
-A datatype consists of a lexical space, a value space and a lexical-to-value
-mapping.
-</p>
-<p>The <dfn>lexical space</dfn> of a datatype is a set of Unicode [[!UNICODE]] strings.</p>
-<p>
-The <dfn>lexical-to-value mapping</dfn> of a datatype is a set of pairs whose
-first element belongs to
-the <a>lexical space</a> of the datatype,
-and the second element belongs to the
- <dfn>value space</dfn> of the datatype:
-</p>
-<ul>
-<li>
-Each member of the lexical space is paired with (maps to) exactly one member
-of the value space.
-</li>
-<li>
-Each member of the value space may be paired with any number (including
-zero) of members of the lexical space (lexical representations for that
-value).
-</li>
-</ul>
-<p>
-A datatype is identified by one or more IRIs.
-</p>
-<p>
-RDF may be used with any datatype definition that conforms to this
-abstraction, even if not defined in terms of XML Schema.
-</p>
- <p>Certain XML Schema built-in datatypes are not suitable for use
- within RDF. For example, the
-<a href="http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#QName">QName</a>
-datatype requires a namespace declaration to be in scope during
- the mapping, and is not recommended for use in RDF.
- [[!RDF-MT]] contains a
-<a href="http://www.w3.org/TR/2004/REC-rdf-mt-20040210/#dtype_interp">more detailed discussion</a>
- of specific XML Schema built-in datatypes. </p>
-
-<div class="note">
-<p>When the datatype is defined using XML Schema:
-</p>
-<ul>
-<li>
-All values correspond to some lexical form, either using
-the lexical-to-value mapping of the datatype or if it is a union
-datatype with a lexical mapping associated with one of the member
-datatypes.
-</li>
-<li>
-XML Schema facets remain part of the datatype and are used by the XML
-Schema mechanisms that control the lexical space and the value space;
-however, RDF does not define a standard mechanism to access these facets.</li>
-
-<li>In [[XMLSCHEMA-1]],
-<a href="http://www.w3.org/TR/2001/REC-xmlschema-1-20010502/#section-White-Space-Normalization-during-Validation">
-white space normalization</a> occurs
-during
-<a href="http://www.w3.org/TR/2001/REC-xmlschema-1-20010502/#key-vn">validation</a>
-according to the value of the
-<a href="http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#rf-whiteSpace">whiteSpace
-facet</a>. The lexical-to-value mapping used in RDF datatyping
-occurs after this, so that the whiteSpace facet has no
-effect in RDF datatyping.
-</li>
-</ul>
-
-</div>
-
-
-<section id="section-XMLLiteral">
- <h3>XML Content within an RDF Graph</h3>
-
- <p class="issue">The canonicalization rules required for XML literals
- are quite complicated. Increasingly, RDF is produced and consumed in
- environments where no XML parser and canonicalization engine is
- available. A possible change to relax the requirements for the
- lexical space, while retaining the value space, is under discussion.
- This is <a href="http://www.w3.org/2011/rdf-wg/track/issues/13">ISSUE-13</a>.</p>
-
- <p>RDF provides for XML content as a possible literal value.
- Such content is indicated in an RDF graph using a typed literal
- whose datatype is a special built-in datatype
- <dfn>rdf:XMLLiteral</dfn>,
- defined as follows.</p>
-
-
- <dl>
- <dt><a name="XMLLiteral-uri" id="XMLLiteral-uri">An IRI for
-identifying this datatype</a></dt>
-
- <dd>is
- <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral</code>.</dd>
-
-
-
-
- <dt><a name="XMLLiteral-lexical-space" id="XMLLiteral-lexical-space">The lexical space</a></dt>
-
-<dd>is the set of all
-strings:
-<ul>
-<li>which are well-balanced, self-contained
-<a href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">
-XML content</a>
-[[!XML10]];
-</li>
-<li>for which encoding as UTF-8
-[[!UTF-8]] yields
-<a href="http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/#def-exclusive-canonical-XML">
-exclusive
-Canonical XML </a> (with comments, with empty
-<a href="http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/#def-InclusiveNamespaces-PrefixList">
-InclusiveNamespaces PrefixList
-</a>) [[!XML-EXC-C14N]];
-</li>
-<li>for which embedding between an arbitrary XML start tag and an end tag
-yields a document conforming to <a href=
- "http://www.w3.org/TR/1999/REC-xml-names-19990114/">XML
- Namespaces</a> [[!XML-NAMES]]</li>
-</ul>
-</dd>
-
-
- <dt><a name="XMLLiteral-value-space" id="XMLLiteral-value-space">The value space</a></dt>
-
- <dd>is a set of entities, called XML values, which is:
-<ul>
-<li>disjoint from the lexical space;</li>
-<li>disjoint from the value space of any other datatype that is not explicitly defined as a sub- or supertype of this datatype;</li>
-<li>disjoint from the set of Unicode character strings [[!UNICODE]];</li>
-<li>and in 1:1 correspondence with the lexical space.</li>
-</ul>
-</dd>
-
- <dt><a name="XMLLiteral-mapping" id="XMLLiteral-mapping">The lexical-to-value mapping</a></dt>
-
- <dd>
-is a one-one mapping from the lexical space onto the value space,
- i.e. it is both injective and surjective.
-</dd>
-
-
-
- </dl>
-
- <p class="note">Not all values of this datatype are compliant
- with XML 1.1 [[XML11]]. If compliance
- with XML 1.1 is desired, then only those values that are
-<a href="http://www.w3.org/TR/2002/CR-xml11-20021015/#sec2.13">fully
- normalized</a> according to XML 1.1 should be used.</p>
-
- <p class="note">XML values can be thought of as the
-[[XML-INFOSET]] or the [[XPATH]]
-nodeset corresponding to the lexical form, with an appropriate equality
-function.</p>
-
- <p class="note">RDF applications may use additional equivalence relations, such as
-that which relates an
-<a href="http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#string"><code>xsd:string</code></a>
-
-with an <code>rdf:XMLLiteral</code> corresponding to
-a single text node of the same string.</p>
-
-</section>
-
-</section>
-
-
-<section id="section-Graph-syntax">
- <h2>Abstract Syntax</h2>
-
- <p>This section defines the RDF abstract syntax. The RDF abstract
- syntax is a set of triples, called the RDF graph.</p>
-
- <p>This section also defines equivalence between RDF graphs. A
- definition of equivalence is needed to support the RDF Test Cases
- [[RDF-TESTCASES]] specification.</p>
-
-<p class="note">This <em>abstract</em> syntax is the
-syntax over which the formal semantics are defined.
-Implementations are free to represent RDF graphs in
-any other equivalent form. As an example:
-in an RDF graph,
-literals with datatype <tt>rdf:XMLLiteral</tt> can be represented
-in a non-canonical
-format, and canonicalization performed during the comparison between two
-such literals. In this example the comparisons may be
-being performed either between syntactic structures or
-between their denotations in the domain of discourse.
-Implementations that do not require any such comparisons can
-hence be optimized.
-</p>
-
-
-<section id="section-triples">
- <h3>RDF Triples</h3>
-
- <p>An <dfn>RDF triple</dfn> contains three components:</p>
-
- <ul>
- <li>the <dfn>subject</dfn>, which is an
- <a>IRI</a> or a <a>blank node</a></li>
-
- <li>the <dfn>predicate</dfn>, which is an <a>IRI</a></li>
-
- <li>the <dfn>object</dfn>, which is an <a>IRI</a>,
- a <a>literal</a> or a <a>blank node</a></li>
- </ul>
-
- <p>An RDF triple is conventionally written in the order subject,
- predicate, object.</p>
-
- <p>The predicate is also known as the <dfn>property</dfn> of the triple.</p>
-
- <p><a title="IRI">IRIs</a>, <a title="blank node">blank nodes</a> and
- <a title="literal">literals</a> are collectively known as
- <dfn title="RDF term">RDF terms</dfn>.</p>
-</section>
-
-
-<section id="section-rdf-graph">
- <h3>RDF Graph</h3>
-
- <p>An <dfn>RDF graph</dfn> is a set of RDF triples.</p>
-
- <p>The set of <dfn title="node">nodes</dfn> of an RDF graph is the set of subjects and objects of
- triples in the graph.</p>
-</section>
-
-
-<section id="section-graph-equality">
- <h3>Graph Equivalence</h3>
-
- <p>Two <a title="RDF graph">RDF graphs</a> <var>G</var> and <var>G'</var> are equivalent if there
- is a bijection <var>M</var> between the sets of nodes of the two graphs,
- such that:</p>
-
- <ol>
- <li><var>M</var> maps blank nodes to blank nodes.</li>
- <li><var>M(lit)=lit</var> for all <a title="literal">RDF literals</a> <var>lit</var> which
- are nodes of <var>G</var>.</li>
-
- <li><var>M(uri)=uri</var> for all <a title="IRI">IRIs</a> <var>uri</var>
- which are nodes of <var>G</var>.</li>
-
- <li>The triple <var>( s, p, o )</var> is in <var>G</var> if and
- only if the triple <var>( M(s), p, M(o) )</var> is in
- <var>G'</var></li>
- </ol>
- <p>With this definition, <var>M</var> shows how each blank node
- in <var>G</var> can be replaced with
- a new blank node to give <var>G'</var>.</p>
-</section>
-
-
-<section id="section-IRIs">
- <h3>IRIs</h3>
-
- <p>An <dfn title="IRI"><acronym title="Internationalized Resource Identifier">IRI</acronym></dfn>
- (Internationalized Resource Identifier) within an RDF graph
- is a Unicode string [[!UNICODE]] that conforms to the syntax
- defined in RFC 3987 [[!IRI]]. IRIs are a generalization of
- <dfn title="URI"><acronym title="Uniform Resource Identifier">URI</acronym>s</dfn>
- [[URI]]. Every absolute URI and URL is an IRI.</p>
-
- <p>IRIs in the RDF abstract syntax MUST be absolute, and MAY
- contain a fragment identifier.</p>
-
- <p>Two IRIs are equal if and only if they are equivalent
- under Simple String Comparison according to
- <a href="http://tools.ietf.org/html/rfc3987#section-5.1">section 5.1</a>
- of [[!IRI]]. Further normalization MUST NOT be performed when
- comparing IRIs for equality.</p>
-
- <p class="note">When IRIs are used in operations that are only
- defined for URIs, they must first be converted according to
- the mapping defined in
- <a href="http://tools.ietf.org/html/rfc3987#section-3.1">section 3.1</a>
- of [[!IRI]]. A notable example is retrieval over the HTTP
- protocol. The mapping involves UTF-8 encoding of non-ASCII
- characters, %-encoding of octets not allowed in URIs, and
- Punycode-encoding of domain names.</p>
-
- <p class="note">Some concrete syntaxes permit relative IRIs
- as a shorthand for absolute IRIs, and define how to resolve
- the relative IRIs against a base IRI.</p>
-
- <p class="note">Previous versions of RDF used the term
- “<dfn>RDF URI Reference</dfn>” instead of “IRI” and allowed
- additional characters:
- “<code><</code>”, “<code>></code>”,
- “<code>{</code>”, “<code>}</code>”,
- “<code>|</code>”, “<code>\</code>”,
- “<code>^</code>”, “<code>`</code>”,
- ‘<code>“</code>’ (double quote), and “<code> </code>” (space).
- In IRIs, these characters must be percent-encoded as
- described in <a href="http://tools.ietf.org/html/rfc3986#section-2.1">section 2.1</a>
- of [[URI]].</p>
-
- <div class="note">
- <p>Interoperability problems can be avoided by minting
- only IRIs that are normalized according to
- <a href="http://tools.ietf.org/html/rfc3987#section-5">Section 5</a>
- of [[!IRI]]. Non-normalized forms that should be avoided
- include:</p>
-
- <ul>
- <li>Uppercase characters in scheme names and domain names</li>
- <li>Percent-encoding of characters where it is not
- required by IRI syntax</li>
- <li>Explicitly stated HTTP default port
- (<code>http://example.com:80/</code>);
- <code>http://example.com/</code> is preferrable</li>
- <li>Completely empty path in HTTP IRIs
- (<code>http://example.com</code>);
- <code>http://example.com/</code> is preferrable</li>
- <li>“<code>/./</code>” or “<code>/../</code>” in the path
- component of an IRI</li>
- <li>Lowercase hexadecimal letters within percent-encoding
- triplets (“<code>%3F</code>” is preferable over
- “<code>%3f</code>”)</li>
- <li>Punycode-encoding of Internationalized Domain Names
- in IRIs</li>
- <li>IRIs that are not in Unicode Normalization
- Form C [[!NFC]]</li>
- </ul>
- </div>
-</section>
-
-
-<section id="section-Graph-Literal">
- <h3>RDF Literals</h3>
-
- <p class="issue">This section is a major departure from RDF 2004
- as <a title="simple literal">simple literals</a> are now treated
- as syntactic sugar for <code>xsd:string</code>
- <a title="typed literal">typed literals</a>. Further changes
- to RDF's literal design are under consideration:
- <a title="language-tagged literal">Language-tagged literals</a>
- may receive a datatype, and
- <a href="http://www.w3.org/TR/rdf-plain-literal/"><code>rdf:PlainLiteral</code>s</a> [[RDF-PLAINLITERAL]]
- may be folded into the design somehow. This is
- <a href="http://www.w3.org/2011/rdf-wg/track/issues/71">ISSUE-71</a>.</p>
-
- <p>A <dfn>literal</dfn> in an <a>RDF graph</a> is either a
- <a>typed literal</a> or a <a>language-tagged literal</a>.</p>
-
- <p>All literals have a <dfn>lexical form</dfn> being a Unicode
- [[!UNICODE]] string, which SHOULD be in Normal Form C [[!NFC]].</p>
-
- <p><dfn title="language-tagged literal">Language-tagged literals</dfn> have
- a <a>lexical form</a> and a non-empty <dfn>language tag</dfn> as
- defined by [[!BCP47]]. The language tag MUST be well-formed according to
- <a href="http://tools.ietf.org/html/bcp47#section-2.2.9">section 2.2.9</a>
- of [[!BCP47]], and MUST be normalized to lowercase.</p>
-
- <p><dfn title="typed literal">Typed literals</dfn> have a <a>lexical form</a>
- and a <dfn>datatype IRI</dfn> being an <a>IRI</a>.</p>
-
- <p>Concrete syntaxes MAY support <dfn title="simple literal">simple
- literals</dfn>, consisting of only a <a>lexical form</a>
- without any language tag or datatype IRI. Simple literals only
- exist in concrete syntaxes, and are treated as
- syntactic sugar for abstract syntax
- <a title="plain literal">typed literals</a> with the datatype IRI
- <code>http://www.w3.org/2001/XMLSchema#string</code>.
- Simple literals and <a>language-tagged literals</a> are
- collectively known as <dfn title="plain literal">plain literals</dfn>.</p>
-
- <p class="note">Earlier versions of RDF allowed
- <a title="simple literal">simple literals</a> in the abstract syntax.</p>
-
- <p class="note">Literals in which the lexical form begins with a
- composing character (as defined by [[CHARMOD]]) are allowed however they may cause
- interoperability problems, particularly with XML version 1.1 [[XML11]].</p>
-
- <p class="note">Earlier versions of RDF permitted tags that
- adhered to the generic tag/subtag syntax of language tags,
- but were not well-formed according to [[!BCP47]]. Such
- language tags do not conform to RDF 1.1.</p>
-
- <p class="note">When using the language tag, care must be
- taken not to confuse language with locale. The language
- tag relates only to human language text. Presentational
- issues should
- be addressed in end-user applications.</p>
-
- <p class="note">The case normalization of
-language tags is part of
- the description of the abstract syntax, and consequently the abstract
- behaviour of RDF applications. It does not constrain an
- RDF implementation to actually normalize the case. Crucially, the result
- of comparing two language tags should not be sensitive to the case of
- the original input.</p>
-
-
-<section id="section-Literal-Equality">
- <h4>Literal Equality</h4>
-
- <p>Two literals are equal if and only if all of the following
- hold:</p>
-
- <ul>
- <li>The strings of the two lexical forms compare equal, character
- by character.</li>
-
- <li>Either both or neither have language tags.</li>
-
- <li>The language tags, if any, compare
- equal.</li>
-
- <li>Either both or neither have datatype IRIs.</li>
-
- <li>The two datatype IRIs, if any, compare equal, character by
- character.</li>
- </ul>
-
- <p class="note">RDF Literals are distinct and distinguishable
- from <a title="IRI">IRIs</a>; e.g. <code>http://example.org/</code> as an RDF
- Literal (untyped, without a language tag) is not equal to
- <code>http://example.org/</code> as an IRI.</p>
-</section>
-
-
-<section id="section-Literal-Value">
- <h4>The Value Corresponding to a Typed Literal</h4>
-
- <p>The datatype IRI refers to a <a href=
- "#section-Datatypes">datatype</a>. For XML Schema <a href=
- "http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#built-in-datatypes">
- built-in</a> datatypes, IRIs such as
- <code>http://www.w3.org/2001/XMLSchema#int</code> are used. The IRI
- of the datatype <a href="#section-XMLLiteral"><tt>rdf:XMLLiteral</tt></a> may be used.
- There may be other, implementation dependent, mechanisms by which
- IRIs refer to datatypes.</p>
-
- <p>The <em>value</em> associated with a typed literal is found by
- applying the lexical-to-value mapping associated with the datatype IRI to
- the lexical form.
- </p>
-
- <p>
- If the lexical form is not in
- the lexical space of the datatype associated with the datatype IRI,
-then no literal value can be associated with the typed literal.
-Such a case, while in error, is not <em>syntactically</em> ill-formed.</p>
-<!--
- <p>A typed literal for which the datatype does not map the lexical
- form to a value is not syntactically ill-formed.</p>
--->
-
-
- <p class="note">
-In application contexts, comparing the values of typed literals (see
-<a href="#section-Literal-Value">
-section
-6.5.2</a>)
-is usually more helpful than comparing their syntactic forms (see
-<a href="#section-Literal-Equality">
-section
-6.5.1</a>).
-Similarly, for comparing RDF Graphs,
-semantic notions of entailment (see
-[[!RDF-MT]]) are usually
-more helpful than syntactic equality (see
-<a href="#section-graph-equality">
-section
-6.3</a>).</p>
-
-</section>
-
-</section>
-
-
-<section id="section-blank-nodes">
- <h3>Blank Nodes</h3>
-
-<p>
-The <dfn title="blank node">blank nodes</dfn> in an RDF graph
-are drawn from an infinite set.
-This set of blank nodes, the set of all <a title="IRI">IRIs</a>
-and the set of all <a title="literal">literals</a> are pairwise disjoint.
-</p>
-<p>
-Otherwise, this set of blank nodes is arbitrary.
-</p>
-<p>RDF makes no reference to any internal structure of blank nodes.
-Given two blank nodes, it is
-possible to determine whether or not they are the same.</p>
-
-
-<section id="section-skolemization">
- <h4>Replacing Blank Nodes with IRIs</h4>
-
- <p>Blank nodes do not have identifiers in the RDF abstract syntax. The
- <a title="blank node identifier">blank node identifiers</a> introduced
- by some concrete syntaxes have only
- local scope and are purely an artifact of the serialization.</p>
-
- <p>In situations where stronger identification is needed, systems MAY
- systematically transform some or all of the blank nodes in an RDF graph
- into IRIs [[!IRI]]. Systems wishing to do this SHOULD mint a new, globally
- unique IRI (a <dfn>Skolem IRI</dfn>) for each blank node so transformed.</p>
-
- <p>This transformation does not change the meaning of an RDF graph,
- provided that the Skolem IRIs do not occur anywhere else.</p>
-
- <p>Systems may wish to mint Skolem IRIs in such a way that they can
- recognize the IRIs as having been introduced solely to replace a blank
- node, and map back to the source blank node where possible.</p>
-
- <p>Systems that want Skolem IRIs to be recognizable outside of the system
- boundaries SHOULD use a well-known IRI [[WELL-KNOWN]] with the registered
- name <code>genid</code>. This is an IRI that uses the HTTP or HTTPS scheme,
- or another scheme that has been specified to use well-known IRIs; and whose
- path component starts with <code>/.well-known/genid/</code>.
-
- <p>For example, the authority responsible for the domain
- <code>example.com</code> could mint the following recognizable Skolem IRI:</p>
-
- <pre>http://example.com/.well-known/genid/d26a2d0e98334696f4ad70a677abc1f6</pre>
-
- <p class="issue">IETF registration of the <code>genid</code> name is
- currently in progress.</p>
-
- <p class="note">RFC 5785 [[WELL-KNOWN]] only specifies well-known URIs,
- not IRIs. For the purpose of this document, a well-known IRI is any
- IRI that results in a well-known URI after IRI-to-URI mapping [[!IRI]].</p>
-</section>
-
-</section>
-
-
-<section id="section-multigraph">
- <h2>Abstract Syntax for Working with Multiple Graphs</h2>
-
- <div class="issue">
- <p>The Working Group will standardize a model and semantics for
- multiple graphs and graphs stores. The
- <a href="http://www.w3.org/2011/01/rdf-wg-charter">charter</a> notes:</p>
-
- <blockquote>The RDF Community has used the
- term “named graphs” for a number of years in various settings,
- but this term is ambiguous, and often refers to what could rather
- be referred as quoted graphs, graph literals, IRIs for graphs,
- knowledge bases, graph stores, etc. The term “Support for Multiple
- Graphs and Graph Stores” is used as a neutral term in this charter;
- this term is not and should not be considered as definitive.
- The Working Group will have to define the right term(s).</blockquote>
-
- <p>Progress on the design for this feature is tracked under multiple
- issues:</p>
-
- <ul>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/5">ISSUE-5: Should we define Graph Literal datatypes?</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/14">ISSUE-14: What is a named graph and what should we call it?</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/15">ISSUE-15: What is the relationship between the IRI and the triples in a dataset/quad-syntax/etc</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/17">ISSUE-17: How are RDF datasets to be merged?</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/22">ISSUE-22: Does multigraph syntax need to support empty graphs?</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/28">ISSUE-28: Do we need syntactic nesting of graphs (g-texts) as in N3?</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/29">ISSUE-29: Do we support SPARQL's notion of "default graph"?</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/30">ISSUE-30: How does SPARQL's notion of RDF dataset relate our notion of multiple graphs?</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/32">ISSUE-32: Can we identify both g-boxes and g-snaps?</a></li>
- <li><a href="http://www.w3.org/2011/rdf-wg/track/issues/33">ISSUE-33: Do we provide a way to refer to sub-graphs and/or individual triples?</a></li>
- </ul>
- </div>
-</section>
-
-</section>
-
-
-<section id="section-fragID" class="informative">
- <h2>Fragment Identifiers</h2>
-
- <p class="issue">This section does not address the case where RDF is
- embedded in other document formats, such as in RDFa or when an RDF/XML
- fragment is embedded in SVG. It has been suggested that this may be
- a general issue for the TAG about the treatment of
- fragment identifiers when one language is embedded in another. This is
- <a href="http://www.w3.org/2011/rdf-wg/track/issues/37">ISSUE-37</a>.</p>
-
- <p class="issue">This section treats the RDF/XML media type as
- canonical for establishing the referent of IRIs that include
- fragment identifier. Today we have many different media types
- that can carry RDF graphs, and HTTP content negotiation is more
- common. Also, the problem addressed in the section
- (context-dependence of fragment identifiers) has to some extent
- gone away when RFC 2396 was replaced by RFC 3986. The latter
- states that the same fragment should be used for the same thing
- in resources that have multiple representations
- (Section 3.5 [[URI]]). This is
- <a href="http://www.w3.org/2011/rdf-wg/track/issues/69">ISSUE-69</a>.</p>
-
- <p>RDF uses <a title="IRI">IRIs</a>,
- which may include fragment identifiers, as
- context free identifiers for resources. RFC 2396 states
- that the meaning of a fragment
- identifier depends on the MIME content-type of a document, i.e.
- is context dependent.</p>
- <p>These apparently conflicting views are reconciled by
- considering that an <a>IRI</a> in an RDF graph is treated
- with respect to the MIME type <code>application/rdf+xml</code>
- [[RDF-MIME-TYPE]]. Given an IRI that includes a fragment identifier,
- the fragment identifer identifies the same thing
- that it does in an <code>application/rdf+xml</code> representation of the
- resource identified by the IRI excluding the fragment identifier. Thus:</p>
- <ul>
- <li>we assume that the IRI excluding fragment
- identifier identifies a resource, which is presumed to have
- an RDF representation. So when <code>eg:someurl#frag</code> is used in an RDF
- document, <code>eg:someurl</code> is taken to
- designate some RDF document (even when no such document can
- be retrieved).</li>
- <li><code>eg:someurl#frag</code> means the thing
- that is indicated, according to the rules of the
- <code>application/rdf+xml</code> MIME content-type as
- a “fragment” or “view” of the RDF document at
- <code>eg:someurl</code>. If the document does not
- exist, or cannot be retrieved, or is available only in
- formats other than <code>application/rdf+xml</code>, then exactly what
- that view may be is somewhat undetermined, but that does not
- prevent use of RDF to say things about it.</li>
- <li>the RDF treatment of a fragment identifier allows it to
- indicate a thing that is entirely external to the document,
- or even to the “shared information space” known as the Web.
- That is, it can be a more general idea, like some particular
- car or a mythical Unicorn.</li>
- <li>in this way, an <code>application/rdf+xml</code> document acts as an
- intermediary between some Web retrievable documents (itself,
- at least, also any other Web retrievable IRIs that it may
- use, possibly including schema IRIs and references to other
- RDF documents), and some set of possibly abstract or non-Web
- entities that the RDF may describe.</li>
- </ul>
- <p>This provides a handling of IRIs and their
- denotation that is consistent with the RDF model theory and
- usage, and also with conventional Web behavior. Note that
- nothing here requires that an RDF application be able to
- retrieve any representation of resources identified by the IRIs
- in an RDF graph.</p>
-</section>
-
-
-<section id="section-Acknowledgments" class="informative">
- <h2>Acknowledgments</h2>
-
- <p class="issue">This section does not yet list those who made
- contributions to the RDF 1.1 version, nor does it list the
- current RDF WG members.</p>
-
- <p>The RDF 2004 editors acknowledge valuable contributions from
- Frank Manola, Pat Hayes, Dan Brickley, Jos de Roo,
- Dave Beckett, Patrick Stickler, Peter F. Patel-Schneider, Jerome Euzenat,
- Massimo Marchiori, Tim Berners-Lee, Dave Reynolds and Dan Connolly.</p>
-
- <p>This specification contains a significant contribution from the
- designers of the RDF typed literal mechanism, Pat
- Hayes, Sergey Melnik and Patrick Stickler. The document draws upon an earlier
- RDF Model and Syntax document edited by Ora Lassilla and Ralph Swick,
- and RDF Schema edited by Dan Brickley and R. V. Guha.</p>
-
- <p>This specification is a product of extended deliberations by the
- <a href="http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-Acknowledgments">members
- of the RDFcore Working Group and the RDF and RDF Schema Working Group</a>.</p>
-</section>
-
-
-<section class="appendix informative" id="changes">
- <h2>Changes from RDF 2004</h2>
-
- <ul>
- <li>2011-08-13: Updated Turtle reference to Turtle FPWD</li>
- <li>2011-07-21: Condensed the 2004 acknowledgements</li>
- <li>2011-07-21: Updated the two sections on literals to reflect the <a href="">ISSUE-12 resolution</a> that simple literals are no longer part of the abstract syntax. Formally introduced the terms “language-tagged literal”, “simple literal”.</li>
- <li>2011-07-21: Updated the introduction, and removed many mentions of RDF/XML. Changed the normative reference for the terms in the RDF namespace from the RDF/XML spec to the RDF Schema spec. Removed any mention of the 1999 version of RDF.</li>
- <li>2011-07-21: Replaced RFC 2279 reference (UTF-8) with RFC 3629</li>
- <li>2011-07-20: Removed informative sections “Motivations and Goals” (see <a href="http://www.w3.org/TR/rdf-concepts/#section-Overview">RDF 2004 version</a>) and “RDF Expression of Simple Facts” (see <a href="http://www.w3.org/TR/rdf-concepts/#section-SimpleFacts">RDF 2004 version</a>)</li>
- <li>2011-06-01: Replaced the URI References section with <a href="#section-IRIs">new section on IRIs</a>, and changed “RDF URI Reference” to “IRI” throughout the document.</li>
- <li>2011-06-01: Changed language tag definition to require well-formedness according to BCP47; added a note that this invalidates some RDF</li>
- <li>2011-05-25: Added boxes for known WG issues throught the document</li>
- <li>2011-05-25: Deleted “Structure of this Document” section, it added no value beyond the TOC</li>
- <li>2011-05-25: Implemented resolution of <a href="http://www.w3.org/2011/rdf-wg/track/issues/40">ISSUE-40: Skolemization advice in the RDF dcocument</a> by adding a section on <a href="#section-skolemization">Replacing Blank Nodes with IRIs</a></li>
- <li>2011-05-25: rdf:XMLLiteral is disjoint from any datatype not explicitly related to it, per erratum <a href="http://www.w3.org/2001/sw/RDFCore/errata#concept-xmlliteral">[concept-xmlliteral]</a></li>
- <li>2011-05-25: Added Conformance section with RFC2119 reference</li>
- <li>2011-05-25: Updated all W3C references to latest editions, and Unicode from v3 to v4</li>
- <li>2011-05-24: Converted to ReSpec, changed metadata to reflect RDF 1.1</li>
- </ul>
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