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improve definition of Literal
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‎spec/index.html

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@@ -733,125 +733,139 @@ <h3>Literals</h3>
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<p>Literals are used for values such as strings, numbers, and dates.</p>
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<p>A <dfn data-local-lt="RDF literal">literal</dfn> in an <a>RDF graph</a> consists of
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two, three, or four elements, as follow:</p>
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two, three, or four elements, as follow.</p>
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<ol>
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<li>a <dfn>lexical form</dfn> consisting of a sequence of
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<a data-cite="I18N-GLOSSARY#dfn-code-point" class="lint-ignore">Unicode code points</a> [[!UNICODE]]
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which are <a data-cite="I18N-GLOSSARY#dfn-scalar-value">Unicode scalar values</a>,
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and therefore do not contain
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<a data-cite="I18N-GLOSSARY#dfn-surrogate" class="lint-ignore">Unicode surrogate code points</a></li>
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<li>a <dfn>datatype IRI</dfn>, being an <a>IRI</a>
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<li>A <dfn>lexical form</dfn>, being a [=string=].
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<li>A <dfn>datatype IRI</dfn>, being an <a>IRI</a>
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identifying a datatype that determines how the lexical form maps
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to a <a>literal value</a></li>
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<li>if and only if the <a>datatype IRI</a> is
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<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code>, a
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to a <a>literal value</a>.</li>
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<li>If and only if the <a>datatype IRI</a> is
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<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> or
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<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>, a
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non-empty <dfn>language tag</dfn> as defined by [[!BCP47]]. The
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language tag MUST be well-formed according to
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<a data-cite="bcp47#section-2.2.9">section 2.2.9</a>
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of [[!BCP47]],
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and MUST be treated consistently, that is, in a case insensitive manner.
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Two language tags are the same if they only differ by case.</li>
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<li>if and only if the <a>datatype IRI</a> is
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Two [[!BCP47]]-complying strings that differ only by case represent the same [=language tag=].</li>
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<li>If and only if the <a>datatype IRI</a> is
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<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>,
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a non-empty <a>language tag</a>
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that MUST be well-formed according to <a data-cite="bcp47#section-2.2.9">section 2.2.9</a>
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of [[!BCP47]],
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and MUST be treated consistently, that is, in a case insensitive manner,
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and a <dfn>base direction</dfn> that MUST be either `ltr` or `rtl`.</li>
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a <dfn>base direction</dfn> that MUST be either<ul>
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<li>`ltr`, indicating that the initial text direction is set to left-to-right, or</li>
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<li>`rtl`, indicating that the initial text direction is set to right-to-left.</li>
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</ul></li>
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</ol>
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<p>A literal is a <dfn>language-tagged string</dfn> if the third element
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is present and the fourth element is not present.
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Lexical representations of language tags MAY be case normalized,
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(for example, by canonicalizing as defined by
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<a data-cite="bcp47#section-4.5">BCP 47 section 4.5</a>).
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</p>
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<p>A literal is a <dfn id="dfn-dir-lang-string">directional language-tagged string</dfn>
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A literal is a <dfn id="dfn-dir-lang-string">directional language-tagged string</dfn>
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if both the third element and fourth elements are present.
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The third element, the language tag, is treated identically as in a <a>language-tagged string</a>,
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and the fourth element, <a>base direction</a>, MUST be either `ltr` or `rtl`, which MUST be in lower case.</p>
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<p>The meanings of the <a>base direction</a> values are:</p>
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<ul>
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<li>`ltr`: indicates that the initial text direction is set to left-to-right.</li>
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<li>`rtl`: indicates that the initial text direction is set to right-to-left.</li>
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</ul>
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<p>Please note that concrete syntaxes MAY support
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<dfn data-lt="simple literal" class="export">simple literals</dfn> consisting of only a
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<a>lexical form</a> without any <a>datatype IRI</a>, <a>language tag</a>, or <a>base direction</a>.
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Simple literals are syntactic sugar for abstract syntax
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<a>literals</a>
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with the <a>datatype IRI</a>
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<code>http://www.w3.org/2001/XMLSchema#string</code>
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(which is commonly abbreviated as <code>xsd:string</code>).
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Similarly, most concrete syntaxes represent
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<a>language-tagged strings</a> and <a>directional language-tagged strings</a> without
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the <a>datatype IRI</a> because it always equals either
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<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> (<code>rdf:langString</code>)
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or <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code> (<code>rdf:dirLangString</code>), respectively.</p>
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<p>The <dfn>literal value</dfn> associated with a <a>literal</a> is:</p>
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<ul>
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<li>If the literal is a <a>language-tagged string</a>,
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then the literal value is a pair consisting of its <a>lexical form</a>
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and its <a>language tag</a>, in that order.</li>
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<li>If the literal is a <a>directional language-tagged string</a>, then the literal value is
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a tuple of its <a>lexical form</a>, its <a>language tag</a>, and its <a>base direction</a>,
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likewise in that order.</li>
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<li>If the literal's <a>datatype</a> is handled by an RDF implmentation,
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<ul>
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<li>if the literal's <a>lexical form</a> is in the <a>lexical space</a>
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of the <a>datatype</a>, then the literal value is the result of applying
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the <a>lexical-to-value mapping</a> of the datatype to the
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<a>lexical form</a>.</li>
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<li>otherwise, the literal is <dfn data-lt-no-plural>ill-typed</dfn> and no literal value can be
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associated with the literal. Such a case produces a semantic
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inconsistency but is not <em>syntactically</em> ill-formed.
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Implementations SHOULD accept [=ill-typed=] literals and produce RDF
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graphs from them. Implementations MAY produce warnings when
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encountering [=ill-typed=] literals.</li>
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</ul>
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</li>
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<li>If the literal's <a>datatype IRI</a> is <em>not</em>
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handled by an RDF implementation, then the literal value is
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not defined by this specification. Implementations SHOULD accept
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literals with unknown datatype IRIs and produce RDF graphs from them.
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</li>
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</ul>
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</p>
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<p><dfn data-local-lt="term-equal">Literal term equality</dfn>:
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Two literals are term-equal (the same <a>RDF literal</a>)
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two literals are term-equal (the same <a>RDF term</a>)
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if and only if:</p>
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<ul>
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<li>the two <a>lexical forms</a> compare equal</li>
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<li>the two <a>datatype IRIs</a> compare equal</li>
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<li>the two <a>language tags</a> (if any) compare equal</li>
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<li>the two <a>base directions</a> (if any) compare equal</li>
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<li>the two <a>lexical forms</a> compare equal,</li>
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<li>the two <a>datatype IRIs</a> compare equal,</li>
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<li>the two <a>language tags</a> are either both absent, or both present and compare equal,</li>
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<li>the two <a>base directions</a> are either both absent, both `ltr`, or both `rtl`.</li>
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</ul>
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<p>Comparison is performed using
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<p>Comparison of the [=lexical forms=] and of the [=datatype IRIs=] is performed using
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<a data-cite="I18N-GLOSSARY#dfn-case-sensitive">case sensitive matching</a>
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(see description of string comparison in
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<a href="#rdf-strings" class="sectionRef"></a>)
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except for language tags, where the comparison is performed using
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(see description of string comparison in
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<a href="#rdf-strings" class="sectionRef"></a>).
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Comparison of the [=language tags=] is performed using
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<a data-cite="I18N-GLOSSARY#dfn-case-sensitive">ASCII case-insensitive matching</a>.
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Thus, two literals can have the same value
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without being the same <a>RDF term</a>.
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For example:</p>
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<pre>
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"1"^^xs:integer
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"01"^^xs:integer
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</pre>
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<p>denote the same <a data-lt="literal value">value</a>, but are not the
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same literal <a>RDF terms</a> and are not
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<a>term-equal</a> because their
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<a>lexical forms</a> differ.</p>
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</p>
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<section>
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<h2>Representation of literals</h2>
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<p>Some concrete syntaxes MAY support
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<dfn data-lt="simple literal" class="export">simple literals</dfn> consisting of only a
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<a>lexical form</a> without any <a>datatype IRI</a>, <a>language tag</a>, or <a>base direction</a>.
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Simple literals are syntactic sugar for abstract syntax
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<a>literals</a>
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with the <a>datatype IRI</a>
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<code>http://www.w3.org/2001/XMLSchema#string</code>
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(which is commonly abbreviated as <code>xsd:string</code>).
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</p>
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<p>
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Similarly, most concrete syntaxes represent
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<a>language-tagged strings</a> and <a>directional language-tagged strings</a> without
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the <a>datatype IRI</a> because it always equals either
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<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> (<code>rdf:langString</code>)
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or <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code> (<code>rdf:dirLangString</code>), respectively.
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</p>
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<p>
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Any [=string=] complying with [[!BCP47]] MAY be used to represent a [=language tag=] in concrete syntaxes or implementation.
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Such strings MAY be case normalized,
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(for example, by canonicalizing as defined by
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<a data-cite="bcp47#section-4.5">BCP 47 section 4.5</a>).
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On the contrary, an implementation MAY preserve the case from the original representation,
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provided that it processes it in a case-insensitive manner.
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</p>
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<aside class=note>
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The treatment of language tags has changed between RDF 1.1 and RDF 1.2.
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In RDF 1.1, `"chat"@fr` and `"chat"@FR` were representing two distinct terms, but implementations had license to replace one with the other (which most did).
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In RDF 1.2, they are now representing the exact same literal, i.e., the case difference in the concrete syntax does not propagate into the abstract syntax.
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</aside>
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</section>
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<section>
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<h2>Literal value</h2>
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<p>The <dfn>literal value</dfn> associated with a <a>literal</a> is defined as follow.</p>
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<ul>
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<li>If the literal is a <a>language-tagged string</a>,
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then the literal value is a pair consisting of its <a>lexical form</a>
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and its <a>language tag</a>, in that order.</li>
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<li>If the literal is a <a>directional language-tagged string</a>, then the literal value is
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a tuple of its <a>lexical form</a>, its <a>language tag</a>, and its <a>base direction</a>,
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likewise in that order.</li>
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<li>If the literal's <a>datatype</a> is handled by an RDF implementation,
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<ul>
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<li>if the literal's <a>lexical form</a> is in the <a>lexical space</a>
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of the <a>datatype</a>, then the literal value is the result of applying
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the <a>lexical-to-value mapping</a> of the datatype to the
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<a>lexical form</a>.</li>
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<li>otherwise, the literal is <dfn data-lt-no-plural>ill-typed</dfn> and no literal value can be
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associated with the literal. Such a case produces a semantic
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inconsistency but is not <em>syntactically</em> ill-formed.
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Implementations SHOULD accept [=ill-typed=] literals and produce RDF
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graphs from them. Implementations MAY produce warnings when
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encountering [=ill-typed=] literals.</li>
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</ul>
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</li>
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<li>If the literal's <a>datatype IRI</a> is <em>not</em>
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handled by an RDF implementation, then the literal value is
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not defined by this specification. Implementations SHOULD accept
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literals with unknown datatype IRIs and produce RDF graphs from them.
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</li>
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</ul>
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<p>
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Thus, two literals can have the same value
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without being the same <a>RDF term</a>.
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For example:</p>
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<pre>
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"1"^^xsd:integer
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"01"^^xsd:integer
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</pre>
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<p>denote the same <a data-lt="literal value">value</a>, but are not the
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same literal <a>RDF terms</a> because their
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<a>lexical forms</a> differ.</p>
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</section>
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</section>
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<section id="section-blank-nodes">

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