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improve definition of Literal #162

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improve definition of Literal
pchampin Feb 26, 2025
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Expand Up @@ -733,125 +733,140 @@ <h3>Literals</h3>
<p>Literals are used for values such as strings, numbers, and dates.</p>

<p>A <dfn data-local-lt="RDF literal">literal</dfn> in an <a>RDF graph</a> consists of
two, three, or four elements, as follow:</p>
two, three, or four elements, as follow.</p>
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This was intentionally a colon. A full-stop puts a bit too much break.

Suggested change
two, three, or four elements, as follow.</p>
two, three, or four elements, as follow:</p>


<ol>
<li>a <dfn>lexical form</dfn> consisting of a sequence of
<a data-cite="I18N-GLOSSARY#dfn-code-point" class="lint-ignore">Unicode code points</a> [[!UNICODE]]
which are <a data-cite="I18N-GLOSSARY#dfn-scalar-value">Unicode scalar values</a>,
and therefore do not contain
<a data-cite="I18N-GLOSSARY#dfn-surrogate" class="lint-ignore">Unicode surrogate code points</a></li>
<li>a <dfn>datatype IRI</dfn>, being an <a>IRI</a>
<li>A <dfn>lexical form</dfn>, being an [=RDF string=].
<li>A <dfn>datatype IRI</dfn>, being an <a>IRI</a>
identifying a datatype that determines how the lexical form maps
to a <a>literal value</a></li>
<li>if and only if the <a>datatype IRI</a> is
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code>, a
to a <a>literal value</a>.</li>
<li>If and only if the <a>datatype IRI</a> is
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> or
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>, a
non-empty <dfn>language tag</dfn> as defined by [[!BCP47]]. The
language tag MUST be well-formed according to
<a data-cite="bcp47#section-2.2.9">section 2.2.9</a>
of [[!BCP47]],
and MUST be treated consistently, that is, in a case insensitive manner.
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@TallTed TallTed Mar 1, 2025 •
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and MUST be treated consistently, that is, in a case insensitive manner.
and MUST be consistently treated in a case insensitive manner.

Two language tags are the same if they only differ by case.</li>
<li>if and only if the <a>datatype IRI</a> is
Two [[!BCP47]]-complying strings that differ only by case represent the same [=language tag=].</li>
<li>If and only if the <a>datatype IRI</a> is
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>,
a non-empty <a>language tag</a>
that MUST be well-formed according to <a data-cite="bcp47#section-2.2.9">section 2.2.9</a>
of [[!BCP47]],
and MUST be treated consistently, that is, in a case insensitive manner,
and a <dfn>base direction</dfn> that MUST be either `ltr` or `rtl`.</li>
a <dfn>base direction</dfn> that MUST be either<ul>
<li>`ltr`, indicating that the initial text direction is set to left-to-right, or</li>
<li>`rtl`, indicating that the initial text direction is set to right-to-left.</li>
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Suggested change
a <dfn>base direction</dfn> that MUST be either<ul>
<li>`ltr`, indicating that the initial text direction is set to left-to-right, or</li>
<li>`rtl`, indicating that the initial text direction is set to right-to-left.</li>
a <dfn>base direction</dfn> that MUST be one of the following:<ul>
<li>`ltr`, indicating that the initial text direction is set to left-to-right</li>
<li>`rtl`, indicating that the initial text direction is set to right-to-left</li>

</ul></li>
</ol>

<p>A literal is a <dfn>language-tagged string</dfn> if the third element
is present and the fourth element is not present.
Lexical representations of language tags MAY be case normalized,
(for example, by canonicalizing as defined by
<a data-cite="bcp47#section-4.5">BCP 47 section 4.5</a>).
</p>

<p>A literal is a <dfn id="dfn-dir-lang-string">directional language-tagged string</dfn>
A literal is a <dfn id="dfn-dir-lang-string">directional language-tagged string</dfn>
if both the third element and fourth elements are present.
The third element, the language tag, is treated identically as in a <a>language-tagged string</a>,
and the fourth element, <a>base direction</a>, MUST be either `ltr` or `rtl`, which MUST be in lower case.</p>

<p>The meanings of the <a>base direction</a> values are:</p>
<ul>
<li>`ltr`: indicates that the initial text direction is set to left-to-right.</li>
<li>`rtl`: indicates that the initial text direction is set to right-to-left.</li>
</ul>

<p>Please note that concrete syntaxes MAY support
<dfn data-lt="simple literal" class="export">simple literals</dfn> consisting of only a
<a>lexical form</a> without any <a>datatype IRI</a>, <a>language tag</a>, or <a>base direction</a>.
Simple literals are syntactic sugar for abstract syntax
<a>literals</a>
with the <a>datatype IRI</a>
<code>http://www.w3.org/2001/XMLSchema#string</code>
(which is commonly abbreviated as <code>xsd:string</code>).
Similarly, most concrete syntaxes represent
<a>language-tagged strings</a> and <a>directional language-tagged strings</a> without
the <a>datatype IRI</a> because it always equals either
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> (<code>rdf:langString</code>)
or <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code> (<code>rdf:dirLangString</code>), respectively.</p>

<p>The <dfn>literal value</dfn> associated with a <a>literal</a> is:</p>

<ul>
<li>If the literal is a <a>language-tagged string</a>,
then the literal value is a pair consisting of its <a>lexical form</a>
and its <a>language tag</a>, in that order.</li>
<li>If the literal is a <a>directional language-tagged string</a>, then the literal value is
a tuple of its <a>lexical form</a>, its <a>language tag</a>, and its <a>base direction</a>,
likewise in that order.</li>
<li>If the literal's <a>datatype</a> is handled by an RDF implmentation,
<ul>
<li>if the literal's <a>lexical form</a> is in the <a>lexical space</a>
of the <a>datatype</a>, then the literal value is the result of applying
the <a>lexical-to-value mapping</a> of the datatype to the
<a>lexical form</a>.</li>
<li>otherwise, the literal is <dfn data-lt-no-plural>ill-typed</dfn> and no literal value can be
associated with the literal. Such a case produces a semantic
inconsistency but is not <em>syntactically</em> ill-formed.
Implementations SHOULD accept [=ill-typed=] literals and produce RDF
graphs from them. Implementations MAY produce warnings when
encountering [=ill-typed=] literals.</li>
</ul>
</li>
<li>If the literal's <a>datatype IRI</a> is <em>not</em>
handled by an RDF implementation, then the literal value is
not defined by this specification. Implementations SHOULD accept
literals with unknown datatype IRIs and produce RDF graphs from them.
</li>
</ul>
</p>

<p><dfn data-local-lt="term-equal">Literal term equality</dfn>:
Two literals are term-equal (the same <a>RDF literal</a>)
two literals are term-equal (the same <a>RDF term</a>)
if and only if:</p>
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if and only if:</p>
if and only if the following are all true:</p>


<ul>
<li>the two <a>lexical forms</a> compare equal</li>
<li>the two <a>datatype IRIs</a> compare equal</li>
<li>the two <a>language tags</a> (if any) compare equal</li>
<li>the two <a>base directions</a> (if any) compare equal</li>
<li>the two <a>lexical forms</a> compare equal,</li>
<li>the two <a>datatype IRIs</a> compare equal,</li>
<li>the two <a>language tags</a> are either both absent, or both present and compare equal,</li>
<li>the two <a>base directions</a> are either both absent, both `ltr`, or both `rtl`.</li>
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<li>the two <a>lexical forms</a> compare equal,</li>
<li>the two <a>datatype IRIs</a> compare equal,</li>
<li>the two <a>language tags</a> are either both absent, or both present and compare equal,</li>
<li>the two <a>base directions</a> are either both absent, both `ltr`, or both `rtl`.</li>
<li>The two <a>lexical forms</a> compare equal.</li>
<li>The two <a>datatype IRIs</a> compare equal.</li>
<li>The two <a>language tags</a> are either both absent, or both present and compare equal.</li>
<li>The two <a>base directions</a> are either both absent, both `ltr`, or both `rtl`.</li>

</ul>
<p>Comparison is performed using
<p>Comparison of the [=lexical forms=] and of the [=datatype IRIs=] is performed using
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For the datatype IRIs, shouldn't this better be covered by IRI equality?

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fair point. I reused existing language, which didn't mention IRI equality. This is equivalent, because IRI equality is also based on string comparison, but this would be clearer.

<a data-cite="I18N-GLOSSARY#dfn-case-sensitive">case sensitive matching</a>
(see description of string comparison in
<a href="#rdf-strings" class="sectionRef"></a>)
except for language tags, where the comparison is performed using
(see description of string comparison in
<a href="#rdf-strings" class="sectionRef"></a>).
Comparison of the [=language tags=] is performed using
<a data-cite="I18N-GLOSSARY#dfn-case-sensitive">ASCII case-insensitive matching</a>.
Thus, two literals can have the same value
without being the same <a>RDF term</a>.
For example:</p>

<pre>
"1"^^xs:integer
"01"^^xs:integer
</pre>

<p>denote the same <a data-lt="literal value">value</a>, but are not the
same literal <a>RDF terms</a> and are not
<a>term-equal</a> because their
<a>lexical forms</a> differ.</p>
</p>

<section>
<h2>Representation of literals</h2>

<p>Some concrete syntaxes MAY support
<dfn data-lt="simple literal" class="export">simple literals</dfn> consisting of only a
<a>lexical form</a> without any <a>datatype IRI</a>, <a>language tag</a>, or <a>base direction</a>.
Simple literals are syntactic sugar for abstract syntax
<a>literals</a>
with the <a>datatype IRI</a>
<code>http://www.w3.org/2001/XMLSchema#string</code>
(which is commonly abbreviated as <code>xsd:string</code>).
</p>

<p>
Similarly, most concrete syntaxes represent
<a>language-tagged strings</a> and <a>directional language-tagged strings</a> without
the <a>datatype IRI</a> because it is always either
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> (<code>rdf:langString</code>)
or <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code> (<code>rdf:dirLangString</code>), respectively.
</p>

<p>
Any [=string=] complying with [[!BCP47]] MAY be used to represent a [=language tag=] in concrete syntaxes or implementations.
Such strings MAY be case normalized
(for example, by canonicalizing as defined by
<a data-cite="bcp47#section-4.5">BCP 47 section 4.5</a>).
Alternatively, an implementation MAY preserve the case from the original representation,
provided that it processes it in a case-insensitive manner.
</p>

<aside class=note>
The treatment of language tags has changed between RDF 1.1 and RDF 1.2.
In RDF 1.1, `"chat"@fr` and `"chat"@FR` were theoretically representing two distinct terms, but implementations had license to replace one with the other via some form of normalization.
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.
Since many RDF 1.1 implementations do normalize language tags internally, they will not be impacted by this change.
</aside>

</section>

<section>
<h2>Literal value</h2>

<p>The <dfn>literal value</dfn> associated with a <a>literal</a> is defined as follows.</p>

<ul>
<li>If the literal is a <a>language-tagged string</a>,
then the literal value is a pair consisting of its <a>lexical form</a>
and its <a>language tag</a>, in that order.</li>
<li>If the literal is a <a>directional language-tagged string</a>, then the literal value is
a tuple of its <a>lexical form</a>, its <a>language tag</a>, and its <a>base direction</a>,
likewise in that order.</li>
<li>If the literal's <a>datatype</a> is handled by an RDF implementation,
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<li>If the literal's <a>datatype</a> is handled by an RDF implementation,
<li>If the literal's <a>datatype</a> is handled by an RDF implementation, then one of the following applies:

<ul>
<li>if the literal's <a>lexical form</a> is in the <a>lexical space</a>
of the <a>datatype</a>, then the literal value is the result of applying
the <a>lexical-to-value mapping</a> of the datatype to the
<a>lexical form</a>.</li>
<li>otherwise, the literal is <dfn data-lt-no-plural>ill-typed</dfn> and no literal value can be
associated with the literal. Such a case produces a semantic
inconsistency but is not <em>syntactically</em> ill-formed.
Implementations SHOULD accept [=ill-typed=] literals and produce RDF
graphs from them. Implementations MAY produce warnings when
encountering [=ill-typed=] literals.</li>
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<li>if the literal's <a>lexical form</a> is in the <a>lexical space</a>
of the <a>datatype</a>, then the literal value is the result of applying
the <a>lexical-to-value mapping</a> of the datatype to the
<a>lexical form</a>.</li>
<li>otherwise, the literal is <dfn data-lt-no-plural>ill-typed</dfn> and no literal value can be
associated with the literal. Such a case produces a semantic
inconsistency but is not <em>syntactically</em> ill-formed.
Implementations SHOULD accept [=ill-typed=] literals and produce RDF
graphs from them. Implementations MAY produce warnings when
encountering [=ill-typed=] literals.</li>
<li>If the literal's <a>lexical form</a> is in the <a>lexical space</a>
of the <a>datatype</a>, then the literal value is the result of applying
the <a>lexical-to-value mapping</a> of the datatype to the
<a>lexical form</a>.</li>
<li>Otherwise, the literal is <dfn data-lt-no-plural>ill-typed</dfn> and no literal value can be
associated with the literal. Such a case produces a semantic
inconsistency, but it is not <em>syntactically</em> ill-formed.
Implementations SHOULD accept [=ill-typed=] literals and produce RDF
graphs from them. Implementations MAY produce warnings when
encountering [=ill-typed=] literals.</li>

</ul>
</li>
<li>If the literal's <a>datatype IRI</a> is <em>not</em>
handled by an RDF implementation, then the literal value is
not defined by this specification. Implementations SHOULD accept
literals with unknown datatype IRIs and produce RDF graphs from them.
</li>
</ul>

<p>
Thus, two literals can have the same value
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Thus, two literals can have the same value
It follows from the above that two literals can have the same value

without being the same <a>RDF term</a>.
For example:</p>

<pre>
"1"^^xsd:integer
"01"^^xsd:integer
</pre>

<p>denote the same <a data-lt="literal value">value</a>, but are not the
same literal <a>RDF terms</a> because their
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same literal <a>RDF terms</a> because their
same literal <a>RDF term</a> because their

<a>lexical forms</a> differ.</p>
</section>
</section>

<section id="section-blank-nodes">
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