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Re: Proposal for ISSUE-12, string literals

From: Richard Cyganiak <richard@cyganiak.de>
Date: Thu, 12 May 2011 12:52:51 +0100
Cc: public-rdf-wg@w3.org
Message-Id: <0D69FD23-A8C5-46F3-A701-26A3AEE0F8FA@cyganiak.de>
To: antoine.zimmermann@insa-lyon.fr
On 12 May 2011, at 11:36, Antoine Zimmermann wrote:
> The XSD specifications define the canonical form of xsd:boolean, xsd:decimal, xsd:float, xsd:double, xsd:dateTime, xsd:time, xsd:date, xsd:hexBinary, xsd:integer, xsd:nonPositiveInteger, xsd:negativeInteger, xsd:long, xsd:int, xsd:short, xsd:byte, xsd:nonNegativeInteger, xsd:unsignedLong, xsd:unsignedInt, xsd:unsignedShort, xsd:unsignedByte, xsd:positiveInteger.
> 
> RDF could simply rely on these definitions.

+1.

But I'd be tempted to go further and make only the primitive types such as xsd:decimal into RDF canonical forms. This would mean that systems MAY canonicalize all numbers to a single numeric datatype.

Best,
Richard



> 
> Le 12/05/2011 12:19, Richard Cyganiak a écrit :
>> On 12 May 2011, at 09:22, Ivan Herman wrote:
>>> - You make the remark on the wiki page on 'extending this to
>>> numeric literals', which I would rather say 'extending this to any
>>> datatype' (eg, xsd:dateTime, too).
>> 
>> Right -- I changed the section heading on the wiki.
>> 
>>> I have the impression that this is also a consequence of what you
>>> write already. You emphasize the 'lexical equality', and you also
>>> say "Implementations MAY replace any literal with a canonical form
>>> if both are syntactically different, but have the same value."
>>> which does not look like being bound to string literals.
>> 
>> The way I wrote it, the only literals marked as canonical forms are
>> plain string literals. So the sentence doesn't license replacement
>> of, say, +00013 with 13, because no numeric literals have been marked
>> as canonical forms. That could be easily changed, of course.
>> 
>>> Do you think there is anything missing in this document to make
>>> that picture complete (except, editorially, to possibly add
>>> non-string examples)?
>> 
>> If we only want to address string literals, then I think the proposal
>> is complete.
>> 
>> If we want to address other XSD literals as well, then some bullet
>> points should be added to the list of equalities, and the canonical
>> lexical form of some XSD datatypes (e.g., "13.0"^^xsd:decimal) should
>> be defined to be canonical forms so that other same-valued literals
>> can be replaced with the canonical form. This requires a detailed
>> reading of the XSD spec (which I have not done so far).
>> 
>> (RDF Concepts should probably contain a paragraph or two introducing
>> the rdf:PlainLiteral datatype and referencing the relevant spec, but
>> let's treat that as a separate issue.)
>> 
>>> - I would also propose to make some tiny changes in the Semantics
>>> document.
>> 
>> I'll let the editors of that document comment.
>> 
>> Best, Richard
>> 
>> 
>>> 
>>> Ivan
>>> 
>>> 
>>> On May 11, 2011, at 23:23 , Richard Cyganiak wrote:
>>> 
>>>> I took an action today to draft text for RDF Concepts that
>>>> resolves ISSUE-12. I put it on the wiki here:
>>>> http://www.w3.org/2011/rdf-wg/wiki/StringLiterals/EntailmentProposal
>>>> 
>>>> 
> A plain text copy is attached below.
>>>> 
>>>> Best, Richard
>>>> 
>>>> 
>>>> 
>>>> SHORT SUMMARY
>>>> 
>>>> 1. RDF Concepts puts more emphasis on the distinction between
>>>> (syntactic) “literal equality” and (semantic, important for
>>>> applications) “value equality” 2. RDF Concepts explicitly points
>>>> out the specific string value equalities that already arise from
>>>> RDF Semantics 3. RDF Concepts declares one of the string literal
>>>> forms as canonical 4. Implementations MAY canonicalize, but don't
>>>> have to 5. The canonical form is plain literals.
>>>> 
>>>> 
>>>> WHY?
>>>> 
>>>> 1. No changes to the abstract syntax required 2. No changes to
>>>> any concrete syntax or parser required 3. No changes to any
>>>> implementations of any of the existing entailment regimes
>>>> required 4. Those who are ok with canonicalization can do that,
>>>> and don't need to deal with entailment 5. Those who don't want to
>>>> canonicalize, have the option of supporting only string value
>>>> equality at query time, without RDFS- and D-Entailment 6. “MAY
>>>> canonicalize” softly discourages the use of xsd:string typed
>>>> literals, without abolishing them outright or declaring them
>>>> archaic 7. Standardizing on xsd:string was never an option
>>>> because of language tags 8. Standardizing on rdf:PlainLiteral was
>>>> never an option because it MUST NOT be used in serializations
>>>> that support plain literals
>>>> 
>>>> 
>>>> CHANGES TO 6.5.2 The Value Corresponding to a Typed Literal
>>>> http://www.w3.org/TR/rdf-concepts/#section-Literal-Value
>>>> 
>>>> 
>>>> §1 Rename it to “6.5.1 The Value Corresponding to a Literal” and
>>>> move it ahead of 6.5.1
>>>> 
>>>> §2 Add to the beginning: “The value of a plain literal without
>>>> language tag is the same Unicode string as its lexical form.
>>>> 
>>>> The value of a plain literal with language tag is a pair
>>>> consisting of 1. the same Unicode string as its lexical form, and
>>>> 2. its language tag.
>>>> 
>>>> For typed literals, …” (continue with rest of section as is)
>>>> 
>>>> §3 Remove the Note at the end of the section
>>>> 
>>>> 
>>>> CHANGES TO 6.5.1 Literal Equality
>>>> http://www.w3.org/TR/rdf-concepts/#section-Literal-Equality
>>>> 
>>>> 
>>>> §4 Rename section to “6.5.2 Literal Equality and Canonical
>>>> Forms”
>>>> 
>>>> §5 Add to the beginning: “Equality of literals can be evaluated
>>>> based on their syntax, or based on their value.”
>>>> 
>>>> §6 Change “Two literals are equal …” to: “Two literals are
>>>> syntactically equal …” in the current first paragraph.
>>>> 
>>>> §7 Add to the end: “In application contexts, comparing the values
>>>> of literals (see section 6.5.1) is usually more helpful than
>>>> comparing their syntactic forms. Literals with different lexical
>>>> forms and with different datatypes can have the same value. In
>>>> particular:
>>>> 
>>>> - A plain literal with lexical form aaa and no language tag has
>>>> the same value as a typed literal with lexical form aaa and
>>>> datatype IRI xsd:string - A plain literal with lexical form aaa
>>>> and no language tag has the same value as a typed literal with
>>>> lexical form aaa@ and datatype IRI rdf:PlainLiteral - A plain
>>>> literal with lexical form aaa and language tag xx has the same
>>>> value as a typed literal with lexical form aaa@xx and datatype
>>>> IRI rdf:PlainLiteral”
>>>> 
>>>> §8 “Some literals are canonical forms. Implementations MAY
>>>> replace any literal with a canonical form if both are
>>>> syntactically different, but have the same value. All plain
>>>> literals, with or without language tag, are canonical forms.”
>>>> 
>>>> 
>>>> CHANGES TO 6.3 Graph Equivalence
>>>> http://www.w3.org/TR/rdf-concepts/#section-graph-equality
>>>> 
>>>> 
>>>> §9 Append this leftover sentence, which was removed from 6.5.1:
>>>> “Note: For comparing RDF Graphs, semantic notions of entailment
>>>> (see [RDF-SEMANTICS]) are usually more helpful than the syntactic
>>>> equivalence defined here.”
>>>> 
>>>> 
>>>> EXTENDING THIS TO NUMERIC LITERALS???
>>>> 
>>>> (While we're at it, we might also cover equalities between the
>>>> built-in numeric XSD types, and between different lexical forms
>>>> of the same built-in XSD datatype.)
>>> 
>>> 
>>> ---- Ivan Herman, W3C Semantic Web Activity Lead Home:
>>> http://www.w3.org/People/Ivan/ mobile: +31-641044153 PGP Key:
>>> http://www.ivan-herman.net/pgpkey.html FOAF:
>>> http://www.ivan-herman.net/foaf.rdf
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>> 
>> 
> 
> 
> -- 
> Antoine Zimmermann
> Researcher at:
> Laboratoire d'InfoRmatique en Image et Systèmes d'information
> Database Group
> 7 Avenue Jean Capelle
> 69621 Villeurbanne Cedex
> France
> Tel: +33(0)4 72 43 61 74 - Fax: +33(0)4 72 43 87 13
> Lecturer at:
> Institut National des Sciences Appliquées de Lyon
> 20 Avenue Albert Einstein
> 69621 Villeurbanne Cedex
> France
> antoine.zimmermann@insa-lyon.fr
> http://zimmer.aprilfoolsreview.com/
> 
Received on Thursday, 12 May 2011 11:53:23 GMT

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