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Re: Dave Reynolds: rdf:text - clarification requested

From: Axel Polleres <axel.polleres@deri.org>
Date: Wed, 10 Dec 2008 16:42:44 +0100
Message-ID: <493FE374.5060006@deri.org>
To: Sandro Hawke <sandro@w3.org>
CC: Dave Reynolds <der@hplb.hpl.hp.com>, public-rdf-text@w3.org, "Seaborne, Andy" <andy.seaborne@hp.com>

Sandro Hawke wrote:
> Dave:
>>>> Please could I get some clarification on the following line from the 
>>>> rdf:text document:
>>>>
>>>> """In addition to the RIF and OWL specifications, this datatype is 
>>>> expected to supersede RDF's plain literals with language tags, cf. 
>>>> [5], which is why this datatype has been added into the rdf: 
>>>> namespace. """
>>>>
>>>> I don't recall any discussion on this notion of "supersede". What 
>>>> exactly is being proposed here? I have been regarding rdf:text as a 
>>>> formalization of RDF plain literals with language tags which 
>>>> simplifies OWL2/RIF's job but is not a change to RDF.  Clearly any 
>>>> change to the RDF specs would have implications for tools developers, 
>>>> especially if there are any round-tripping requirements, and wouldn't 
>>>> be something to make likely. I don't think it appropriate to hint at 
>>>> such a change in the rdf:text document without more details.
>>>>
>>>> Apologies for not having noticed this line earlier.
> 
> Axel:
>>> My personal opinion: I is not the intention to change/affect the 
>>> existinfg RDF specs, but the datatype is indeed intended to fix the 
>>> mismatch between plain literals and language tagged literals for 
>>> impementations which adopt it.
> 
> Dave:
>> Is that something that needs "fixing"?
>>
>> In i18n terms then internationalized text and strings are quite 
>> different things. The differences between the two in RDF have not been a 
>> problem in implementations or practice that I'm aware of. Is there any 
>> evidence to suggest otherwise?
>>
>> I thought rif:text, now rdf:text, was invented to simplify including 
>> internationalized strings in RIF not to fix some problem with RDF.
>>
>>> Any suggestion for a rewording that would rather convey this message?
>> We need clarity on what is being proposed before thinking of a wording.
>>
>> Are you intending or expecting that RDF implementations should 
>> explicitly support rdf:text as a datatype?
>>
>> So that they would regard:
>>
>> (a)    eg:a eg:p  "foo"@en .
>>
>> and
>>
>> (b)    eg:a eg:p  "foo@en"^^rdf:text .
>>
>> as equivalent graphs?

IMO yes, implementations that are rdf:text-aware should treat these 
equivalently.

> I'm not totally sure which kind of equivelence is right here, but I'm
> inclined to make it as close to identity as possible.  That is, (a) and
> (b) are two different ways to serialize the same triple.
+1

> So it's a lot like these two Turtle documents
> 
> (c)    @prefix foo: <http://example.org/abc#>
>        foo:a foo:p  "hello"
> 
> and
> 
> (d)    @prefix bar: <http://example.org/abc#>
>        bar:a bar:p  "hello"
>         
> which are different text but serialize the same RDF graph.
> 
> For simplicity of implementation, I think RDF serializations should
> mandate use of one style of language tagging or the other.  In order to
> handle legacy syntaxes which were created before rdf:text and so could
> not pick, I think we should probably say rdf:text SHOULD NOT be used in
> any RDF syntax which has built-in support for language tagging (in order
> to avoid all the problems you name, below).
> 
> That is, in RDF/XML, N-Triples, N3, and Turtle, 

note: the latter three are "only" member- or team submissions. a 
standard emerging from these could fix that, and likewise could the 
upcoming re-launch of DAWG (in case that there is support to add 
datatype support to SPARQL in that round).

> one SHOULD NOT use
> rdf:text.  (Happily, this aligns with rdf-syntax saying "Any other names
> are not defined and SHOULD generate a warning when encountered, but
> should otherwise behave normally.")  Meanwhile, the various RIF syntaxes
> and the newer OWL syntaxes do not directly support language tagging, so
> one has to use rdf:text.  Perhaps a Turtle 1.1 would remove type-a
> language tagging and mandate rdf:text instead. Similarly, APIs are free
> to pick one or the other (or some other, equivalent) approach, but
> should probably just provide one, and certainly not distinguish between
> the two.
> 
>> Would there be any expectation on round tripping so that an RDF 
>> processor receiving a graph in form (b) would be expected to return it 
>> in the same form and not normalize it to form (a)?
> 
> With the above formulation, this problem doesn't come up.  
> 
>> When we originally proposed rif:text I was expecting to translate RDF 
>> lang-tagged literals to rif:text as part of a translator and rif:text 
>> would not appear as a datatype in the RDF.
>>
>> Implementing rif:text as a RDF datatype is clearly possible but the 
>> discontinuity introduced by changing RDF would be a serious concern. We 
>> could end up in a state where some RDF producers thought form (b) was a 
>> legal way to exchange an internationalized text fragment in RDF while a 
>> fraction of deployed RDF consumers would not consume it (at least not 
>> with the required semantics).
> 
> Agreed, this is a problem we should avoid.
> 
>> If only RIF and RDF were involved then my preferred phrasing would be 
>> something like:
>>
>> "Note that the rdf:text datatype is purely intended for use within RIF 
>> and it is not intended that RDF processors should support this as an RDF 
>> datatype. Consumers of RDF-RIF combinations are expected to map between 
>> RDF language-tagged literals and rdf:text literals as part of the RIF 
>> translation process."
>>
>> Indeed it might be better still to substitute "SHOULD NOT" (in the 
>> RFC2119 sense) for "not intended".
>>
>> However, that doesn't cover OWL2. I don't understand enough of OWL2's 
>> requirements here, and how interoperation with deployed RDF is 
>> envisaged, to be able to suggest anything specific.

I see the concerns, however, supporting it in OWL2 and not in RDF sounds 
kind of weird, assuming that OWL2 will have an RDF serialization... no?

Axel

-- 
Dr. Axel Polleres
Digital Enterprise Research Institute, National University of Ireland, 
Galway
email: axel.polleres@deri.org  url: http://www.polleres.net/
Received on Wednesday, 10 December 2008 15:43:55 UTC

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