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Re: Text for FAQ re rdf:datatype="&rdfs;#XMLLiteral"

From: Frank Manola <fmanola@mitre.org>
Date: Mon, 11 Aug 2003 09:13:02 -0400
Message-ID: <3F37965E.7030508@mitre.org>
To: Patrick.Stickler@nokia.com
CC: w3c-rdfcore-wg@w3.org

Patrick.Stickler@nokia.com wrote:

>>-----Original Message-----
>>From: ext Frank Manola [mailto:fmanola@mitre.org]
>>Sent: 11 August, 2003 15:32
>>To: Stickler Patrick (NMP/Tampere)
>>Cc: w3c-rdfcore-wg@w3.org
>>Subject: Re: Text for FAQ re rdf:datatype="&rdfs;#XMLLiteral"
>>Patrick Stickler wrote:
>>>Can I specify rdfs:XMLLiteral as the value of rdf:datatype?
>>I believe it's rdf:XMLLiteral, rather than rdfs:XMLLiteral 
>>(if not, both 
>>Primer and Concepts need to be revised, among other documents).
> Err... That's right. We changed it.
> s/rdfs/rdf/
>>>Yes. Though it should be done with caution, particularly if
>>>writing RDF/XML by hand, to ensure that the specified lexical
>>>form is fully valid, as RDF parsers are not required to
>>>check explicitly specified lexical forms.
>>Could you clarify "as RDF parsers are not required to check 
>>specified lexical forms"?  Our general pitch on datatypes is that the 
>>proper lexical forms for a datatype are defined by the datatype.  So 
>>while RDF can't determine whether a datatype URIref actually 
>>refers to a 
>>datatype, and can't determine whether a lexical form is valid with 
>>respect to a particular datatype, software designed to process that 
>>particular datatype can (and presumably must).  In the case of 
>>rdf:XMLLiteral, the datatype in question is defined as part 
>>of RDF, and 
>>if we follow the same general model as for other datatypes, 
>>it seems to 
>>me that RDF software must be capable of both determining whether the 
>>datatype URIref rdf:XMLLiteral actually refers to a datatype 
>>(it does), 
>>and determine whether a lexical form is valid with respect to that 
>>datatype (that is, assuming we allow explicitly writing an 
>>rdf:XMLLiteral value as a typed literal, rather than using the 
>>parseType).  If we're making an exception in the case of 
>>then it seems to me that the next FAQ needs to be "why" (I'm 
>>not saying 
>>there may not be good reasons, but this seems to break the 
>>general model)?
> Perhaps it remains an open question whether we require RDF
> parsers to check the validity of lexical forms explicitly
> specified with rdf:datatype="&rdf;XMLLiteral". I (perhaps
> mistakenly) thought we didn't want to impose such a requirement
> on RDF parsers. Perhaps we do.
> Patrick

I'm not necessarily suggesting that we *ought* to impose such a 
requirement, just pointing out that it seems to break the model that we 
describe for other datatypes.  And I think it'll be natural for people 
to wonder exactly how (or when) the validity of an instance of 
rdf:XMLLiteral gets determined, if RDF software doesn't do it.  If we 
continue our litergy that says we're not defining a processing model, I 
don't see how this can be dependent on whether we imagine an RDF/XML 
parser on top of an XML parser (for example).  Even though we define 
RDF/XML as the standard language for serializing RDF graphs, that 
language is defined in terms of how the RDF/XML generates triples.  It's 
at the level of triples that this validity checking would have to be 
done.  Presumably it's at this level that whether subjects and 
predicates are valid URIrefs is determined, and whether any literals 
specified are valid literals.

I'd note that an alternative would be to treat rdf:XMLLiteral sort of 
like we treat other datatypes, except that we (RDF) have defined a 
built-in name for it.  That is, we don't define XML (any more than we 
define xsd:integer) but we know people are presumably going to want to 
use XML literals, so we provide this datatype, which is likely to be 
widely available (but if it is made available in a particular RDF 
implementation, the implementation has to provide the appropriate 
software to support the RDF-defined datatype model, and the datatype 
semantics).  I don't really care, except if we're going to break the 
model, I think we need to say so.


Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-875
Received on Monday, 11 August 2003 08:48:27 UTC

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