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Re: [Fwd: Re: [DTB] Datatypes and Built-ins first run to clean up and extend the initial list]

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 04 Mar 2008 08:39:41 +0100
Message-ID: <47CCFCBD.3030702@deri.org>
To: Michael Kifer <kifer@cs.sunysb.edu>
CC: "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>

Michael Kifer wrote:
>>>>> A given IRI is interpreted as some abstract object in a given 
>>>>> interpretation.  I do not understand how a casting function for 
>>>>> such abstract objects can be defined in a meaningful way.
>>>> yes, it seems hairy/impossible to define it properly within the 
>>>> current FLD framework, because I can, even if I assume fixed 
>>>> interpretations for cast-functions, not "access" the lexical
>>>> representation from this fixed interpretation...
>>> It seems to me that the problem is with an inappropriate use of IRIs.
>>>  You seem to be treating them as a data type, while IRIs are like RDF
>>>  resources, and thus are not data types. Maybe you need to introduce
>>> a data type for what you want (not sure).
>> hmmmm. ouch.
>> Just to clarify a point here: It is not *me* who is treating IRIs like 
>> datatypes here, it is just how it is done in FOAF and in other RDF 
>> vocabularies... most actually. not my fault.
> I am just elaborating on the problem that Jos pointed out.
>>   To avoid this misunderstanding, I had quoted the FOAF spec in my 
>> original mail in the part which was cut out in the reply::
>>> foaf [1] suggests to encode telephone numbers as rdf:resources using
>>> a tel: scheme qualified URI:
>>> "Property: foaf:phone phone - A phone, specified using fully
>>> qualified tel: URI scheme (refs:
>>> http://www.w3.org/Addressing/schemes.html#tel)."
>>> in all RDF encodings of vCard I am aware of telephone numbers are
>>> encoded in String literals, instead.
>>> Now, if there is no way to get out the actual string of that URI in a
>>> builtin, I have no clue how to address this simple ontology mapping
>>> use case, see also [2], slide 4 and slide 10.
>>> I don't want to go into the philosophical problem of that URLs and
>>> URIs are intermingled in that FOAF encoding of telephone numbers, I
>>> just want to write a rule which does that mapping.
>> This was my original question, how do I do this mapping from
>> URIs to strings, if I need to (e.g. asking for a certain substing 
>> appearing in a URL)? yes, this mapping is not possible in the datatype 
>> system of FLD at the moment.
>> So, what do we want? Do we want to fix this, or do we want to convince 
>> all people out there who use RDF in that way that they are doing wrong,
>> i.e. shall I change my foaf file from
> I do not know. If they really mean rdf:resource then they have a semantic
> problem (I think it does not sit well with RDF semantics). 

Well, might be the case, but that's just how people use it. And there is 
increasingly RDF data being published on the Web, which encodes URLs 
(Locators) as rdf:resource and not as xsd:anyURI.

> But in your
> example below you seem to be using the xsd:anyURI *data type* -- not an
> rdf:resource -- to encode phone numbers. That has no problem. You can
> define a builtin that operates on anyURIs.

Of course, that alternative version wouldn't have any problem, sure.
But what I wanted to say is that *nobody* does it like that.
Everybody uses option 1 below and not option 2.

Option 1:

>> <http://www.polleres.net/foaf.rdf#me>
>>     foaf:phone <tel:+35391495723> ;
>>     foaf:homepage <http://www.polleres.net/> .

Option 2:

>> <http://www.polleres.net/foaf.rdf#me>
>>       foaf:phone    "tel:+35391495723"^^xsd:anyURI ;
>>       foaf:homepage "http://www.polleres.net/"^^xsd:anyURI .

But I am unsure whether we can by any means accomodate for that.
Anyway, I think that casts are not trivial to define even for typed
literals or no? Since the lex-to-val mapping is not injective in 
general, how can I define for instance  the cast

    xsd:string( "01"^^xsd:integer)


In our framework, we have to give a fixed interpretation.
It says that this function is only defined for certain datatypes as 
"input", but in out framework the "input" is always a TERM and for 
evaluating I_F(xsd:string( "01"^^xsd:integer) )
I need to consider the actual domain value which "01"^^xsd:integer
is mapped to and reconvert it to a string. For that example, it is still 
easy, because there is a canonical string "1" which we can return.

Now what about

    xsd:string( "http://www.example.org/theNumberOne"^^rif:iri)

??? Now what is the truth value of this? Depends on the current 
interpretation, or no? If the curent interpretation maps the iri
http://www.example.org/theNumberOne to an element in the value space of
e.g. xsd:integer the function should return a value, if it doesn't map
http://www.example.org/theNumberOne to an element in a compatible value
space for string conversions, the function returns an error. Yes?

I am not sure at the moment whether this is a problem, but it is at 
least a bit contrived.


>> So how do I convince all the rest of the world to change that likewise?
>> Even the RDF Primer [2] (Example 6):
>> ----------------------------------------------------------------------
>>       <rdf:Description rdf:nodeID="abc">
>>          <exterms:fullName>Dave Beckett</exterms:fullName>
>>          <exterms:homePage rdf:resource="http://purl.org/net/dajobe/"/>
>>       </rdf:Description>
>> ----------------------------------------------------------------------
>> or the SPARQL spec [3] (examples in Section 6):
>> ----------------------------------------------------------------------
>>   @prefix foaf:       <http://xmlns.com/foaf/0.1/> .
>>   @prefix rdf:        <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
>>   _:a  rdf:type        foaf:Person .
>>   _:a  foaf:name       "Alice" .
>>   _:a  foaf:mbox       <mailto:alice@example.com> .
>>   _:a  foaf:homepage   <http://work.example.org/alice/> .
>> ----------------------------------------------------------------------
>> promote the use of rdf:resources for URLs (homepages, email-adresses, 
>> etc.). It might be arguable with respect to the concepts, but people 
>> *do* use URIs like that and it is even done in the specs of RDF as we 
>> see here.
>> How do we cater for it?  By just stating I cannot get a substring out of 
>> a homepage-URL or email-address... just because rdf:resource (and 
>> rif:iri, respectively) are not datatypes?
>> I knew somehow why I put that in my signature ;-)
>> Axel
>> 1. http://xmlns.com/foaf/spec/
>> 2. http://www.w3.org/TR/REC-rdf-syntax/
>> 3. http://www.w3.org/TR/rdf-sparql-query/
>> -- 
>> Dr. Axel Polleres
>> email: axel@polleres.net  url: http://www.polleres.net/
>> rdfs:Resource owl:differentFrom xsd:anyURI .

Dr. Axel Polleres
email: axel@polleres.net  url: http://www.polleres.net/

rdfs:Resource owl:differentFrom xsd:anyURI .
Received on Tuesday, 4 March 2008 07:40:09 UTC

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