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DC-ARCHITECTURE  October 2009

DC-ARCHITECTURE October 2009

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Subject:

Re: Proposal #2: replace rdf:value with skos:prefLabel

From:

David Wood <[log in to unmask]>

Reply-To:

DCMI Architecture Forum <[log in to unmask]>

Date:

Thu, 22 Oct 2009 12:13:41 -0400

Content-Type:

text/plain

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Parts/Attachments

text/plain (169 lines)

On Oct 20, 2009, at 5:15 PM, Thomas Baker wrote:

> On Thu, Oct 15, 2009 at 09:35:07AM +0100, Pete Johnston wrote:
>>> 2. Instead of using rdf:value to relate a value to a
>>>   DCAM Value String [1, section 4.6], David suggests using
>>>   skos:prefLabel [3].
> ...
>>> -- The domain of skos:prefLabel was also left unspecified [3],
>>>   so its use does not imply that the subject of a statement is
>>>   a SKOS concept.  On the other hand, I believe the
>>>   correct use of rdf:value has long been unclear.
>>>   So #2 seems like a good idea too, though as part of such a
>>>   change we would need to understand better where the problem
>>>   with rdf:value lies.
> ...
>> (a) According to
>>
>> http://www.w3.org/TR/skos-reference/#L1581
>>
>> the range of skos:prefLabel is the class of plain literals. The  
>> DCAM notion of
>> value strings currently includes typed literals. See e.g. the  
>> example in 4.3 of
>>
>> http://dublincore.org/documents/2008/01/14/dc-rdf/
>>
>> where both a plain and typed literal are provided.
>>
>> So (with the current concept of "value string") I don't think  
>> skos:prefLabel
>> would work as a straight substitute for the use of rdf:value?
>>
>> I see SKOS also has a skos:notation property but that seems to be  
>> explicitly
>> for "non-natural-language" literals?
>
> SKOS Reference [1] says:
>
>    Note that the range of skos:prefLabel, skos:altLabel and
>    skos:hiddenLabel is the class of RDF plain literals
>    [RDF-CONCEPTS].
>
>    By convention, RDF plain literals are always used in the
>    object position of a triple, where the predicate is one of
>    skos:prefLabel, skos:altLabel or skos:hiddenLabel.  If a
>    graph does not follow this usage convention an application
>    may reject such data but is not required to.  See also the
>    note below.
>
> and
>
>    By convention, the property skos:notation is only used with
>    a typed literal in the object position of the triple, where
>    the datatype URI denotes a user-defined datatype
>    corresponding to a particular system of notations or
>    classification codes.
>
> The example 4.3 of DC-RDF [2] does indeed show a
> "non-natural-language" code in the object position of the
> triple:
>
>    ex:subject32 rdf:value "EA32^^ex:SubjectEncoding"
>
> I'm wondering whether rdf:value could in principle be replaced
> with skos:notation _or_ skos:prefLabel depending on whether used
> with datatypes or plain literals.  Would any significant use
> cases be left unaddressed (e.g., datatypes using natural
> language)?
>
> There are other dimensions to this question (e.g., the notion of
> "preferred" as raised by Karen), but I'm wondering whether the
> distinction between skos:prefLabel and skos:notation would at
> least address the issue of plain versus typed literals.  (And
> are there indeed arguments for using two distinct properties
> instead of the undifferentiated rdf:value?)
>
> I'm also wondering whether there are also reasons to move _away_
> from using rdf:value.  As discussed in a thread on the
> public-swd-wg mailing list in January [3], the inconsistent use
> of rdf:value was recognized and discussed in 2001-2002 [4] and
> AFAICT never really clearly resolved [5].
>
> On Mon, Oct 19, 2009 at 10:33:49AM -0400, David Wood wrote:
>> The two changes listed below by Tom would indeed solve my most
>> pressing issues.
>
> Dave, it would be helpful to know what it was about rdf:value
> that created an issue for you.


rdf:value was underspecified, as noted above.  However, a read of the  
RDF spec at [6] suggests that the intended use of rdf:value is to be  
found in the RDF Primer [7] ("The intended use for rdf:value is  
explained intuitively in the RDF Primer document").

The Primer presents a use of rdf:value for expounding structured  
relations, such as this example:

[[
      <exterms:weight rdf:parseType="Resource">
        <rdf:value rdf:datatype="&xsd;decimal">2.4</rdf:value>
        <exterms:units rdf:resource="http://www.example.org/units/kilograms 
"/>
      </exterms:weight>
]]

The DCAM seems to use rdf:value consistently with the intended usage  
in some places.  However, rdf:value is used in the DCAM to describe  
labels, values and notes.  The semantics for those elements are quite  
unclear.

So, my general objection to rdf:value is that it is underspecified.   
My more specific argument is that the DCAM's use of rdf:value is not  
always consistent with the apparent intended usage as inferred in the  
RDF Primer and pointed to be the RDF spec.

The lack of clarity with rdf:value has real consequences.  For  
example, we recently saw an description of properties with both  
valueString and valueURI written in a DSP as:

[[
$valueURI rdf:value $valueString
]]

...although the intention seems to have been something like this:

[[
[skos:broader $valueURI; skos:prefLabel $valueString]
]]

I have a hard time believing that using rdf:value to relate a valueURI  
to a valueString is what we want to do, but a read of the DCAM seems  
to suggest that it is a valid interpretation.

By contrast, the intended usage of skos:notation and skos:prefLabel is  
more clearly identified in the SKOS Reference.  The suggested use of  
skos:notation and skos:prefLabel for datatypes and plain literals  
respectively makes sense to me as a mechanism for clarifying the  
semantics.

Ideally, I would like to see the DCAM normatively reference RDF and be  
changed to solely extend RDF (not redefine it).

Thanks to James Leigh and Stuart Sutton for offline conversations  
regarding this issue.

Regards,
Dave


[6]  http://www.w3.org/TR/rdf-mt/#rdfValue
[7]  http://www.w3.org/TR/rdf-primer/#rdfvalue



>
> Tom
>
> [1] http://www.w3.org/TR/skos-reference/#L1581
> [2] http://dublincore.org/documents/2008/01/14/dc-rdf/
> [3] http://lists.w3.org/Archives/Public/public-swd-wg/2009Jan/ 
> 0027.html
> [4] http://lists.w3.org/Archives/Public/public-swd-wg/2009Jan/ 
> 0038.html
> [5] http://www.w3.org/2000/03/rdf-tracking/#rdfms-replace-value
>
> -- 
> Thomas Baker <[log in to unmask]>
>

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