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Re: Should rdf:value have a semantics??

From: Graham Klyne <GK@NineByNine.org>
Date: Wed, 04 Dec 2002 10:50:52 +0000
Message-Id: <5.1.0.14.2.20021204104236.00a2f9f0@127.0.0.1>
To: pat hayes <phayes@ai.uwf.edu>
Cc: fmanola@mitre.org, w3c-rdfcore-wg@w3.org

At 04:47 PM 12/3/02 -0600, pat hayes wrote:
>>Back up a minute.  The "historic" use of rdf:value was for the cases
>>like
>>
>>my:cat rdf:type ex:DomesticCat .
>>my:cat ex:weight _:x .
>>_:x rdf:value "15" .
>>_:x ex:unit ex:Kilogram
>>
>>Now that we have datatypes, the extension of this usage to
>>
>>my:cat rdf:type ex:DomesticCat .
>>my:cat ex:weight _:x .
>>_:x rdf:value "15"^^xsd:integer .
>>_:x ex:unit ex:Kilogram
>>
>>seems perfectly straightforward.  You're being more precise about what
>>the value is (it's a number, rather than a string), and you need other
>>properties anyway to specify the units (and anything else).  I'm not
>>such a fan of rdf:value, but it seems to me that restricting rdf:value
>>to preclude this usage might seem somewhat artificial to those already
>>using it.
>
>Yes, OK, fair enough. Sigh. Then I really don't know what can be said 
>about it in the MT. So maybe I wont say anything.
>
>Neeeverrr Mind.

OK, even if we can't define it formally, shouldn't we at least try to say 
something normative about its *intended* meaning -- otherwise discussions 
like the above will continue to dog any use of rdf:value, and we might be 
better off deprecating it now.

The trouble I have is that I don't know what "principle value" means.

   Q: How long is the wire?

   A: 10 miles

I might argue that "miles" is in some sense more "principal" than the 
associated number 10;  e.g. as opposed to metres.

Another question, would it be wrong to write:
   ex:pieceOfWire rdf:value "10" .
   ex:pieceOfWire rdf:value "miles" .
?

This is suggesting to me an idea that rdf:value might just be a stand-in 
for *any* property;  a kind of universal super-property (like rdf:Resource 
is for classes)?  That corresponds to it having no (useful) formal 
semantics, yet means that we can write expressions that use it without 
wondering about their validity.  Is there any more we *can* say normatively 
about rdf:value?

[later...]

I've just looked at Pat's revised words in [1], section 3.2.4:
[[
The intended use for rdf:value is to associate some quantity with a literal 
representing the 'amount' of the quantity, such as a weight in kilograms or 
a length in yards. Examples of this kind of use are given in [RDF-PRIMER]. 
Since the subject of the relevant triple can be any quantitity, and the 
object can either be a plain literal indicating a textual representation of 
the amount or a typed literal denoting the numerical value of the amount, 
there is no way to state this condition formally in a model theory.
]]

These words suggest that rdf:value is a superproperty over all properties 
that express *quantities*;  i.e. numbers, etc., and their lexical spaces.

Unfortunately, there's a counter-example in the original M&S, section 7.3, 
which uses (something very like) the following:

    ex:docURI dc:subject _:x .
    _:x rdf:value "020 - Library science" .
    _:x ex:classification "DeweyDecimalCode" .

#g
--

[1] http://www.coginst.uwf.edu/~phayes/RDF_Semantics_finalCall.html


-------------------
Graham Klyne
<GK@NineByNine.org>
Received on Wednesday, 4 December 2002 05:55:43 EST

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