W3C home > Mailing lists > Public > semantic-web@w3.org > July 2010

Re: Subjects as Literals

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 30 Jun 2010 22:14:15 -0500
Cc: Hugh Glaser <hg@ecs.soton.ac.uk>, Robert Sanderson <azaroth42@gmail.com>, Linked Data community <public-lod@w3.org>, Semantic Web <semantic-web@w3.org>
Message-Id: <38430386-A8BC-44E6-95B2-DFBDD7870EA6@ihmc.us>
To: Ross Singer <rossfsinger@gmail.com>

On Jun 30, 2010, at 8:14 PM, Ross Singer wrote:

> I suppose my questions here would be:
>
> 1) What's the use case of a literal as subject statement (besides
> being an academic exercise)?

A few off the top of my head.

1. Titles of books, music and other works might have properties such  
as the date they were registered, who owns them, etc..
2. Dates may have significant properties such as being the day that  
someone was shot or when war broke out.
3. Dates represented as character strings in some known date format  
other than XSD can be asserted to be the same as a 'real' date by  
writing things like

"01-02-1481" sameDateAs "01022010"^^xsd:date .
"01-02-1481" isDateIn :MuslimCalendar .

I am sure that you can think of many more. In general, allowing  
strings as subjects opens the door to a wide range of uses of RDF to  
'attach'  information to pieces of text. Another example which occurs  
to me: this piece of text is the French translation of that piece of  
text, expressed as a single RDF triple with two literals.

4. It has been noted that one can map datatyping into RDF itself by  
treating the datatypes as properties, and there are several use cases  
for this. The natural way to do it involves having literals as  
subject, since the dataype map goes from the string to the value:

"23" xsd:number "23"^^xsd:number .

5. Also, allowing this "purely academically" has the notable advantage  
of simplifying RDF(S) inferencing, including making the forward- 
chaining rules simpler. Right now, there is a strange oddity involving  
blank node instantiations. One can say things like 'the number of my  
children is prime" by using an blank node:

:PatHayes hasNumberOfKids _:x .
_:x :a :PrimeNumber .

But this legal RDF can't be instantiated in the obvious way:

:PatHayes hasNumberOfKids "3"^^xsd:number .
"3"^^xsd:number :a "PrimeNumber .   XXXX

This trips up RDFS reasoners, which can often produce inferences by a  
kind of sneaky use-a-bnode-instead maneuver even when the obvious  
conclusion cannot be stated because of the restriction. (There are a  
few examples in the RDF semantics document.) Removing the restriction  
would enable reasoners to work more efficiently with a smaller set of  
rules. (I gather that at least some of the RDFS rule engines out there  
already do this, internally.)

> 2) Does literal as subject make sense in "linked data" (I ask mainly
> from a "follow your nose" perspective) if blank nodes are considered
> controversial?

Seems to me that from the linked data POV, anything that can be an  
object should also be useable as a subject. Of course, that does allow  
for the view that both of them should only ever be IRIs, I guess.

Pat Hayes

>
Received on Thursday, 1 July 2010 03:16:16 UTC

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