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

Re: Subjects as Literals

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 5 Jul 2010 17:43:17 -0500
Cc: Linked Data community <public-lod@w3.org>, Semantic Web <semantic-web@w3.org>
Message-Id: <EB7BFEBA-7A8B-4E9F-85FF-550B79413CCB@ihmc.us>
To: Sampo Syreeni <decoy@iki.fi>
Not wanting to keep beating this particular drum, but some things just  
have to be responded to.

On Jul 5, 2010, at 1:36 PM, Sampo Syreeni wrote:

> On 2010-06-30, Hugh Glaser wrote:
>
>> RDF permits anyone to say anything about anything . . . except a  
>> literal if it is the subject of the property you want to use for  
>> the description.
>
> The way I see it, the main reason for this restriction is that the  
> data is supposed to be machine processable. Literals rarely are,  
> especially in their original, plain form.

This objection strikes me as completely wrong-headed. Of course  
literals are machine processable.

> I mean, suppose we allowed literals as subject, predicate and  
> object. What does it really mean if I say "Sally"@en "likes"@en  
> "Mike"@en?

Well, nobody is suggesting allowing literals as predicates (although  
in fact the RDF semantics would easily extend to this usage, if  
required, and the analogous structures are allowed, and do have  
genuine use cases, in ISO Common Logic.) But it is easy to give  
'ridiculous' examples for any syntactic possibility. I can write  
apparent nonsense using nothing but URIs, but this is not an argument  
for disallowing URIs in RDF.

> I'd argue pretty much nothing processable. That's because literals  
> tack on an arbitrary, limited number of type specifiers (type and  
> perhaps language) to textual data, and neglect everything beyond  
> that. That is not how full disambiguation is done; it's how a human  
> processor *minimally* disambiguates a piece of text, without making  
> it unambiguous.

This is WRONG. The type specifiers *completely* disambiguate the text  
in the body of the literal. Really, this is not a topic for debate in  
a public email list. Just check the actual RDF specifications, in  
particular http://www.w3.org/TR/rdf-mt/#dtype_interp . For plain  
literals, the meaning of the literal is the string itself, a unique  
string of characters. Again, this is stated in the RDF specification  
documents as a normative part of the RDF spec.

> With Schema derived or otherwise strictly derived types, the level  
> of disambiguation can be the same as or even better than with URI's,  
> true. But then that goes the other way around, too: URI's could take  
> the place of any such precise type.

No, they cannot. For numbers, for example, one would need infinitely  
many URIs; but in any case, why bother creating all these URIs? We  
have (universally understood) names for the numbers already, called  
numerals. For dates, times and so forth, there are many formats in use  
throughout human societies, of course. That is WHY the work of  
establishing datatype standards work was done. To ignore all this, to  
reject a widely accepted standard, and advocate reversion to a home- 
made URI scheme seems to me to be blatantly irresponsible.

Pat Hayes

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Received on Monday, 5 July 2010 22:44:20 UTC

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