W3C home > Mailing lists > Public > www-rdf-interest@w3.org > February 2001

Literals and URIs (2)

From: Graham Klyne <GK@NineByNine.org>
Date: Thu, 15 Feb 2001 09:40:43 +0000
Message-Id: <5.0.2.1.2.20010215091902.035fb7a0@joy.songbird.com>
To: "Bill de hOra" <bill@dehora.fsnet.co.uk>
Cc: "RDF interest group" <www-rdf-interest@w3.org>
Bill,

[This is a follow-up to an earlier response]

Ah, herein lies the rub!

A URI may denote a unique _resource_, but is that a unique _entity_.  A 
literal can be a unique resource without always having the same meaning.

In a sense, I think this is a more generalized case of the well-used 
example of todays weather "http://weather.org/Todays-weather/".  This has a 
quite clearly defined "conceptual mapping" [RFC2396] to an non-fixed data 
entity.  Why should one not permit a URI with a conceptual mapping to a 
string of characters that can be used in different contexts to imply 
different actual meanings?

I might define urn:mylexicon:Paris to refer to a lexicon entry containing 
all of the meanings you gave previously;  said URI might then reasonably be 
used as the object of a statement to reference one of those meanings.  I 
think this is an unarguably valid use of RDF (even though you may not like 
the style), which is very similar to the use of a literal "Paris" or a 
corresponding data URI discussed previously.   (I speculate that this could 
be similar to what happens with the DMOZ open directory description in RDF.)

#g
--

At 12:20 AM 2/15/01 +0000, Bill de hOra wrote:
>But there is an algorithm for matching URIs precisely because they are
>considered to be unique entities: literals are not unique. "Green" is not the
>same as "Green" above the syntactic level. It's not safe to convert string
>literals (semantically or pragmatically) into URIs because it's not safe to
>provide machines with algorithms for matching them.
>
>Two identical literals could stand for very different things, albeit the 
>creator
>of the data hasn't made that intent sufficiently obvious to machine. For 
>similar
>reasons, XML now provides namespaces to differentiate between elements and
>attributes that syntactically are identical, but aren't semantically 
>intended to
>be identical.

------------
Graham Klyne
(GK@ACM.ORG)
Received on Thursday, 15 February 2001 05:11:51 GMT

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