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Re: datatypes and MT

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Fri, 02 Nov 2001 11:54:25 +0000
Message-ID: <3BE28971.7030803@hplb.hpl.hp.com>
To: Pat Hayes <phayes@ai.uwf.edu>
CC: w3c-rdfcore-wg@w3.org, pfps@research.bell-labs.com


Pat Hayes wrote:

[...]


> 2. Bnodes as literal values.
> 
> Let me put together here under one heading a variety of variations of 
> the following theme: that occurrences of literals in value position in a 
> triple should be understood as an abbreviation of a pair of triples with 
> an 'intermediate' bnode, where the bnode denotes the triple value and 
> the literal label itself is relegated to a minor role of somehow 
> 'illustrating' how that value could be written in some notation. For 
> example: rewrite
> 
> aaa bbb lit .
> 
> as
> aaa bbb _:1 .
> _:1 rdf:value lit .
> 
> Before analyzing this, I note that it has the advantage of putting a 
> node that denotes the literal value in subject position, where other 
> properties (such as rdf:type) can be asserted of it. This is indeed a 
> major advantage - I think the *only* advantage - of this proposal, but 
> we could render this moot simply by declaring that RDF shall allow 
> literals in subject position. I will return to this point later.
> 
> The first of these triples seems easy to interpret: it means exactly 
> what the original triple meant, in fact. It's the second triple that 
> seems rather odd. Since _:1 is supposed to denote the literal value, it 
> would seem to have the same value at both ends.


That is not how I would read it.  The blunt end denotes a value from the value 
space of whatever type of literal we have, e.g. an integer.  The sharp end 
denotes a string.  The range of rdf:value would be xsd:string or some such, and 
this fact would likely be built into processors.


> 
> Another way is to insist that the literal label in the second triple is 
> treated in a nonstandard way: rather than denoting a literal value, it 
> is being mentioned rather than used; it simply indicates the literal 
> itself. (Or, equivalently, all literals are treated as strings.) The 
> intuitive meaning of rdf:value is then a kind of inversion of the 
> denotation mapping itself: it assigns a literal label to the literal 
> value that is the semantic value of that label in the given 
> interpretation. This seems to be what Sergey has in mind, and it is also 
> I think what Dan C. suggested a while back as the best way to handle 
> literals.
> 
> Now, this seems to me to have a fatal flaw, which arises from the fact 
> that the value spaces of two different datatypes might overlap. For 
> example, suppose that there are datatypes xxd:octal and xxd:decimal, 
> then the following would seem to be perfectly true:
> 
> _:1 rdf:type xxd:octal
> _:1 rdf:type xxd:decimal
> _:1 rdf:value "32"
> _:1 rdf:value "26"


But that is not how Sergey would write it.  He is proposing:

   _:1 xxd:octal   "32" .
   _:1 xxd:decimal "26" .

[...]

Brian
Received on Friday, 2 November 2001 06:59:13 EST

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