W3C home > Mailing lists > Public > www-rdf-logic@w3.org > November 2001

RE: literals and typing

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Fri, 9 Nov 2001 13:22:24 -0600
Message-Id: <p0510102bb811dbeb8b3f@[65.212.118.147]>
To: "Geoff Chappell" <geoff@sover.net>
Cc: www-rdf-logic@w3.org
>  > -----Original Message-----
>>  From: www-rdf-logic-request@w3.org
>>  [mailto:www-rdf-logic-request@w3.org]On Behalf Of Pat Hayes
>>  Sent: Thursday, November 08, 2001 11:05 PM
>>  To: Geoff Chappell
>>  Cc: www-rdf-logic@w3.org
>>  Subject: Re: literals and typing
>>
>>
>>  >I've been taking a look at the current MT draft and the proposed
>>  >changes for datatyping. I've been trying to understand what it might
>>  >be like to work with from an implementation standpoint. Any MT gurus
>>  >out there want to take a look at the following dump of my
>>  >thinking and shoot it full of holes where appropriate?
>>  > [...]
>>  >It seems that it is the job of the reasoning system to actually
>>  >extract the literal value from the literal
>>
>>  Well....not exactly. I see the job of the RDF reasoning system being
>>  to figure out what datatype scheme is supposed to be associated with
>>  the literal (if that needs figuring out, cf. the options below), and
>>  to check the result for consistency (eg if the literal is said to be
>>  one datatype but the range of the property is a different,
>>  incompatible, type then something is wrong. ) But actually figuring
>>  out the 'real value' of the literal is something else's job;
>>  something that has access to the innards of the datatyping scheme.
>>  For example, I wouldn't expect an RDF engine to know all the details
>>  of XML datatyping, only to be able to consistently record a datatype,
>>  infer it where necessary, and deliver the results to something that
>>  does know the details. As far as RDF is concerned, 'xsd:integer' is
>>  just a URI that gets handed to some engine that knows about
>>  XMLschema, maybe along with a literal that is known to be of that
>>  type.
>
>Are you distinguishing between an RDF reasoning system - that performs only
>logical inference - from, say, an RDF query and inference system that may
>have knowledge of specific datatypes and functions to deal with them?

Yes.

>I'd
>lumped them together and assumed that an RDF reasoning system would have
>knowledge of some subset of datatypes -- for those types, it could deliver
>the real literal value (assuming sufficient and consistent type info); for
>others, it could just return what it knows about the value.

I'd call that something like an RDF+datatype engine.

>  >
>[...]
>>  >Thoughts?
>>
>>  I think I agree with most of the above, modulo above comment. (I
>>  presume that you are taking 'literal' to be synonymous with 'string',
>>  ie a kind of label, a syntactic entity, right? )
>
>hmmm...I guess I'm using 'literal' (lower case) in most cases to mean the
>label on a node that denotes a thing that is not necessarily a string.

Right. What it DENOTES is not necessarily a string. But it itself, 
the literal label in the RDF graph, is a string. (Some proposals want 
us to have non-strings as literal labels)

>I'd
>been thinking that the thing denoted was not type rdfs:Resource - but does
>that have to be the case? are Literals and Resources disjoint under the MT?

No, the MT  tries to be careful to be agnostic on this point. 
DAML+OIL requires they be disjoint, but RDF needn't make that 
commitment.

>is rdfs:Literal the base class of all things not rdfs:Resource?.

No. It is consistent to assume that everything is an rdfs:Resource.

>
>
>>However, I think the
>>  current Big Issue over how best to do datatyping is really about how
>>  'far apart' the literal and its datatyping information is allowed to
>>  be. The chief proposals on the table range from:
>>
>>  X: very close indeed, in fact part of the same label, so a 'literal'
>>  looks like "xsd:integer:10" (Patrick Stickler)
>>
>>  S: pretty damn close, in that the datatype links a bNode representing
>>  the value of the literal to the literal itself, ie one writes (sorry,
>>  I use Ntriples S P O ordering)
>>  aaa eg:prop _:x .
>>  _:x xsd:integer "10" . (Sergey Melnik)
>>
>>  P: arbitrarily far away, provided that RDFS can make the connection,
>>  eg by specifying a property range to be a datatype.  (Me and Peter
>  > Patel-Schneider).
>>
>>  Only the last of these requires the rather elaborate extensions to
>>  the model theory that are being contemplated, and the coreWG is still
>>  debating the options, which is why the new version of the MT is not
>>  yet released.
>
>From my perspective, the only of those that is inline with (the intent of)
>current practice is P. Though it seems that X is perhaps a viable case under
>P - i.e. there would be nothing stopping an individual from declaring under
>P that the range of property <p> is something like xmldatatype and then
>saying  <subject> <p> "<xsd:integer>10</xsd:integer>". The value would be
>opaque to the rdf inference process but could still be processed and be
>meaningful externally. But it doesn't seem to make sense (to me) to have the
>inference process looking within the literal labels.

Well, take a look at the blizzard of discussion on the RDF Core WG archive.

To elaborate on the above, BTW, I just posted a longer comparison:
http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Nov/0295.html

Pat
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Received on Friday, 9 November 2001 14:22:21 UTC

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