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

RE: literals and typing

From: Geoff Chappell <geoff@sover.net>
Date: Fri, 9 Nov 2001 07:43:19 -0500
To: "Pat Hayes" <phayes@ai.uwf.edu>
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? 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.

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

> Pat

Thanks for the feedback.

Received on Friday, 9 November 2001 09:19:39 UTC

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