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Re: addressing requirements around daml:collection (rdfms-seq-representation)

From: Graham Klyne <Graham.Klyne@MIMEsweeper.com>
Date: Sat, 20 Apr 2002 17:28:59 +0100
Message-Id: <>
To: Dan Connolly <connolly@w3.org>
Cc: w3c-rdfcore-wg@w3.org, Lynn Andrea Stein <las@olin.edu>
The question I'm asking myself is:  how does this pan out for RDF systems 
that treat parseType='collection' as parseType='literal'?

I thought it might be kind-of cool if datatyping could be used in such a 
way that a collection "literal" denotes a list... but we can't do that with 
the current approach.

But, on the other hand, treating the list as a literal may mean that some 
intended inferences are not available, but I don't see that it would 
sanction any invalid or unintended inferences [**] -- so maybe it's a 
"benign" incompatibility?


[**] My rationale is that two different lists will always be two different 
literals.  These will not be related by any properties that are defined to 
relate list values.  The case when two literals are equal would also have 
the corresponding collections being equal.  Hmmm.. but we *might* have 
properties that relate different literals and not the corresponding 
collections:  under that circumstance, we would have unintended truths.  Is 
this marginal enough not to be a serious problem?  Maybe so:  I can't think 
of *any* RDF properties that relate XML literals.

At 12:54 PM 4/19/02 -0500, Dan Connolly wrote:
>I picked up the ball on this 28 Mar:
>"Volunteers: DanC, Lynn Stein (some timing issues need to be resolved),
>De Roo,
>Will participate in RDF core working group discussion."
>I got some inspiration from today's RDF Core telcon
>(and in discussion that followed after the formal
>telcon was adjourned).
>In case Lynn finds time to participate, I'm also summarizing
>the history here. (Brian, maybe you could link this
>message or some of the stuff I cite here from the
>rdfms-seq-representation issue)
>daml:collection is specified in:
>   section 3. rdf:parseType="daml:collection"
>   DAML+OIL (March 2001) Reference Description
>   W3C Note 18 December 2001
>   http://www.w3.org/TR/2001/NOTE-daml+oil-reference-20011218#3
>Here's the hello-world example:
>   <daml:oneOf parseType="daml:collection">
>     <daml:Thing rdf:about="#Eurasia"/>
>     <daml:Thing rdf:about="#Africa"/>
>     <daml:Thing rdf:about="#North_America"/>
>     <daml:Thing rdf:about="#South_America "/>
>     <daml:Thing rdf:about="#Australia"/>
>     <daml:Thing rdf:about="#Antarctica"/>
>   </oneOf>
>Now RDF Core has decided[26Feb] that using parseType as an
>extension mechanism doesn't really work; M&S
>says daml:collection should be treated like
>parseType="Literal" which is different
>from -- inconsistent with, in some cases -- expanding
>it to first/rest.
>[26Feb] http://www.w3.org/2001/sw/RDFCore/20020225-f2f/#d-2002-02-26-1
>Where to go next? WebOnt clearly needs closed containers.
>[I should be able to derive that from the requirements
>document; I hope you don't mind if I pass over that for now...]
>This seems pretty closely related to
>   http://www.w3.org/2000/03/rdf-tracking/#rdfms-seq-representation
>which we decided to close by punting/postponing
>[oops; didn't we? I see it's "currently: for discussion";
>ah... perhaps I'm confusing it with...
>which is closed, as of 15Feb.]
>RDF/xml has special syntax for containers, but experience
>trying to exploit it to come to intuitive conclusions
>has exposed problems. Take the class above... say
>Continent is the subject of that oneOf property.
>If we know
>         ex:Bob daml:differentIndividualFrom ex:Eurasia.
>         ex:Bob daml:differentIndividualFrom ex:Africa.
>         ex:Bob daml:differentIndividualFrom ex:North_America.
>         ex:Bob daml:differentIndividualFrom ex:South_America.
>         ex:Bob daml:differentIndividualFrom ex:Australia.
>         ex:Bob daml:differentIndividualFrom ex:Antarctica.
>         ex:NotContinent daml:complementOf ex:Continent.
>then we should be able to conclude
>         ex:Bob rdf:type ex:NotContinent.
>Now this works perfectly well* when the oneOf claim
>is spelled out long-hand using first/rest/nil.
>*[I should back this claim with running code,
>but in the interest of getting this and other
>stuff done today, I'm skipping that for now.
>I think Jos has the relevant code running.]
>But RDF/xml's collection syntax expands to these
>rdf:_1 rdf:_2 etc. triples, which don't say
>enough to come to the relevant conclusion
>without doing closed-world stuff.
>So there have been some proposals for closing
>RDF collections [apologies for not crediting
>the sources; I did a little surfing and
>couldn't find them]:
>* mint a new "end-marker" term; nothing occurs
>after the end-marker in a collection.
>I don't like that because you can't have a
>collection containing the end-marker.
>And it grates against my design intuitions generally.
>* add a "count" property to bags.
>Exploiting a count property involves all sorts
>of equality and integer processing that seem
>like overkill and/or intractible inferencing.
>* add a 'highest index' property to bags:
>it tells you the highest index that's used
>to relate a collection to one of its members.
>This is only slightly better than a "count" property,
>to my mind.
>None of those seems worth doing to me.
>Now we come to the discussion in today's telcon...
>In Amsterdam, WebOnt seems to have come to the
>conclusion that RDF/xml is an acceptable transfer
>syntax, but it shouldn't be regarded as user interface;
>authoring tools and "presentation syntaxes" are how
>most folks should see WebOnt's language.
>So maybe it's OK to just spell out WebOnt constructs
>like oneOf longhand, using first/rest/nil
>in normal RDF/xml syntax, even though it's
>not pretty. [I think PeterPS
>sent one of these monsters to WebOnt a while
>ago... I can't find it in a few minutes
>of surfing.]
>Let's call that the first proposal... I dub it 'longhand-OK'.
>But Jeremy pretty much talked me out of that:
>daml:collection is a pretty mature mechanism
>with considerable implementation experience
>behind it, and to tell the users "nope; it's
>gone. Deal." is quite a step backwards.
>So the next proposal is:
>   * add parseType="collection" to RDF/xml;
>   * add rdf:first, rdf:rest, rdf:nil too.
>   * specify that parseType="collection"
>   is notation for first/rest/nil triples
>   ala daml:collection.
>Let's call that parse-collection-STD,
>i.e. standardize this way of parsing collections.
>[I'm leaving an equivalent of daml:item
>out of the proposal; it relates to
>rdf:first by inference rules, which don't
>belong in the syntax spec; I could
>consider rdfs:item ala rdfs:member...
>or even using rdfs:member as the
>analog of daml:item... but I'm
>not proposing that just now.]
>Now... let's look at some costs/benefits:
>+ longhand-OK is almost no work for RDFCore
>- longhand-OK is kinda rude to the folks
>   who are using DAML+OIL and considering
>   WebOnt; it's not clear that it'll be
>   acceptable to the WebOnt WG, let alone
>   the (potential) WebOnt user community.
>+ parse-collection-STD recognizes the implementation
>   experience in the DAML+OIL community
>- parse-collection-STD raises the bar for the general
>   RDF implementation community, who has been told
>   that we're just working out bugs, and is not
>   expecting us to demand more features of them.
>   (does anybody in the WG know of anybody in
>    the general RDF implementation community
>    who hasn't already added support for daml:collection?)
>- pase-collection-STD introduces more ways to
>   write collections; this is more work for our
>   primer, more work for RDF users deciding which
>   way to go, etc.
>- parse-collection-STD puts more work in the
>   critical path for getting the RDF/xml syntax
>   spec to last call.
>- longhand-OK introduces risk that RDF/xml
>   syntax spec won't successfully exit last
>   call, cuz WebOnt WG (or DAML+OIL users)
>   will object.
>On balance, I prefer parse-collection-STD right now.
>But I need to sleep on it or something to be sure.
>Other thoughts? Lynn? Jos? DaveB?
>Appendix: some stuff I found while researching
>this message. I thought I'd cite them in
>the message, but they didn't turn out to be
>directly relevant. But rather than throwing
>them away, I think I'll append them.
>Entailment and bags (was:Re: Agenda items for the f2f)
>From: Pat Hayes (phayes@ai.uwf.edu)
>Date: Mon, Feb 04 2002
>a theory of rdf:Bags
>From: Dan Connolly (connolly@w3.org)
>Date: Sun, Feb 17 2002
>hm... what ever became of "ACTION: 2002-02-15#4 PatH: Send a few
>paragraphs to the list to address this" --
>that action isn't mentioned in the next week's minutes.
>Dan Connolly, W3C http://www.w3.org/People/Connolly/

Graham Klyne
Received on Sunday, 21 April 2002 15:37:40 UTC

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