- From: Pat Hayes <phayes@ai.uwf.edu>
- Date: Mon, 4 Feb 2002 13:31:06 -0600
- To: Dave Beckett <dave.beckett@bristol.ac.uk>
- Cc: w3c-rdfcore-wg@w3.org
> >> Agenda request: collections >> (esp. entailment stuff around bags. I have >> an idea I haven't written up yet...) > >+1 > >The MT urgently needs this - collections are deployed. Agreed. Let me summarize my own take on this, as I really think that this is the only rational possibility without making some very fundamental and far-reaching changes in the RDF model. We have containers that have ordered selectors, ie the ordering is built into the very syntax of the selectors. So the containers simply *are* ordered in any RDF interpretation, whether we want to treat that order as significant or not. So we cannot make rdf:bags into unordered entities in the RDF model theory. For more on this see (Sorry Im off-web so cannot give URIs) the thread "Bags (was: closing semantic issues)" starting 7 Nov 01. Key excerpts: <excerpt> --------------------- DanC: > _:b rdf:type rdf:Bag. > _:b rdf:_1 "abc". > _:b rdf:_2 "def". > >entails > > _:b rdf:type rdf:Bag. > _:b rdf:_2 "abc". > _:b rdf:_1 "def". > >and generally: the _n arcs of a bag can be permuted. PatH: That doesn't work. If it entails that, then it must also entail: _:b rdf:type rdf:Bag. _:b rdf:_1 "abc". _:b rdf:_2 "def". _:b rdf:_2 "abc". _:b rdf:_1 "def". so just saying 'can be permuted' isn't enough. The fact is that we really shouldn't have indexed selectors on things that are supposed to be bags, since once you use those selectors once, the bag is ordered, whether you like it or not. You can only 'permute' it if you have state, and RDF has no states to permute over. Monotonic inference only adds conclusions, it can't do things like permutation. Graham: > >I was thinking that a bag represented using a list-type expression >might have the same problem: If you want it actually BE a bag, right, it will break. It cant be both a list and a bag. (It could be a list pretending to be a bag, though. Two different lists can pretend to be same bag, of course.:-) --------------------- Pat H: I've always understood the idea to be that RDF *describes* containers, rather than implementing them or exemplifying them. So it is OK for RDF to keep a 'private' ordering and basically treat all containers as sequences, but if it describes a container as being an rdf:Bag then it is saying that this ordering isn't supposed to be taken to be 'significant' in some sense. (I think this is close to what the M&S says.) But the ordering is still *there* in the RDF. After all, this is exactly how LISP would handle bags: you would encode them as lists and provide bag-access functions that pretended that the list ordering didn't matter, by doing things like checking for permutations under the hood. RDF doesn't implement this kind of thing, of course, but then that's not its role; it just has to carry information around without damaging it. If it delivers all the members of a container and the container type, that's all it needs to be able to do, right? Something else can figure out what to do with it from there on out. ---------------------- </excerpts> I guess that sums it up. We cannot possibly (1) use ordered selectors to access elements of an unordered container, (2) use monotonic inference, and also (3) expect that the containers will be identical when the ordering is permuted, all at the same time. We have to give up on one of these. I suggest that (2) is more important than containers, that (1) is now a given, since this syntax is deployed even if it is brain-damaged, so we have to give up on (3). And also that this is not such a bad idea, if we document it properly. It amounts to saying that RDF really treats all containers in the same way (which it does) but is able to record - not use, but record - the information needed by any external applications that may worry about whether two containers are or are not distinct. Thats the difference between saying that container is a bag or a sequence: not that it 'acts like' a bag in RDF reasoning. If this is acceptable then it is trivial to add rdf:seq and rdf:bag to the MT. I've already written the relevant section. (Adding rdf:Alt to the MT is something else altogether: I would suggest that we simply abandon rdf:Alt.) Pat -- --------------------------------------------------------------------- IHMC (850)434 8903 home 40 South Alcaniz St. (850)202 4416 office Pensacola, FL 32501 (850)202 4440 fax phayes@ai.uwf.edu http://www.coginst.uwf.edu/~phayes
Received on Monday, 4 February 2002 14:30:50 UTC