- From: Dan Connolly <connolly@w3.org>
- Date: Tue, 04 Sep 2001 19:42:50 -0500
- To: pat hayes <phayes@ai.uwf.edu>
- CC: w3c-rdfcore-wg@w3.org
pat hayes wrote: > > >pat hayes wrote: > > > > > > >Jan Grant wrote: > > > >[...] > > > > > While I have held, in principle, what I'd characterise as > >DanC's opinion > > > > > here (or the more extreme version: "alt is totally broken") > > > > > > > >I'm not saying it's broken; I'm just saying it's not magic. > > > >It's very mundane; from the MT perspective, > > > >it means no more or less than any > > > >other class (Apple, Bananna, Integer, ...). > > > > > > Then why bother even mentioning it in the M&S, let along spending > > > pages on it? > > > >For the same reason that C programming books mention printf(). > >Yes, everybody could develop their own print routine > >in vanilla C, but it's cost-effective for the community > >to agree on a standard library of terms sometimes. > > Well then let us refer to it in that way, ie as part of a 'standard > library', rather than as part of the core language itself. Very well. > Right now, > as I understand it, an RDF engine that did not handle containers in a > particular way would not be in conformance to the M&S spec. Nor would a C compiler whose runtime didn't include printf() conform to the ANSI C spec. The special handling is in the surface syntax only; i.e. <rdf:_1> can be abbreviated <rdf:li>. > >The widespread availability of printf() does not impact > >the syntax and semantics of the C programming language > >at all. It does, however, increase its utility significantly. [...] > >At its core, the language has *neither* construct. > >It has only > > -- logical constants (URIs) > > -- existentially quantified variables > > -- two-place predicates > > -- conjunction > > > >That's it. > > > >Bag and Alt are just two logical constants. > > That is a point of view, but it does not seem to be the one expressed > in the M&S. I think it is in there, though sorta hidden. I hope the clarified M&S spec makes this view more clear. But maybe others disagree; I dunno. > I have no axe to grind here: I would be happy to banish containers > and reification from RDF, myself. But I would like us to be clear on > the matter. If they are part of the language, with an intended > meaning, then the model theory - the definitive semantics of the > language - should define that meaning as far as possible. If they are > not part of the language then let's remove them from the language > (and maybe have an explicit status for them as part of a 'library' of > handy constructions). If they are part of the language but have no > defined meaning, then let us say so, and be ready to smile fixedly at > the mockery that would then be our due. I like the library idea. > > > It's > > > not a question of 'magic', but of understanding why there would be a > > > totally meaningless distinction built into the syntax. > > > >Just because the distinction is not specified in a model > >theory doesn't make it meaningless. > > I disagree. If the distinction is part of the syntax of the langauge > specification, and it has an intended meaning, then that meaning > should be reflected in the model theory. Otherwise the model theory > is not a semantic theory of the language, in which case it is > effectively useless. If a parser recognises a construct and treats it > specially, then it ought to be at least mentioned in any semantic > specification (if only to say that the semantics assigns it no > particular special interpretation.) I'm happy with a "no special interpretation" notice. > >The distinction > >between Apples and Bananas is not in the model theory, > >but there is a distinction: in many interpretations, > >their extensions are different. > > Right, and the MT indeed shows what that claim means. It also, > however, shows that one cannot actually state the distinction in RDF. > But one can state the distinction between Bag and Alt, apparently; in RDF? I'd be very surprised to learn that the distinction between Bag and ALT can be expressed in RDF 1.0. > so > what does *that* mean, it seems reasonable to ask? I think I'm lost at this point. > > > This line amounts to treating all containers alike in the MT, > > > >exactly; which, I think, is why you were actioned to remove > >containers from the MT altogether. > > That was not my understanding. > > Can anyone clarify this point? Was there a consensus that the MT > treatment of containers was wrong or inadequate? Perhaps I overstated the case; I don't think there was any decision of that sort... > My understanding was simply that the action to remove containers and > reification from the version of the MT in the working paper was > because it was felt that there were open issues at this stage; That much is so... > but > that once these issues were resolved, that the model theory could be > extended to the entire language. Is that understanding incorrect? I don't think anyone can say for certain yet. > (Or is the point that there is no *need* to mention containers > explicitly, since if this is all they amount to, then they have no > extra semantics over and above that which arises from the core RDF > meaning of rdf:_n in any case? That's my understanding. > If so, I would suggest that it would > do no harm to mention them in the model theory document, if only to > emphasise that the MT makes only this minimal assumption, and does > not mandate any of the 'distributive' meanings discussed at such > length in the M&S. ) That's fine with me. > > > ie they > > > are thingies that have elements which are accessed by applying rdf:_n > > > to them, and that's all. > > > >Yes! > > OK, but we can put that in the model theory. In fact it is there now, > without the 'and that's all'. It really amounts simply to some domain > restrictions on rdf:Bag, rdf:Alt and rdf:Seq (ie that they are > disjoint), but that isn't otherwise expressible in RDF, so it is a > genuine extension to the language. Good point. > (There is also the business of > there being a default first member of any alt, but I can't handle > defaults in the MT, I confess.) Yeah; who knows what to do with that. [...] > >Just not in the core model theory. > > But why on earth not do so, if we can do it? OK, to a certain extent, a treatment of containers is probably in order, eventually. -- Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Tuesday, 4 September 2001 20:42:53 UTC