- From: Dan Connolly <connolly@w3.org>
- Date: Fri, 02 Feb 2001 17:17:33 -0600
- To: Drew McDermott <drew.mcdermott@yale.edu>
- CC: www-rdf-logic@w3.org
Drew McDermott wrote: [...] > Let me be more constructive. Yes, this is good stuff... > Here are some issues we could talk about > instead of triples. In each case, when I ask "Should the language > have X?" what I probably mean is "Should there be a subset of the > language that allows X?" > > > Should the language have types? I think the answer is a strong Yes, > but many AI languages have used sets instead. The difference is that > types behave more like a syntactic restriction on variables and > predicates, whereas sets are objects in the domain. er... the PCC/PCA work http://www.w3.org/DesignIssues/Logic#PCA shows that stuff I want to do can be done with typed lambda calcluclus. I'm guessing, but I suppose that's sorta what you mean by types. I have no idea whether the same stuff can be done without types. Folks have been asking about model-theoretic semantics for RDF; when considered as a cut-down FOPC, that's easy enough to do in terms of regular old ZF-set theory and such. I went looking for model-theoretic semantics for typed lambda calculus and found hairballs. Is that what you mean by sets? i.e. sets in the model theory? Or sets ala RDF Classes? > > What sorts of quantifiers are allowed? Do lambda-expressions appear > as explicit constructs? Again, they do in the proof-checking-based work that I've seen that looks promising. But again, I don't know that we can't do the relevant stuff without them. > > Should the language have functions? They are often very handy. In > PDDL, for instance, we have recently added functions that denote > fluents, so that volume_in(tank3) might denote the time-varying amount of > fluid in tank3. PDDL? pointer? > > Is the language extensional or intensional? [...] Er... I'll have to read that 3 or 4 more times to see if I understand it. [...] > Some of the participants in a the discussion assume that > intensionality = quotation. It's true that quotation is one way to > implement intensionality, but it's not the only way. The other is > just to use possible-world semantics. pointer? [...] > > As bad as RDF might seem, all the other languages I've > seen in this design space are worse. Either (a) they don't > connect to URI space, so why bother, or (b) they're > even more baroque than RDF. Check out XMI, for example. > http://www.oasis-open.org/cover/xmi.html > http://www.google.com/search?q=XMI > > I took a glance at it. (Not easy; there are a lot of stale links > around XMI; is it dead?) I'm not sure what makes XMI more baroque > than RDF. It looks like they just took UML and XML-ified it. That's > more or less what I'm advocating with respect to knowledge > representation. XMI doesn't look baroque to you? Hmm... maybe I better look at it again. From that perspective, one might say we just took semantic networks and XML-ified it. But there were some other issues... like embedding in HTML... that influenced the syntax of RDF. > > (Step 2 should take > > about 15 minutes.) > > Hah! I think you display a lack of experience in > consensus building. > > Not a complete lack. I realize that there can be infinite discussion > over trivial syntactic issues. The way to settle them is to appoint a > syntax czar whose decisions are final. Hmm... I haven't been able to pull of anything like that in the Web community for some time; no one person is trusted that much any more. Sigh. > > What exactly is the role RDF plays in all this? > > It's a little teeny formal system based on URIs and XML: > two place predicates only, conjunctions and existentials but no > negation, disjunction, universals, etc. > > The problem is that RDF is sometimes the small language we plan to > use, and other times it's just the medium in which the actual language > is going to be written. What's driving me a little crazy is that its > advocates switch back and forth between these two positions. OK, yes, your point is well-made. But (a) people do change their minds. I certainly do. and (b) I hope you're not expecting all RDF advocates to speak with one voice. We're not quite there yet. > When its > inadequacies as a language are pointed out, the RDFites say, "We can > implement any language we like." But then the next day they're using > RDF as the actual language again. > > I don't understand the power of "triples." Doesn't XML already have > "triples"? If I say > > <foo u="x"> > <baz v="y"/> > </foo> > > isn't there a triple x-baz-y? The nodes in that triple aren't URIs. > What's the big deal? Well... triples look like an important primitive when your (my) background is in the Web, where links have two ends and a type. The subject-verb-object thing seems to be an important primitive in human communication (cf Chomsky). RFC822 header fields have the same property/value structure, with an implicit subject. Likewise library catalog cards. OOP programming uses object.prop = value all the time. Triples are an idiom that show up all over the place, in my experience. They look like a pretty important and useful modelling primitive. I think the object/property/value will continue to be an important idiom for communication with users, whether or not it continues to be The Ultimate Primitive in a universal language for the web. > I feel like I > must be in the presence of a mystery on a par with the Holy Trinity; > mere mortals can only genuflect, not understand. > > Let me repeat the problem: If RDF is just a mechanism for describing > the syntax of some other language, then it's irrelevant. Not totally; you have to have some syntax to put in email and to put into and out of tools. There's a certain investment in software and wetware in RDF. Mabye not indispensable, but not irrelevant either. > If it is the > actual language, then it's inadequate. Yes, indeed; that's generally the case with releases with .0 on the end, no? ;-) > -- Drew -- Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Friday, 2 February 2001 18:17:37 UTC