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

Re: universal languages

From: Dan Connolly <connolly@w3.org>
Date: Fri, 02 Feb 2001 17:17:33 -0600
Message-ID: <3A7B400D.9500CF40@w3.org>
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
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.


>    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

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