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Re: xlxp-dev: RDF and XLink

From: Paul Prescod <paul@prescod.net>
Date: Tue, 13 Apr 1999 20:47:46 -0500
Message-ID: <3713F3C2.F3B463DC@prescod.net>
To: eliot@isogen.com
CC: xlxp-dev@fsc.fujitsu.com, www-rdf-comments@w3.org
Eliot Kimber wrote:
> 
> But this misses the point of why you need an XLink (or a HyTime or an
> HTML) *in addition to* an RDF: they are operating at different levels of
> generality and addressing different detailed applications.

It is clearly the case that the RDF *data model* is more general than the
XLink data model. RDF can make assertions about relationships and also
about literals. (you can emulate literal text in a relationship-only
system by having "text nodes" that exist only to be pointed to once...I
don't think it is the same)

So the relevant questions are:

 a) do we still need XLink or 

 b) can we build XLink on top of RDF or

 c) why not?

That's what I meant to say in the last message.

One confusing issue is that RDF's stated scope is not as wide as its data
model would allow.

> It's like saying that you can get from Austin to Dallas by air so you
> have no need for cars.  Sometimes you use cars, sometimes you use
> planes. They both reflect the same basic data model (vehicle transports
> people) but meet different requirement sets.

So you are arguing that we still need XLink. I'm willing to believe that
but need it substantiated: what requirements do XLink meet that RDF does
not? What does XLink make either possible or easy that RDF makes
impossible or hard?

> 1. RDF is general to the point of uselessness in the absence of a
> specific application. 

We will know whether or not RDF is "general to the point of uselessness"
when we have RDF APIs, query languages, validators, databases and so
forth. In its current state, yes it seems useless but so does XML until
you have XML tools.  XML is also ultra-general but the availability of
parsers, APIs, query languages, validators, databases etc. makes the
generality profitable.

For RDF all we have is "parsers".

> I've heard statements like "it's just tupples".
> Hot tip: So are all data structures. Doesn't help. There must be more to
> it.

Again, I don't see anything wrong with the generality per se. "XML is a
way of encoding trees and links." One could ask "so what?" Well, when you
have DOMs, architectures, XQL, SDQL and XSL it becomes apparent that
unifying the encoding and data model of trees with links buys you some
good stuff.

> 2. The RDF specification is not clear itself about what RDF is: is it a
> syntax? Is it a modeling formalism? Is it an amorphous pile of goo? All
> three? None of the above? Very difficult to tell.

The RDF specification is not explicit because it is hiding behind the
metadata label. Nevertheless it seems to me that RDF is a data model for
logical assertions. I have never researched the kinds of things you get
"for free" with a standardized model for logical assertions but many
people have no idea what you got "for free" from a standardized model for
hyperlinks so I am willing to give the KR people the benefit of the doubt. 

Presumably they've have "assertion query languages", "assertion
databases", "assertion validators" and so forth. I presume that they've
also found those tools to be useful in managing information.

-- 
 Paul Prescod  - ISOGEN Consulting Engineer speaking for only himself
 http://itrc.uwaterloo.ca/~papresco

By lumping computers and televisions together, as if they exerted a 
single malign influence, pessimists have tried to argue that the 
electronic revolution spells the end of the sort of literate culture 
that began with Gutenberg’s press. On several counts, that now seems 
the reverse of the truth.

http://www.economist.com/editorial/freeforall/19-12-98/index_xm0015.html
Received on Wednesday, 14 April 1999 11:53:25 GMT

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