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meaning of rif:External (was Re: Diatribe on why rif:iri consts should be left alone)

From: Sandro Hawke <sandro@w3.org>
Date: Fri, 10 Apr 2009 11:21:49 -0400
To: Jos de Bruijn <debruijn@inf.unibz.it>
cc: Dave Reynolds <der@hplb.hpl.hp.com>, kifer@cs.sunysb.edu, RIF WG Public list <public-rif-wg@w3.org>
Message-ID: <19995.1239376909@ubehebe>

> Dave Reynolds wrote:
> > Surely the extensibility argument could equally well be applied to the
> > predicates and functions in DTB. Those are denoted by rif:iris and we
> > are giving them a fixed interpretation, at least as externals.
> 
> It is precisely because of the extensibility issue that we require
> External() to be written around external predicates and functions. If
> the name, say, func:string-join is used outside External(), it is simply
> an uninterpreted symbol, and DTB has nothing to say about it.

This makes me suspect we have some different ideas about how the
Semantic Web is supposed to work.  Let me try to state my understanding,
and then return to External().  I don't actually understand how this
relates to ISSUE-93, so I don't talk about that here.

I think the key architectural point about IRIs is that whenever you use
one, you are automatically (to some extent) subscribing to some external
semantics.  They might not be written down yet, or they might be written
only in natural language.  They might even change, possibly in
uncontrolled, hostile ways.  When you chose to use an IRI, you have to
chose carefully.  Generally you either pick one in your own namespace or
you pick one in the namespace of an organization you trust to maintain
both the web content and the community usage (eg W3C).

The analogy to the HTML web holds here; when you make a link out of a
web page you care about, you have to think carefully about where you are
linking to.  What happens to your reputation, your services, and your
users if that site suddenly turns into something offensive or dangerous
(eg a phishing site)? 

It's also not a good plan to have one IRI refer to two different things,
such that you can only tell which it refers to by context.  In what you
say above, what happens if someone provides documentation in metadata
about a predicate that is used in both an external and a non-external
context?  Which predicate does the documentaiton describe?  In my mind,
it's still about both, because they're actually the same thing.  I don't
see External as changing what the IRI denotes, but as signalling the
consumer that it should know how to reduce the enclosed expression to a
Const (for terms) or true/false (for formulas).

I suspect that's not actually the meaning given to External in BLD in
the LC1 draft, but in the press to get to LC, I figured we had about the
same idea about how External worked and we could hammer out technical
differences later when we started having test cases, implementations,
etc.   

With the insight of this conversation in mind, I suggest we rename
External to Evaluated.  It's not clear to me that it should appear in
the model theoretic semantics; it seems more like a pragmatic check for
consumers, allowing them to reason more effectively and give appropriate
errors when they need to.

On Michael's point about Extensibility in [1], it's not the dialects
that give meaning to IRIs, it's the IRI's owner (in some sort of
collaboration with the rest of the world).  So any given predicate IRI
or function IRI conceptually means the same thing in every RIF
document, it's just that in some dialects we can assume consumers know
that meaning and in others we can't.  Arbitrary use of the IRI doesn't
signal an assumption that its meaning is known, but use inside
External/Evaluated does.

    -- Sandro

[1] http://lists.w3.org/Archives/Public/public-rif-wg/2009Apr/0029.html
Received on Friday, 10 April 2009 15:22:00 GMT

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