Re: A comment on issue-148

Let me feed the debate with my couple of € cents.


The debate is on this sentence:
"IRIs have global scope: Two different appearances of an IRI denote the 
same resource."

There are various ways of interpreting this, but one may be tempted to 
say that it is equivalent to: "A given IRI *always* denotes the same thing."

Of course, when formulated in this way, it is more subject to arguments. 
The word "always" suggests that we are talking about time and changes 
over time. If this is what we mean by "always", then it is clear that 
the formal semantics does not give any credit to such claim, as much as 
it does not pretend it is false. RDF Semantics simply do not say 
anything about time and changes.

But "always" may mean that what an IRI denote is independent of where it 
appears. That is, what an IRI denote does not depend on whether it is in 
subject, predicate or object position of any triple in any graph. This 
is how global scope should be interpreted. And with this notion of 
scope, the spec is correct, no matter what David Booth says.

However, there is another interpretation of "always" here: it can be 
understand as "there is no possible situation under which an IRI could 
denote a thing, while it denotes another thing under a different 
situation". In this case, the statement is false. What an IRI denote is 
subject to interpretation, and therefore, when you change perspective, 
what an IRI denotes may change. But this is taking the notion of scope 
too far.

If I compare this to programming languages, in which it is often 
possible to define variables with global scope (a.k.a., global 
variables), the objection of David Booth would imply that global 
variables do not have global scopes. Indeed, two executions of the same 
programme would assign the "global" variables to different areas of 
memory. A programme that does not use the package having the global 
variable could define the same variable as local or make it global with 
different values. Surely, what the variable refers to depends on the 
context of execution.

But in a specification, there is no reason to extend scope outside the 
borders of a single system (even though the system is distributed and 
open) or outside the borders of a single perspective, or context.

The notion of "scope" that David Booth is using (to justify that a 
single IRI can denote several things) is trans-perspective, or 
trans-context.  In RDF Concepts, the definitions are given assuming one 
perspective.  This is *not* in contradiction with RDF Semantics, even if 
the formal semantics defines an infinite set of interpretations, with 
infinite ways of denoting.  The set of interpretations has to be defined 
because a system does not know what is the one perspective that has to 
be assumed when processing RDF.  But the formalism makes it clear at 
least what perspectives are plausible and which are impossible. Ideally, 
the RDF graph is sufficiently detailed that there is only one possible 
interpretation of the data.

Now the situation can be made more complicated by the fact that there 
are many cases when one wants to reason about several perspectives at 
the same time.  One may want to reason across contexts.  This is fair 
enough, but it is out of the scope of RDF Semantics.  It is, however, 
within the possible scope of RDF Dataset semantics, but this is another 
story.

To continue with the programming comparison, one can write 
meta-programmes that are analysing programmes and their variables. In 
this case, for the meta-programme, each global variables from the 
programmes become local. Similarly, when David Booth assumes IRIs can 
denote several things, he is thinking at a meta-level that RDF Concepts 
does not have to describe.



AZ.

Le 24/10/2013 20:28, Pat Hayes a écrit :
> (Just to get this on the record.)
>
> If we find ourselves debating the pros and cons of this issue, I want
> us to make a clear distinction between two distinct theses, which are
> conflated in the text of the comment.
>
> Thesis 1. In actual practice, a given IRI may be used on the Web to
> refer to two different things. This can happen in a variety of ways,
> including an IRI Collision in the sense described by
> http://www.w3.org/TR/webarch/#URI-collision, but also by IRIs being
> used in RDF with different intended meanings.
>
> Thesis 2. The phenomenon described in Thesis 1 can be usefully
> analyzed using the RDF semantics, by saying that the IRI might refer
> to different things in different graphs.
>
> I agree with (1) but not with (2), for reasons which I can explain to
> anyone who actually wants to know. If the WG accepts the truth of (1)
> it is important that it not do so in a way which implies that it is
> accepting (2) as well.
>
> Pat
>
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>
>
>
>
>

-- 
Antoine Zimmermann
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Received on Friday, 25 October 2013 08:03:50 UTC