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Re: Web Semantics of Datasets (v0.2)

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 10 Oct 2011 15:19:59 -0500
Cc: public-rdf-wg <public-rdf-wg@w3.org>
Message-Id: <78ABBEEE-F9E5-406C-AFCB-D206A81FF008@ihmc.us>
To: Sandro Hawke <sandro@w3.org>

On Oct 10, 2011, at 6:30 AM, Sandro Hawke wrote:

> Here's some revised wording for the proposal, getting a bit closer to
> spec text.   It's still somewhat informal, and mixing normative and
> non-normative bits, and best-practice.   And it's not as clear as it
> should be about handling change over time.
> 
>    -- Sandro
> ===
>  A dataset D is true

You have to say, true in an interpretation, and modify the rest of the phrases accordingly.

> iff (1) its default graph is true and (2) for
>  every pair of <N,G> in D, N names something (a "resource", sometimes
>  called a "g-box") which, at every time T in R, has G as its current
>  state.

What is R? 

> 
>  It follows from AWWW that if N is an IRI which can be dereferenced,
>  a successful, correct dereference of N at any time T in R must yield
>  a serialization ("representation") of G.

No, that does not follow. There is no immediate connection between being a name of something and dereferencing to something. If we want this relationship, we have to state it explicitly. Also, you have stated this using a modality ("can be") which opens up a huge can of worms. (What if it could be dereferenced but never, in fact, was? Did it name the graph under those circumstances?) I would strongly suggest that we avoid this kind of language.

> 
>  In order to know whether a dereference occurs at a time in R, it is
>  useful to have R declared in the default graph of D, or in another
>  nearby, easy-to-find data source.  Where possible, is is helpful to
>  have R be All Time; that is, having N name a resource whose state,
>  by definition, never changes.

Why is that the same condition? Surely this begs the question of whether or not G can change. (?)

> 
>  In RDF data, N may be used (1) directly, to name the g-box,
>  expressing things like the license that applies to its state, or who
>  controls it; and (2) indirectly, to refer to G as the current state
>  of the g-box.  

How are these two uses distinguished? There is currently no provision for this kind of indirection in the RDF semantics. 

> Indirect reference can be used to express things
>  about an RDF Graph (a "g-snap"), like that it was the graph some
>  entity asserted at some time.  Indirection is done in the semantics
>  of the predicates with which N is used.

?? It is? How? 
> 
>  When N is used indirectly, the reference to G only holds inside time
>  range R, of course.  Care must be taken not to use N as if it
>  necessarily referred to G, outside of R.  Since R is defined to be
>  the same for all elements of D, indirect reference is safe in the
>  default graph.   

How does time-dependent reference work in the semantics? (Or is that my job to figure out? If so, I can tell you now it will be a major change, which will require rewriting the OWL and RIF standards, among others.)

Pat

> 
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Received on Monday, 10 October 2011 20:20:30 GMT

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