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Re: Signal for semantic extensions

From: David Booth <david@dbooth.org>
Date: Fri, 17 May 2013 23:05:04 -0400
Message-ID: <5196EFE0.7050705@dbooth.org>
To: public-rdf-comments <public-rdf-comments@w3.org>
This comment raises an issue that is somewhat theoretical at present.  I 
mentioned it over a year ago (message below) but have not seen any 
discussion about it.  I have not seen it be a problem in practice yet, 
so I do not think it is urgent for the working group to address.  But if 
RDF gains popularity over the coming years, and more semantic extensions 
are introduced, it could become a practical consideration, given the 
long time span between RDF versions.

At present there is no standard way in RDF to unambiguously signal the 
expectation of a particular semantic extension.  I'll explain further 
what I mean, and make a specific proposal.  Perhaps others will think of 
a better way to solve the problem, but hopefully this will at least 
explain what it is.

Suppose an RDF consumer receives a graph written by an RDF author and 
(roughly speaking) the RDF consumer wants to be able to fully 
"understand the author's intended meaning" of that graph.  More 
precisely, the RDF author has used certain semantic extensions that 
imply certain entailments, and wishes to allow consumers of that graph 
to be able to automatically (by machine) determine these entailments. 
In turn, the RDF consumer wishes to be able to compute all of those 
entailments.  Note that this is *not* suggesting that the RDF consumer 
be *required* to compute the RDF author's intended entailments.  It is 
only about *enabling* the RDF consumer to do so if desired.

For semantic extensions that are well known, such as OWL, the RDF 
consumer can detect the presence of well known URIs (such as OWL 
predicates) to know that those well known semantic extensions are 
intended.  But for semantic extensions that are *not* well know -- 
non-standard semantic extensions -- the RDF consumer has no standard 
automatable way to know that certain URIs are intended to signal the use 
of particular semantic extensions.  Thus, the RDF consumer has no 
standard way of determining whether or not he/she/it has computed all of 
the entailments that the RDF author intended to convey.

When the RDF consumer processes an RDF graph, the processor should be 
able to clearly indicate to the user either: "I have computed all of the 
author's intended entailments" or "I cannot compute all of the author's 
intended entailments because I do not have the module for semantic 
extension 'http://example/BobsFavoriteExtension'.  Please load it and 
try again."  But this is only possible if the RDF author has an 
unambiguous standard way to signal the intended semantic extensions.

The motivation for this use case is to enable the vision of the semantic 
web to work, even in the presence of new semantic extensions.  This 
means that: (a) the RDF consumer cannot be expected to have any other 
communication with the RDF author (other than obtaining the graph that 
the author had provided); and (b) the RDF consumer must be able to 
perform these steps automatically (by machine).

I suggest the RDF working group define a standard predicate rdf:requires 
(or whatever name the group chooses) that an RDF author can use to 
indicate that a particular semantic extension is intended.  It could be 
used like this:

  <> rdf:requires <http://example/BobsFavoriteExtension> .

which would indicate that the current document uses semantic extension 
<http://example/BobsFavoriteExtension> .  Hence, to be assured of 
determining all of the document author's intended entailments, the RDF 
processor must understand that semantic extension.

Furthermore, for backward compatibility with OWL, it would be good to 

   owl:imports rdfs:subPropertyOf rdf:semanticExtension .

and recommend that RDF processors also recognize owl:imports as 
signaling a semantic extension.

Again, since I have not yet seen this issue arise in practice, I would 
consider it a low priority to fix, and would not mind if the working 
group decides to defer it to a future RDF version.  On the other hand, 
it is a very easy gap to fix.


On 03/30/2012 06:18 PM, David Booth wrote:
> -------- Forwarded Message --------
> From: David Booth <david@dbooth.org>
> To: Pat Hayes <phayes@ihmc.us>
> Cc: Jonathan A Rees <rees@mumble.net>, Jeni Tennison
> <jeni@jenitennison.com>, www-tag@w3.org List <www-tag@w3.org>
> Subject: Re: The TAG Member's Guide to ISSUE-57 Discussion - F2F reading
> Date: Fri, 30 Mar 2012 18:17:06 -0400
> Hi Pat,
> On Wed, 2012-03-28 at 14:24 -0500, Pat Hayes wrote:
>> FWIW, I am willing to work actively (on- or off-list) with anyone who
>> wants to try reconciling any proposal with the RDF semantics, or just
>> to explore any semantic issues. This is particularly timely as the
>> RDF2 WG is right now debating issues which impinge on the RDF
>> semantics framework, so it would be good to get any pending issues or
>> problems out into the open.
> I would suggest that the RDF WG look at Part 3 "Determining Resource
> Identity" of "Resource Identity and Semantic Extensions: Making Sense of
> Ambiguity":
> http://dbooth.org/2010/ambiguity/paper.html#part3
> That section proposes a standard process for determining resource
> identity.  As far as I know, I did not invent this process.  I simply
> documented what seemed to be the general ideas floating around.
> However, I did identify one specific gap in the RDF specs:
> [[
> At present there is a minor gap in the RDF standards, in that there is
> no standard way for an RDF processor to recognize that a particular URI
> is intended to signal an opaque semantic extension: the knowledge of
> which URIs are intended to signal opaque semantic extensions must be
> externally supplied to the RDF processor.  The RDF processor must
> magically know about them in advance.  It cannot alert the user to the
> need for a new opaque semantic extension that was previously unknown.
> This gap could be addressed by defining a standard predicate, such as
> rdf2:requires, to explicitly indicate when a particular semantic
> extension is required.  However, since it currently seems unlikely that
> many semantic extensions will be needed that cannot be defined using
> standard inference rules, this does not seem like a major gap.
> ]]
> I will forward this message separately to the RDF comments list, since I
> cannot post to the regular RDF list.
Received on Saturday, 18 May 2013 03:05:35 UTC

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