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Re: [OWLWG-COMMENT] ISSUE-67 (reification): real semantic-free RDF-comments

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 27 Nov 2007 15:48:52 -0600
Message-Id: <p06230903c3723d1e49fe@[]>
To: "Michael Schneider" <schneid@fzi.de>
Cc: "Owl Dev" <public-owl-dev@w3.org>, "Ian Horrocks" <horrocks@cs.man.ac.uk>, "Jeremy Carroll" <jjc@hpl.hp.com>, <bmotik@cs.man.ac.uk>

>[Comment to OWL-WG discussion; posted to involved WG-members]
>Hi, Ian and Jeremy!
>Just a small speculative comment on Jeremy's idea to use XML comments for
>representing real semantic-free comments in OWL's RDF syntax:
>   http://lists.w3.org/Archives/Public/public-owl-wg/2007Nov/0461.html
>Ian Horrocks answered to Jeremy Carroll:
>>>  More constructively, what I am hearing, I think, is that the 
>>>  requirement is for comments that have no semantics and just fit 
>>>  into the specification in the right way.
>>>  In RDF/XML there has always been the capability to have such 
>>>  comments - they look like:
>>>  <!--
>>>   This is a comment, it has no bearing on the formal semantics of 
>>>  the document.
>>>  -->
>>>  It may be possible to provide say, an informative GRDDL transform 
>>>  from an XML version of the axioms, to RDF/XML, and back again, that 
>>>  round trips comments appropriately.
>>  As you probably recall, this was discussed and dismissed in the 
>>  WebOnt working group for the reason that it is much too low-level (it 
>>  is a feature of XML), and that such comments may be lost when 
>>  documents are processed. What you propose w.r.t. GRDDL sounds more 
>>  like a hack than a realistic solution. Moreover, this mechanism would 
>>  hardly satisfy the requirement to have a more comprehensive framework 
>>  that allowed (at least) for annotating axioms as well as entities.
>What comes to my mind is what's actually missing are "RDF comments", i.e.
>comments which belong to an RDF graph, but which are not interpreted (all
>triples within an RDF graph are always interpreted according to RDF

Webont/RDFWG discussed this idea. They were 
referred to as "dark triples", ie RDF triples 
which are not required to conform to the RDF 
semantics. (They have several other potential 
uses in addition to commenting: at one time it 
was thought that they were required to allow OWL 
to be encoded into RDF.) However, they raise many 
other issues, most importantly being how an RDF 
processor is supposed to be able to distinguish 
them from ordinary triples. To me, the most 
cogent argument against this idea is that it 
doesn't seem to actually be necessary. Suppose it 
were somehow arranged, and then someone 
accidentally thought that a dark triple were a 
real RDF triple. What harm would be done? Since 
RDF is so weak, I suggest that the answer is: no 
harm at all. The only thing that would follow 
from (be RDF-entailed by) such a dark/comment 
triple would be that the comment exists (i.e. the 
same triple with the comment replaced by a 
bnode); and since the comment is presumably a 
string, this seems a harmless entailment in any 
conceivable circumstance.

>I am thinking about comments for URI resources, data values,
>bNodes(?), whole triples, and possibly even subgraphs.

To do subgraphs, it is necessary to somehow refer 
to or indicate the subgraph. This seems to 
require either a complete revision of the basic 
RDF graph model, or using named graphs. Ian's 
point above about axioms as well as entities is 
relevant here, of course.

>  This would make it
>possible to really map both entity and axiom annotations in the Functional
>Syntax to semantic-free comments in RDF. Of course, for every RDF
>serialization language there would also have to be some specific
>serialization for comments, but this would be an RDF issue, and would be
>completely transparent to OWL.
>Well, as I said, this is a very speculative discussion, because there are no
>such RDF comments at the moment, and so this does not help us any further in
>the current situation. But should RDF get its own "RDF-1.1"-WG in some near
>future, this might be a new feature to be considered. And then, perhaps,
>already the next OWL-WG could use this new feature for creating a proper
>RDF-mapping of real semantic-free comments (and perhaps deprecate the
>annotation mapping which comes out from the current WG :-)).
>Ah, and please don't ask me about any technical details of such an RDF
>comment feature!

Well, but it might be worth thinking about it. It 
is rather a tar-pit. Is it *really* this 
important to avoid semantics? A commenting triple 
asserts... something (and that a character string 
exists, but we knew that already.) What, exactly, 
is not specified. So, is this really so dangerous 
that we need to revise the world to avoid it?


>All the comments in this post were just... speculative. ;-)
>Dipl.-Inform. Michael Schneider
>FZI Forschungszentrum Informatik Karlsruhe
>Abtl. Information Process Engineering (IPE)
>Tel  : +49-721-9654-726
>Fax  : +49-721-9654-727
>Email: Michael.Schneider@fzi.de
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>FZI Forschungszentrum Informatik an der Universität Karlsruhe
>Haid-und-Neu-Str. 10-14, D-76131 Karlsruhe
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Received on Tuesday, 27 November 2007 21:49:15 UTC

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