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Re: [UCR] Rich KR Use Case

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Wed, 22 Feb 2006 21:18:20 +0000
Message-Id: <298aa87fa89e3f757a86d18c7c78bd27@cs.man.ac.uk>
Cc: public-rif-wg@w3.org
To: Jim Hendler <hendler@cs.umd.edu>

On 20 Feb 2006, at 17:58, Jim Hendler wrote:

>
> I objected at the beginning to the way this was going, I object now to 
> how it sounds.  The problems are that (1) this implies that Rich KR is 
> somehow better and more valuable on the web (largely unproven),

Jim,

The set of use cases lumped together under this title are pretty 
heterogeneous. Some do, I agree, emphasise KR on the web more than KR 
for the web, but I have been assured that the success of this kind of 
application constitutes a success for the semantic web as a whole, and 
so could be seen as a relevant input for RIF (not to mention the fact 
that publishing/sharing ontolgies on the web is an important aspect of 
many of these use cases). Of the examples available on the Wiki, only 
the "Uncle" example has been chosen (not by me) for inclusion in the 
UCR document. I guess that this is because it is well known and 
relatively easily understood (although I note your comments below). 
Similar examples also arise in the web services domain (amongst 
others), and it would be a good idea to at least refer to these in 
order to strengthen the motivation in general and the "for the web" 
motivation in particular.


> (2) it takes an unflattering approach to describing OWL in its 
> emphasis,

This didn't really strike we when I first read it, but you could be 
right. It would certainly be a good idea to emphasise the fact that OWL 
can also express things that are not typically expressible in rule 
languages, i.e., that the two paradigms have complementary 
expressiveness. The sentence "A typical example, as detailed below, is 
the use of rules to describe complex relationships between binary 
predicates which cannot be captured in ontology languages alone." 
should also be fixed - it may be true of OWL, but it isn't true of 
ontology languages in general.


>  and (3) it doesn't emphasize the important point it is trying to make 
> (that sometimes extending OWL with rules is a good thing, but the 
> rules syntax needs to be commensurate with the syntax OWL is in,

For my taste the *really* important point (which also isn't emphasised) 
is the need for semantic compatibility with RDF and OWL. I'm not sure 
that I understand what is meant by "commensurate with the syntax OWL is 
in", and if it means what I think it means (i.e., RDF/XML syntax), then 
I'm not sure that I agree.


> (4) The syntax used in the example is not the normative exchange 
> syntax for OWL  and this further obfuscates the point being made.

Again, I guess that you are asking for the example to be given in 
RDF/XML syntax. The trouble with this is that human readability would 
then be very poor, even for those reasonably familiar with RDF (which 
many in the WG and beyond may not be). I guess that the abstract syntax 
was chosen in an effort to *avoid* obfuscation (I'm not sure about this 
as I wasn't the originator of this example).

>
> I do not object to having a section (Renamed "extending OWL") which

I'm not very keen on the Rich KR name either and, speaking for myself, 
would be happy to see it changed to "extending OWL".

>  1 - shows how rules can extend something in OWL (I think the uncle 
> example is very misleading, by the way, unless you want to get into a 
> discussion of safe grounding - seems odd to have to insist on 
> including the "person(?y) and person(?x) in the rules when the WG 
> hasn't even addressed this issue)

I agree that "person(?y) and person(?x)" may not be necessary, but it 
doesn't seem to be hurting - surely it is reasonable to expect the 
sibling and child in question to be Persons given that an Uncle must be 
a Man?

>  2 - that does not have any italics nor use the incorrect word 
> impossible  (btw, it is more than possible to do this in OWL,, where I 
> can extend syntax, it is not possible to do this in OWL DL as written)

Well, I am as big a fan of OWL as the next person, but I do believe 
that it is "impossible to describe the desired relationship between the 
hasSibling, hasChild and hasUncle properties in OWL" (either DL or 
Full). The desired relationship is such that the uncle relationship is 
entailed for any pair of individuals in the relevant "indirect" 
relationship. Of course OWL could be extended to include such 
expressive power (and in fact efforts are already underway to define an 
extension to OWL DL that includes a limited form of role composition 
that does not make the language undecidable) - perhaps it would be 
worth mentioning/discussing this in the text.

>  3 - makes the last sentence, which is more important than all the 
> rest of the example, more evident

I agree that this important point is not very prominent (maybe it was 
deemed to be implicit given the context).

>  4 - is retitled to emphasize that this is about the standard format 
> for extending, not about the extending per se (there are many other 
> ways to consider extending languages w/or w/o rules)

Again, I would say that the crucial point is semantic compatibility. I 
agree that there are many other ways in which OWL could be extended, 
and that this could/should be mentioned in the text. It doesn't seem 
unreasonable to focus on rule extensions in this case, however.

>
> btw, less anyone wishes to claim my objections here are more political 
> than technical, I confess you are absolutely right.  THe WG process is 
> inherently political, and that is why I pay to belong to the W3C so I 
> can help get the politics right.

I hope I focussed on the technical objections - I prefer to leave 
political to the experts :-)

One point that I remember well from our WOWG days is that, in response 
to criticism of proposed WG texts, you would usually ask the critic to 
suggest alternative wording. Perhaps you would like to do so in this 
case?

Regards,

Ian


>
>  -JH
>
>
> -- 
> Professor James Hendler			  Director
> Joint Institute for Knowledge Discovery	  	  301-405-2696
> UMIACS, Univ of Maryland			  301-314-9734 (Fax)
> College Park, MD 20742	 		  http://www.cs.umd.edu/~hendler
> Web Log: http://www.mindswap.org/blog/author/hendler
>
Received on Wednesday, 22 February 2006 21:18:21 GMT

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