[foaf-dev] Group vs Class

Dan Brickley danbri at danbri.org
Fri Jun 11 21:42:27 CEST 2010


On Fri, Jun 11, 2010 at 6:07 PM, Ian Davis <me at iandavis.com> wrote:
> Hi Dan,
>
> I was hoping you would have thoughts on this :)

Thoughts are easy; figuring out where to take the design, and how to
decide - a bit harder!

> On Fri, Jun 11, 2010 at 2:03 PM, Dan Brickley <danbri at danbri.org> wrote:
>>
>> The original reasoning was that groups are in a lowercase-sense
>> 'reified' to be things-in-the-world so we can attach extra info to
>> them, such as homepages, icons, founders, etc. So in that context is
>> more than just the mathematical set of members. However there are many
>> cases in FOAF where we want to talk about a group and it is much more
>> like a set, like the set of W3C staff for example. In this situation
>> the membershipClass construct is somewhat longwinded, and hasn't as
>> far as I can see got much adoption. So I think we're missing out here
>> on a lot of the power of OWL and the tooling built around it, and I've
>> been wondering how to bridge that; perhaps just by saying that it's ok
>> to treat a foaf:Group as a class.
>
> That would imply foaf:member could be regarded as the inverse of
> rdf:type wouldn't it?

Yes, although whether we state that explicitly in the RDFS/OWL might
be a different matter. Some approaches (DL mainly) freak out when you
cross the streams and mix mortal stuff (application level vocab) with
the W3C spec builtins. Others don't care...



>> I mentioned this to Dave Reynolds recently, and he mentioned that the
>> main problem is around identity conditions; in OWL (or DL flavours at
>> least?) classes are identical by membership.
>
> Identical or equivalent?

Identical. But my memory is fading. I think final verdict was that the
OWL flavours went one way, RDFS went the other.

http://www.w3.org/TR/rdf-mt/

"The use of the explicit extension mapping also makes it possible for
two properties to have exactly the same values, or two classes to
contain the same instances, and still be distinct entities. This means
that RDFS classes can be considered to be rather more than simple
sets; they can be thought of as 'classifications' or 'concepts' which
have a robust notion of identity which goes beyond a simple
extensional correspondence."


> I suspect the nuance here is that classes are only distinguished by
> their members whereas groups may have other distinguishing
> characteristics (like a name)

Yes, though it's possible we could make foaf:Group have several
flavours: groups that are basically classes, and groups that are
things like social and/or online entities, having dates, logos,
founders etc.

>> [snip]

cheers,

dan


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