- From: McBride, Brian <bwm@hplb.hpl.hp.com>
- Date: Fri, 15 Sep 2000 11:18:05 +0100
- To: "'Graham Klyne'" <GK@Dial.pipex.com>
- Cc: "'RDF Interest (E-mail)'" <www-rdf-interest@w3.org>

I've reorderd stuff in this response from the original: > I'm not sure why you feel you need the _mapping_ between > statements and > reifications. OK. Maybe I don't - I'm not sure at the moment. Its 2.45am here - I'm trying to open the control panel to adjust my body clock - but the damn thing is stuck. I used the notion of a mapping because I felt it gave a clean explanation of what I had in mind. I didn't feel it was introducing a new concept incompatible with m&s. For me the mapping is just a convenient device to explain what is going on. Obviously this is not the case for all. I feel that this is something we can come back to if these ideas ever begin to make it beyond this discussion. An issue to revisit. > Consider, if you have an RDF model (per M&S section 5 formal > model) containing: > > [A] --type------> [Statement] > [A] --subject---> [S] > [A] --property--> [P] > [A] --object----> [O] > > [B] --type------> [Statement] > [B] --subject---> [S] > [B] --property--> [P] > [B] --object----> [O] > > [S] --P---------> [O] > > There is no way to know if [A] or [B] is the reification of > "[S] --P--> [O]". This is a really interesting question. I had written earlier that there is a unique resource that represents the reification of a statement. Back to that decision in a minute. In a sytem where the reification of a statement in unique, then [A] and [B] above denote the same resource. What if we gave them URI's that are different? No matter, both URI's denote the same resource, assuming that a resource can have more than one URI. If you say that a resource has only one URI, then you would have an invalid model. Earlier, I mention identifying properties, i.e. a property which is defined to have a unique value for each resource. What happens if we add an identifying property to [A] and [B] above but with different values. Then again, we have an invalid model - i.e. a model that is consistent with the base RDF abstract model, but violates other constraints. Clearly such models could be created without using reification. I do think there is a choice about the uniqueness of a reifed statement. And right now, I don't have an argument to support the choice of it being unique. What I had in mind at the time was that statements form a set, something I don't think, Graham, if recall correctly, you agree with. So that might be a root of any differences in our understanding here. > It seems plausible to me. My only reservation is that it > introduces a new > type of symbol into the formal model (anon:S doesn't really > work as a URI, > I think). But I think that may be handled in other ways > while retaining > your basic approach to partial reification. Absolutely. anon:S is not a URI, and it might well be better to adopt a different syntax to make that clear. Yes anon:S is a new type of symbol not in the original m&s formal model. I never said the formal model shouldn't change. Clearly what I have suggested is different. The issue is what sort of changes are we allowing. I was trying for something that can be loosely described as a refinement - i.e. selecting one of serveral possible interpretations of the original. Expressing that interpretation does require introducing new notation and terminology. Brian > -----Original Message----- > From: Graham Klyne [mailto:GK@Dial.pipex.com] > Sent: 14 September 2000 23:52 > To: McBride, Brian > Cc: 'RDF Interest (E-mail)' > Subject: RE: abstract model > > > At 10:10 PM 9/14/00 +0100, McBride, Brian wrote: > [...] > > > I > > > think I am > > > forced to conclude that it represents an *extension* to the > > > RDF model as > > > defined. > > > >That may well be :( > > > > > Specifically, the "mapping called Reification" is a > > > new concept > > > that is not present in the original, and the only way to find > > > that mapping > > > using the original model is to find the set of 4 statements > > > that comprise > > > the reification. > > > >Please forgive my stupidity, but I don't follow the argument. > >M&S does have the idea that a reified statement is represented > >by a resource. I think the mapping called Reification is a > >component of the abstract model described by m&s. I think it > >can be positioned as a clarification of what is already > >defined. > > I agree that M&S has the concept of a reified statement > represented by a > resource. > > My view is that the _mapping_ -- the function or relation > that associates > the resource representing a reified statement with the actual > statement it > represents -- is not there. > > Consider, if you have an RDF model (per M&S section 5 formal > model) containing: > > [A] --type------> [Statement] > [A] --subject---> [S] > [A] --property--> [P] > [A] --object----> [O] > > [B] --type------> [Statement] > [B] --subject---> [S] > [B] --property--> [P] > [B] --object----> [O] > > [S] --P---------> [O] > > There is no way to know if [A] or [B] is the reification of > "[S] --P--> [O]". > > Thus, I suggest the formal model does not contain the > information needed to > construct the mapping you describe. > > (However it could be said that either [A] or [B] reifies the > statement "[S] > --P--> [O]".) > > >I suspect my problem is that we are thinking in different modes. > >For me, as a mathematical abstraction it can work as defined. > >I'm thinking of abstract model here, not of implementation. > > > >Maybe you could spell out the problem for me. > > Let me also take a stab from a mathematical abstraction PoV. > > IIRC, a mapping from some domain {d[i] in D} to some range > {r[j] in R} is > defined a set of pairs > > { <d[ik],r[jk]> } > > that defines the mapping correspondence between members of D > and R. It is > this set of pairs that is absent in the formal RDF model. > > >Let me go through briefly the results of a conversation with a > >colleague of mine. Stuart asked me what would the semantics of > >an incomplete reification be. I think the answer hangs together > >reasonably well and may be useful to this discussion. > > Sure, I think it hangs together just fine. > > [...] > >If we apply the same thinking to reification, and I'm assuming > >here that the resource modeling the reified statement is > >anonymous, then given a set of statements like: > > > > S = anon:S > > { > > (S, [rdf:type], [rdf:Statement]), > > (S, [rdfs:subject], [http://aldbaran.hpl.hp.com/bwm]), > > (S, [foo:assertedBy, "Graham") > > } > > > >This is a statement about an incomplete reification of a > >statement. What does it 'mean'. Informally, using the same > >reasoning as above, its semantics would be "Graham has > >made a statement about my internal HP homepage", but it > >doesn't say exactly what statement. > > > >That's the intiution. What do you think? If it stands up > >to initial scrutiny, I'll maybe have a go at expressing > >it in a more formal language. > > It seems plausible to me. My only reservation is that it > introduces a new > type of symbol into the formal model (anon:S doesn't really > work as a URI, > I think). But I think that may be handled in other ways > while retaining > your basic approach to partial reification. > > I'm not sure why you feel you need the _mapping_ between > statements and > reifications. > > #g > > ------------ > Graham Klyne > (GK@ACM.ORG) >

Received on Friday, 15 September 2000 06:18:10 UTC