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Subject: RE: [xdi] Minutes: XDI TC Telecon Thursday 1-2PM PT 2009-02-26
Dear Bill, thanks for this mail. There are many issues involved and it is not easy to discuss them using email (maybe we should keep track of issues in the wiki). However I've tried to provide some first comments below (prefixed by G:). Kind Regards, Giovanni on Sat, 28 Feb 2009 at 04:00:32 -0500 Barnhill, William wrote: >>That there was an clearer typing relationship between predicate and object in XDI XRIs was one of the useful differences from RDF. My thoughts on why are below, following a response to Giovanni's example of why he believes this should not be: >>Giovanni said... >>"Giovanni's first point was that what =giovanni and =markus represent in statements... >> =drummond/+friend/=giovanni >> =drummond/+friend/=markus >>...are not instances of +friend. They are instances of the "individual" class, i.e. class identified by the equals symbol (=). What we can read from the statements =drummond/+friend/=giovanni and =drummond/+friend/=markus is simply that two instances of the individual class +people, namely =giovanni and =markus, are referenced by =drummond/+friend." >>I agree completely..but it makes total sense to me to add that =Drummond/+Friend is a sub-class of +Friend, meaning an individual referenced by =Drummond/+Friend is also referenced by +Friend/$member (is this the right $ word to represent membership in a class, or is it $is or $has ?). G: I do not disagree with this (apart from the notation: $member). But, to me, being referenced doesn't mean necessary "being an instance of". >>Giovanni also said... >>"Actually =giovanni and =markus would exists also if they were not +friend, and if they were instances of +friend this would be not possible." >>I'm not sure what you mean by this Giovanni. If you mean that they are in the graph rooted at =Drummond because of the friend relationship and without that they would not be in the graph then I'd agree that they wouldn't be in =Drummond graph without it..but if there's no connection to =Drummond then they shouldn't be in the graph, right? G: Simply I mean that =giovanni and =markus are digital subjects whose have been assigned the XRIs =giovanni and =markus. As such, they are in the global XDI graph, rooted in "=". The fact that they are referenced by =drummond/+friend is irrelevant to the fact that they exist (but obviously they are happy to be referenced by that predicate :-). >>If you mean globally..how is it not possible for =Markus to exist in global graph regardless of whether =Markus is an instance of +Friend? +Friend membership doesn't mean one Drummond's friends, or friends with everyone, it means you are the friend of someone (does not necessarily mean they are a friend of yours, unless we define it that way in the dictionary). G: I probably do not understand this. I think that it is perfectly possible for an individual existing in the graph without being instance of +friend or +relative, etc. the minimum requirement is just to be an individual. Maybe I'm missing something here? >>Our predicates as currently defined are not going to be individuals (@ or = global identifiers) (but see next para) and not going to be local identifiers. That means they will either be $ words, $$ variables, or + word classes. >>As a sidenote, @ identifiers could conceivably be used as a predicate to indicate shared membership in a community. This has some interesing implications but has not been discussed on list yet and violates general usage in examples so far. If you did this then by my reasoning =Bill.Barnhill/(@Oasis*TC*XDI)/=Drummond => [(@Oasis*TC*XDI)/$member/=Drummond (@Oasis*TC*XDI)/$member/=Bill.Barnhill], which makes sense. G: I almost agree with your first statement, predicates should be limited to "$" words or "+" words or variables $$. I do not understand the second one very well. $ words are fundamental properties in that they have a range of any resource, so all we can say about the class of the object is that it is a subClassOf either xdi:Community or xdi:Individual (my terms, as in email before an XDI:Entity is either a community/collection or an individual, see owl:unionOf ). G: if you mean that any resource in XDI is considered an XDI:Entity then I agree. The conclusion that XDI:Entity are classified into individuals and communities makes some sense to me (but we probably need to discuss a bit more on this) >>We can not say anything about the class of the range for $$ variables except that it is a subClassOf XDI:Entity since XDI:Entity is the root class and all classes are subClasses of it (assumption here, that bit has not been discussed on the list yet, it in my doc to be posted). G: Since I'm working on this topic as well, it would be useful for me if we could discuss together this document. Can we coordinate in order to have a phone call or a chat on this? >>That leaves + word concepts. If we are going with a +word can be used as both a class and as a property (which we have been in the examples), then the following statements are all valid: >>=Bill.Barnhill/+Friend/=Drummond >>=Bill.Barnhill/$is$a/+Friend >>+Friend/$has$a/+Name >>If that's the case then benefit of having an implicit range class be either an XDI:entity (in case of $ words and $$ variables) or the predicate as a class is that you reduce the complexity of the dictionary. G: Interestingly, can you report a bit more on this? >>The cost of doing this is a restriction that an object must be either an XDI:Entity (which is always will be) if the predicate is not a + word, and that the object must be an instance of the predicate as a class if predicate is a + word. G: I think that the "instance" you are referring here is not =Drummond, instance of the individual class ("="), but =Bill.Barnhill+Friend=Drummond, an instance of =Bill.Barnhill+Friend class (which could be in turn an instance of +Friend). Consider this example: +Friend/$has/+since =Bill.Barnhill+Friend=Drummond/+since/"02-10-1998" (i.e. the individual =Drummond, casted to +Friend, in the context of =Bill.Barnhill, has the property +since - which is obviously context-dependent, i.e. changes with the context, =Bill.Barnhill+Friend=Giovanni/+since/"05-03-2007") But would =Drummond/+since/"02-10-1998" or =Drummond/+since/"05-03-2007" make any sense? Another way to say this is that =Drummond would exist even without being a friend of mine, or a friend of anyone. It exists because it is an instance of the individual class ("="). The issue is whether =Bill.Barnhill+Friend=Drummond and =Drummond should point to the same XDI resource, e.g. whether =Bill.Barnhill+Friend=Drummond/$is/=Drummond or =Bill.Barnhill+Friend=Drummond/$is$a/=Drummond (as the implication suggested by Drummond +x/$has/+y --> +x+y/$is$a/+y would suggest). Probably we need to elaborate a bit more on this. >>I haven't thought of an example where the second restriction is not going to apply anyway. G: see my example above >>In the case of @example/+Dept/*telecom anything other than that *telecom is an instance of +Dept seems counter-intuitive to me. G: Please consider that this is a different case than =Bill.Barnhill/+Friend/=Drummond. @example-university/+dept/*telecom is actually describing a composition (@example-university is made of +dept of *telecom, +dept of *science, etc.), thus *telecom is an instance of +dept by definition. We used the lcs "*" to remark this.
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