2007/5/19, Evgeny Egorochkin <<a href="mailto:phreedom.stdin@gmail.com">phreedom.stdin@gmail.com</a>>:<div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> On Saturday 19 May 2007 14:37:57 you wrote: > 2007/5/18, Evgeny Egorochkin <<a href="mailto:phreedom.stdin@gmail.com">phreedom.stdin@gmail.com</a>>: > > On Friday 18 May 2007 12:27:30 Mikkel Kamstrup Erlandsen wrote: > > > > >  * should we allow for multiple inheritance (ie multiple parents > > > > > for fields)? > > > > > > > > I believe there were two issues intermixed: multiple parents for > > > > fields > > > > > > and > > > > multiple types or as you say categories for files. > > > > > > True. That is two issues, but I got the impression that the > > > > Strigi/Nepomuk > > > > > camp where in favor of both? > > > > > > As I consider multiple inheritance (both cats and/or fields) to be a > > > somewhat big feature request it needs to be founded on solid reasoning > > > > if > > > > > we should go with it. > > > > I don't consider multiple file types/categories a big feature. Suppose a > > file > > has type/category Audio. This means it belongs to the following > > categories: > > File, Media, Audio. So it already has multiple types. The question is > > whether > > we allow these types to be outside of strict hierarchy. > > It all boils down to whether or not we allow cycles in the ontology tree. > It is a lot easier to parse/update a tree structure if there are no cycles. > That is why I consider it a big feature. Cycles? Not sure what you are talking about. We should not allow any type to be a parent of itself(indirectly), true, but this is possible even in single ineritance case if onto is malformed. Or maybe you should elaborate more?</blockquote><div> In my terminology this is a cycle: A   : parent = None B1 : parent = A B2 : parent = A C   : parent = B1, B2  </div> <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> > Multiple field inheritance, is too in my opinion is not a big feature > > > request > > if inheritance is implemented as such. It might be useful if we link > > multiple > > external ontologies. If we stick with a relatively simple core ontology, > > it > > may not be required. Time will tell. > "We" in this context is *only* the nepomuk project mind you (correct me if > I'm wrong please). There are no plans what so ever for integrating with > general ontologies in xesam. You can extend the xesam ontology with other > xesam-compliant ontologies and that's it. Sorry? I was under impression that Tracker and Beagle wanted to reuse existing ontologies as much as possible? So I proposed to make a core xesam-specific ontology with mappings to DC, EXIF etc, since it's impossible to cleanly link</blockquote><div> I think we agree here :-) I might have been unclear. What I meant is that xesam should not necessarily be interoperable with any old ontology off the web. Priority ones like EXIF and such is another case that I do think we should expose/consume/embed/extend (/me don't consider DC an ontology). We should be interoperable with desktop-related widespread standards IMHO, but these should be nailed down before hand.  </div> <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">> Xesam should of course not restrict Nepomuk from doing this. You're under wrong impression that I'm lobbying nepomuk-specific features to make life for nepomuk easier. In fact, the simpler is xesam onto(no matter how badly screwed it is), the easier it will be for nepomuk to map it. The reason is that the only mapping needed is Nepomuk->Xesam and not vice versa. So Nepomuk doesn't have to decipher and work around any Xesam onto simplifications/deficiencies(as compared to Nepomuk).</blockquote><div> Oh, I was not aware of what the Nepomuk needs actually where, but if they only need a map Nep->Xes then our life is easier :-)  </div> <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Actually, the easiest thing would be to claim that DC is the best and all-encompassing onto and we don't need anything else since Nepomuk already has a DC mapping.</blockquote><div> I don't think anybody wants this :-) </div> <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> > > Unfortunately we didn't really get to discuss any > > > > > practical use cases in the IRC meeting. > > > > > > I have not been able to come up with a good use case (of multi inh.) > > > myself, but maybe some one here can? > > > > Source code: It is a text document(contains text) and software(has > > dependencies on other software). > > You mean that it might ref some .h files fx? If that is what you meant I > can't see why a simple subclass SourceCode->TextFile (or something) isn't > enough..? Software has dependencies, maintainer, project it belongs to. All multiple-inheritance issues can be resolved by moving offending fields higher in the hierarchy. This doesn't hurt because they all are optional. Also, you can eliminate single inheritance and file types as such, without much fuss. The problem with this approach is that software no longer knows which type is particular file and consequentially what fields to expect etc. The advantage of multi- vs single- inheritance is that you describe aspects of a file with types, e.g it's a text, software and network resource. Software then knows what fields to expect and what it is processing.</blockquote><div> I think you are confusing the matters here. One thing is if a category can have multiple parents in the spec. Another thing is if a specific file can belong to several categories... </div> <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">If the onto is quite generic, multiple-inheritance may not be needed. I don't insist that we must use it. My point is that it's easy to implement and it may be useful. Whether/when it will be useful, time will tell.</blockquote><div> Ok. I find it hard to get a clear view of the pros and cons on this with only the two of us arguing. My biggest problem is that I'm not clear on the implementation burden of a multi-inheritance system. Both ontology-parsing and the actual searching is affected by multi-inh and I don't know how well all backends Lucene, Xapian, Trackers custom SQLLite based, handle this... Maybe a few words by the experts can shed some light on the matter; Jos, Joe, Jamie? </div>Cheers, Mikkel </div>