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Subject: Re: RDF/Topic Maps: late/lazy reification vs. early/preemptivereification(was: Re: [topicmaps-comment] RE: OASIS vs W3C)
Steve, Way cool! Interesting side note: The most recent draft of the RDF Model Theory document, http://www.w3.org/TR/rdf-mt/, does not reach the issue of reification. (Warning, this is weekend reading, probably from hard copy with some blank paper at hand.) Patrick "Steven R. Newcomb" wrote: > > For me, at least, the shortest, most compelling and > cogent demonstration of a certain critical difference > between Topic Maps and RDF was Michael > Sperberg-McQueen's wrap-up keynote at the Extreme > Markup Languages Conference (www.extrememarkup.com) > last August. > > N.B.: This note is about *what I learned* from > Michael's presentation, and it does not > necessarily reflect Michael's views, or even > constitute an accurate account of Michael's > presentation. It's merely what I remember about > it. (I love Michael's wrap-ups at the Extreme > conferences. It's a good thing he traditionally > speaks last, because he's a hard act to follow.) > > Michael brought colored ribbons and other paraphernalia > to the podium, in order to illustrate his words. > > "Tom buttered the bread," was the statement Michael > wanted to represent. There being no volunteers in the > audience named "Tom", Michael appointed our Conference > Chair, Tommie Usdin, to represent the "Tom" node. Syd > Bauman, as I recall, was appointed to represent the > "bread" node. A blue ribbon between Tommie and Syd > represented the arc representing the statement that Tom > buttered the bread. > > [Use a monospace font, such as courier, if you want > to see the ASCII art as it was intended to be seen.] > > Tommie -----------> Syd > ("Tom") (blue ("the bread") > ribbon) > > So far, so good. "Now," Michael said, "What if I want > to say that Tom buttered the bread with a knife? In > order to attach the knife to this statement, I need a > node for the knife, and I also I need a node to > represent the buttering itself. (There must be some > sort of a 'buttering event' going on here.)" After > everyone had finished laughing over our internal > visualizations of "a buttering event", Kate Hamilton > was appointed to be the node that represented the > "buttering event". A differently colored ribbon was > used to connect Tommie to Kate, and Kate to Syd. Now > there was a triangle of ribbons, because Tommie was > still *also* buttering Syd by virtue the original blue > ribbon. > > Kate ("the Buttering") > /\ > / \ > / \ > / \ > / \ > / \ > / \ > Tommie -----------> Syd > ("Tom") (blue ("the bread") > ribbon) > > Now, with Kate in existence, it was possible > to use yet another ribbon color to connect the knife to > Kate (the "buttering event"). > > knife --------- Kate ("the Buttering") > /\ > / \ > / \ > / \ > / \ > / \ > / \ > Tommie -----------> Syd > ("Tom") (blue ("the bread") > ribbon) > > So now Kate was holding one end of each of three > ribbons: one to Tom, one to the bread, and one to the > knife. Michael then proposed to further modify the > statement: "Tom buttered the bread with a knife *on > Friday*". Yet another volunteer became "Friday", and > yet another ribbon was given to Kate, the other end of > which was "Friday". > > knife --------- Kate ("the Buttering") > / /\ > Friday ------+ / \ > / \ > / \ > / \ > / \ > / \ > Tommie -----------> Syd > ("Tom") (blue ("the bread") > ribbon) > > It was clear that Michael could have gone on to attach > any number of things to Kate; the "buttering event" had > a limitless capacity to be related to other things. > Indeed, by the end of Michael's wrap-up keynote, Kate > was already holding one end of several ribbons, > including the two ribbons needed to connect Tommie > (Tom) to Syd (the bread). > > It was also clear that, once "the buttering event" > existed as a distinct node, it was no trouble at all to > say anything about that event. However, *before* Kate > was appointed to be that node, there was no way to say > anything about the buttering event. > > After the "buttering event" node represented by Kate > was brought into existence, the combination of itself > with its arcs to Tommie (Tom) and to Syd (the bread) > was sufficient to represent the fact that "Tom buttered > the bread". Therefore, once the "buttering event" > existed, there was no further need for the original > blue ribbon connecting Tommie (Tom) and Syd (the > bread). The blue ribbon was redundant, and it > unnecessarily complicated the graph of ribbons and > nodes. The blue ribbon should go away, right? > > knife --------- Kate ("the Buttering") > / /\ > Friday ------+ / \ > / \ > / \ > / \ > / \ > / \ > Tommie Syd > ("Tom") ("the bread") > > In Topic Maps, there is no way to say "Tom buttered the > bread" without creating an explicit "buttering event" > -- a "buttering association" between Tom and the bread. > Instead of making a direct connection between Tom and > the bread, Topic Maps forces us to create a "buttering > event" node, and to connect "Tom" and "the bread" to > that node. The advantage here is that we can always > say something new about anything that already exists, > because even the "verbs" in Topic Maps (such as "to > butter") are necessarily already "noun-ified" (such as > "the buttering") and are ready to be addressed as the > ends of additional arcs. This has significant > advantages: it simplifies the process of amalgamating > facts and opinions when you can't know in advance which > things anyone will want to express a new fact or > opinion about. If someone wants to say something about > "Tom"'s buttering of "the bread", there is guaranteed > to be something to which those remarks can be attached. > > In RDF, we are not forced to create a "buttering event" > node in order to say "Tom buttered the bread". We can > simply connect "Tom" to "the bread" directly. This has > significant advantages if it can be accurately assumed > that nobody will need to say something about the > buttering: > > * There are many fewer nodes and arcs to worry about. > > * Perhaps more significantly, verbs remain verbs. Many > people, especially computer jockeys who have not been > steeped in the traditions of markup languages, > application-independent information interchange and > self-describing documents, are more comfortable with > verbs (processes) than with nouns. This is not a bad > thing. It is only the simple truth that, if you're > focusing on implementing the application of butter to > bread, it would only be distracting and annoying to > try to provide for unanticipatable commentaries and > constraints on specific "butterings". > > RDF provides a process, called "reification", whereby > an arc can be alternatively represented as a node when > it is discovered that someone wants to say something > about it. ("Reification" literally means > "thing-ification" or "noun-ification" -- transformation > into a thing. The term "reification" is derived from > the Latin noun "res" (pronounced like "race"), which > means "thing".) When Michael used Kate Hamilton (the > "buttering event") to be the surrogate of the arc > represented by the blue ribbon, he was reifying the > blue ribbon. The arc became a node (and two new arcs). > > In RDF, reification involves changing the graph that > results from processing interchangeable RDF statements. > In Topic Maps, however, everything is already reified. > No existing arcs need be changed when new information > comes along. New arcs and nodes are added, and these > additions are the only changes that are required. This > comparative changelessness can be extremely important. > If you find something in a graph, and you make a record > of the arcs you traversed in order to find it, you may > want to be able to use that same set of arcs to find > the same thing at some future date. If some of those > arcs disappear, you may not be able to retrace your > steps. If, on the other hand, the process of > reification does *not* cause the arcs whose functions > have been duplicated to disappear, then we have a > situation in which a considerable amount of redundant > information is contributing to our infoglut problem. > Either way, a policy of "late reification" (or maybe we > should call it "lazy reification") causes problems for > the usefulness of continuously-amalgamated knowledge. > > Does this mean that I'm pro-Topic Maps and anti-RDF? > No, not at all! These two paradigms have great need > for each other. > > * RDF needs Topic Maps in order to make scalable > management of knowledge emanating from disparate > sources simple, practical and predictable. > Enlightened self-interest dictates that the RDF camp > consider Topic Maps as an important and basic RDF > application, > > * Topic Maps needs RDF in order to have a popular, > widely-accepted basis upon which to describe exactly > what a topic map means, in a fashion that will be > immediately processable by a significant number of > existing and well-funded tools. The PMTM4 model is > an example of a model of the meaning of Topic Maps > that can easily be translated into RDF -- once and > for all topic maps. > > If the PMTM4 model is adopted for this purpose, the > corresponding RDF arcs will never need to be reified, > even the very first time someone needs to make an > assertion about a "buttering". > > In the past, I myself have considered RDF as the > competitor of Topic Maps. Happily, I was wrong -- at > least in fundamental technical terms. Indeed, I now > believe that if there were no RDF, the Topic Maps camp > would have to invent something like it in order to make > the Maps paradigm predictably comprehensible by the > programmers who are pioneering the development of the > Internet. > > There are other interesting comparisons to be made > between RDF and Topic Maps, but ever since Michael's > demonstration of the difference between early vs. late > (preemptive vs. lazy) reification, I have been meaning > to document both the difference and the demonstration. > Thanks for reading it. > > -Steve > > -- > Steven R. Newcomb, Consultant > srn@coolheads.com > > voice: +1 972 359 8160 > fax: +1 972 359 0270 > > 1527 Northaven Drive > Allen, Texas 75002-1648 USA > > ---------------------------------------------------------------- > To subscribe or unsubscribe from this elist use the subscription > manager: <http://lists.oasis-open.org/ob/adm.pl> -- Patrick Durusau Director of Research and Development Society of Biblical Literature pdurusau@emory.edu
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