[sc34wg3] New TMCL slides: pinging LMG's intuition about role-type subjects

Steve Newcomb srn at coolheads.com
Tue Nov 24 15:58:33 EST 2009

Lars Marius Garshol wrote:
> * Steve Newcomb
>> So, if the role "buyer" (as reified by a topic we'll call "buyer") appears in another *kind* (type) of relationship -- i.e., one that is characterizable as a different set of role types -- then that topic must have two different subjects. And that's forbidden in topic maps. If we start to wink at topics that have more than one subject, all is lost, it seems to me.
> You can certainly make this argument, although personally I'm not sure I'm convinced. Can't there be more than one kind of "buyer", all collectively represented by one topic? If we allow subtyping of association types and role types, then there's bound to be role types which can appear in different relationships, at least via their subtypes.
Subtyping is good. Two different subtypes of a single type can be 
represented by two different topics. There's no problem if we're talking 
about different topics in each different-set-of-roles context. There's a 
problem only if we're using the *same* topic to represent *different* 

Maybe I'm just missing the boat, here. (It wouldn't be the first time!) 
If so, I need your help, because you're a key player in moving the TMDM 
ball toward the goalposts. Think of me as a cheerleader working for your 
team. I'm wondering what the cheer is supposed to be. Can you suggest a 
way to explain the semantics of TMDM associations that:

(a) allows role-type topics to have different subjects depending on 
which references to them we happen to be looking at, and which 
references we're happening to ignore, when we need to understand whether 
its subject is the same as, or different from, the subject of some other 
topic, OR

(b) disclaims the "roles-in-a-drama" metaphor as a way of explaining 
what associations are, or maybe that somehow makes reasonable a claim 
that the same role can appear in different dramas, OR

(c) prevents customers from someday regretting having chosen TMDM for 
applications that really require role-type topics to have exactly one 
subject, and role-types to be analogous to roles-in-a-drama?

Learning to cheerlead,


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