Re: [xtm-wg] Suggested public topic: "singleton"

[Geir Ove Grĝnmo <grove@ontopia.net>]

* Nikita Ogievetsky
| There have been several proposals on how to link
| association with a topic link.
| 1)turn association into a topic
| 2)bring "id"-s on associations into the standard
| 3)add "topic" attribute on associations
| 
| I have reservations against all three suggestions.
| Firstly, suggestion 1) depletes (as far as I am concerned)
| the main concept of TopicMaps, which is:
| "anything whatsoever" can be a topic.
| And topic's participation in associations is just another piece of
| information about a topic.
| 
| >From this point of view topic "marriage" participates in
| association between "husband" and "wife" by playing role of the subject of
| this association.
| 
| It can be expressed by means of "type" attribute on association or
| adding an assocrl with type="this-assoc-subject" or
| "topic-this-assoc-is-instance-of"
| 
| Suppose you want to supply any additional information about
| a particular marriage.
| Then you create a topic corresponding to this marriage, let's say
| "marriageAtoB".
| This "marriageAtoB" is of course of type "marriage".
| Now the marriage association between A and B has a narrower subject of
| "marriageAtoB".
| As simple as that.
| 
| There is one peculiarity though:
| Only a single association in a given scope can reference this topic.
| This can be achieved by adding "singleton" to the super-types of
| "marriageAtoB"
| If there are two associations pointing to the same singleton type in the
| same scope, they should be merged.
| <topic id="marriageAtoB" types="marriage singleton">..</topic>
| <assoc type="marriageAtoB">
| ..
| </assoc>
| or
| <topic id="marriageAtoB" types="marriage singleton">..</topic>
| <assoc>
| <assocrl type="this-assoc-subject" href="marriageAtoB">
| ..
| </assoc>
| 
| ------
| 
| Suggestions 2) 3) above yield to jeopardizing integrity which is clear from
| the following:
| 2)
| <topic id="t2" types="some-other-assoc-type">
| <locator href="a" type="is"/>
| </topic>
| <topic id="t1" types="some-assoc-type ">.</topic>
| <assoc id="a" type="t1">...</assoc>
| 3)
| <topic id="t2" types="some-other-assoc-type">.</topic>
| <topic id="t1" types="some-assoc-type ">.</topic>
| <assoc topic="t2" type="t1">...</assoc>
| 
| Integrity is broken in both cases if
| Topic "t2" is not an instance of "t1"
| 
| So having simultaneously both {id and type} or {topic and type}
| on association (or on any other non-topic xtm element)
| is a very subtle matter and can not be used without
| rigorous xtm schema constructs (to be defined yet).

I can't agree with you more. I had a discussion about this issue the
other day and the outcome was that the implications of non-topic topic
map object having both an id, a type and a characterizing topic was
totally _unclear_.

The singleton idea I really like. It makes the confusion go away.

Geir O.


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