[topicmapmail] Default occurrence type in TMDM

[Lars Marius Garshol]

* Lars Marius Garshol
>
> You can see some initial notes toward a meta-PSI set at
>   http://www.garshol.priv.no/blog/40.html

* Robert Cerny
>
> I read that when you posted it and i was deeply confused and sad :)  
> In the TMDM spec from 2005-07-13, there was the section "7.5.3  
> Subjects for defined terms". I thought that would cut it pretty  
> well ?!? Since it is not in there anymore, i obviously missed  
> something.

That draft did have PSIs for all the defined terms in it, yes.  
However, the trouble is that we were not really ready to spell out  
the precise relationships between all of those PSIs, nor really the  
processing semantics of all of them. So we had to send TMDM out  
without them.

The blog posting is really the first step towards trying to develop  
such a PSI set with the correct semantics. As you can tell, we have  
some steps left to go.

> [why is a default occurrence type needed]
>
> For two reasons:
>
> -) If I would want to import a XTM 1.0 map into a TMDM store, it is  
> one of the things that is referenced, and therefore must exist.

This is kind of complicated to answer. Firstly, the only document  
that says what XTM 1.0 means is the XTM 1.0 specification. The XTM  
1.1 spec (from before we upped the version number to 2.0) used to  
also define the mapping from XTM 1.0 to TMDM, but it no longer does  
that.

Now, there are two ways you could get a "default occurrence type" in  
XTM 1.0:

  (1) You create it yourself.

  (2) You use an untyped occurrence. (XTM 1.0 specifies a default PSI  
for
      this case.)

So you could say that XTM 1.0 has what you need here.

> -) If I would be occurrence, i would ask the editors: "Why does  
> name get one and i don't?" :) And the rest of the family would  
> cheer in, in particular role and association.

This is because in XTM 2.0 you have to specify the type of  
associations, roles, and occurrences, but not names. The rationale  
for this is two-fold:

  (1) Names used not to have types at all. Going straight from there to
      requiring them is quite a step. Having a default type helps.

  (2) One of the benefits of names in Topic Maps is that you know what
      label to use for every topic, but this requires a default type so
      that you can choose between the names of a topic.

None of this applies to the other three.

--
Lars Marius Garshol, Ontopian               http://www.ontopia.net
+47 98 21 55 50                             http://www.garshol.priv.no