[tmcl-wg] Consistency of TMCL Schemas ?

[Dmitry]

On Dec 30, 2003  Bernard Vatant wrote:

 >I'd not looked at the TMCL requirements document for a while, but 
returned
 >to it today, still trying to figure out what could be the advantages of
 >using TMCL schemas (whenever they are there) vs using OWL ontologies 
(which
 >are there already) to constrain topic maps.

Firstly, I think that OWL and TMCL have different goals. Because of 
that priorities and preferences
are different. It seems to me that OWL is concentrated on providing web 
oriented platform for automatic inference  in area that can be covered 
by Descriptive Logic. Because of that OWL has limited abilities to 
represent domain constraints and detail constructs for expressing 
relationships between classes.

TMCL, from my perspective, has three main goals:
- define what kind of information is expected to know about topic of 
specific type
- provide some boundaries for filling in expected information 
(cardinality, min/max values, type constraints etc)
- define set of (may be complex) constraints which instance topic map 
has to be validated against.

I call them schema + validation goals.

 From user perspective it means that TMCL will allow:
- build "smart" user interfaces which help users to view/create/modify 
topic map instances.
- be sure in  quality  of  topic map instance before/during/after 
processing it.

Because of that TMCL needs more powerful constraint language and has 
less requirements for detailed  class definitions.

Secondly, TMCL and OWL rely on different data models or probably, 
better to say, they use different categories to create models.
I can imagine rewriting OWL to make it compatible with Topic Maps set 
of categories (such as names, occurrences, associations, subject 
identifiers). But, I think, it will be actually much more complicated 
than existing OWL.

I personally see possibility in creating tools which will map/translate 
OWL ontologies into TMCL schemas. Some information can be lost 
(detailed class relationships, for example) during this translation. 
But most of information will be preserved and mapped to TM-compatible 
view. On the other hand I think that TMCL authors will prefer to extend 
OWL-mapped to TMCL- Schemas with additional constraints which are not 
(naturally) expressible in OWL.

 >Besides specific details (you can express this, you can't express that)
 >there is something I just figured out and that suddenly strikes me as 
a big
 >flaw in the requirements document. Nowhere is expressed that a TMCL 
schema
 >should be internally logically consistent, IOW that all constraints in 
a
 >given schema taken together do not entail contradictions.

 >Since among the requirements is the capacity to express complex 
constraints
 >using logical connectors and quantifiers, seems to me that such a
 >requirement of logical consistency should be a must. Before checking
 >something to be consistent against a set of rules, one should be able 
to
 >check if the set of rules is consistent.

Actually, "R8 Merging of schemas" mentioned ability to identify 
conflicts in schemas which I personally prefer to see in a relaxed 
form.
I think  we would like to represent complex constraints using TMCL. And 
in this case we can not require automatic schema consistency checking. 
I think, TMCL Lite which will have limited constraint language  may 
support some form of schema consistency checking.

I personally think that schema consistency checking is a "helper" 
function of software tools. I look at schema creation as a creation of 
some theory in natural science. Some theory features can be checked  on 
internal consistency. But what is more important is checking schema 
against real samples, and try to find samples which invalidate existing 
schema. Contradictions are resolved by refining schema. At some point 
schema become more or less stable. Core of the schema is not modified 
anymore. But peripheral constraints are refined over all schema live 
time. If some of the steps in "theory" creation/refining can be 
supported by software at some level it is good.

 >Remind you that OWL, or at least OWL-DL, provides such features, and 
that
 >it was not easy stuff to build. And TMCL pretends to handle some 
logical
 >constructs way more complex than those one could express in OWL.

 >So ?

I think it depends on the goals of ontology/schema language. From my 
perspective it is more important to concentrate on schema + validation 
rather than inference now. Inference can be added later to Topic Maps 
as additional specification. TMQL (with "update" part) and TMCL will 
provide a solid basis for that.

And... I would probably look at more robust inference mechanism than 
Descriptive Logic. This inference mechanism must have ability to assert 
exceptions and have ability to reason with contradictory arguments.  
These kind of things that CYC/OpenCYC can do now (and may be beyond 
that).



Dmitry