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From: Michael Schneider <m_schnei@gmx.de>
Date: Tue, 07 Aug 2007 17:12:25 +0200
Cc: public-owl-dev@w3.org
Message-ID: <20070807151225.250830@gmx.net>
To: bmotik@cs.man.ac.uk
Hi, Boris!

I still try to find out what declarations should be good for, and what they provide. In all your posts, including those you posted some months ago to this topic here in the group, and in a paper you wrote (forgot its name, but you will probably know what I mean), I think I always saw these two main lines of argumentation:

1) We need some means for doing "structural consistency" checking for OWL files, and declarations provide such a means. Structural consistency is defined in


the following way:

    "An ontology O is structurally consistent
    if each entity occurring in an axiom from the axiom closure of O
    is declared in O."

2) When mistyping an URI in an ontology, we get a hard to find semantic change. For instance, If I write

    SubClassOf(Class1 Clas2)

where I meant "Class2", the resulting ontology is probably completely consistent (has a model), while definitely not what I intended. I will later (with luck!) find strange behavior when doing inference, but it will be hard to tell what the problem is. If I had non-semantical declarations, I could add a declaration

    Class2 declaredAs owl:Class

(and, hopefully, I do not misspell it, too :)), and would then be reported by a declaration aware parser that there is some class "Clas2" in the ontology, for which there is no declaration.

First, to the 2nd point, because I can faster come to a personal conclusion here. While I definitely see that such URI misnamings (or what you prefer to call them) are an ugly problem in principle, I had already argued in my previous mail that I do not see these typos to be a realistic problem, because when authoring an OWL ontology with a modeling tool, it is not very likely to produce such misnamings. But if I really manage to accidentally create two classes "Class2" and "Clas2", declarations wouldn't help, because my tool would either /not/ produce any such declaration at all, if it does not know about declarations. Or if it automatically produces declarations, it would then produce /two/ of them, for both, the correct and the wrong class name! Right? So, declarations would provide no help here, but instead would provide some non-justified sense of safeness.

And this was only the case of a single declaration-using person creating a complete ontology with a single full featured modeling tool. What, if I share ontology development with other people, who use other tools, which are not declaration aware? Or with people who did not hear about declarations yet? Or people who do not like declarations and have turned them off in their tools? Or those smart colleagues who try to maintain the set of declarations /manually/, which of course leads to mistakes? What is with this imported OWL-1.0 ontology from the SemWeb, for which there will never be any kind of declarations? Or this OWL-1.1 ontology, where its creator doesn't care about declarations?

So, to conclude the 2nd point: Neither see I any practical need for declarations with regard to URI misnamings (such misnamings are not really possible with tool support), nor would declarations help me much in practice.

Now to the first point:

I strongly agree that it is a good thing to have an ontology, which allows me for every name (i.e. URI) used within this ontology to determine, if this name denotes either a class, or an individual, or an object property, or a datatype property (I will call this the "resource kind" of a named entity from now on). I really want to have this property of an OWL file, because otherwise my modeling tool gets into problems, if it isn't able to determine the resource kind for each named entity in the ontology. So what I like to see from a good tool will be some means to check an ontology at read time, if for each name the resource type can be detected. If the resource kind cannot be detected for some named entity, than I expect a warning from my tool. Of course, this check should be efficient enough to not bother me as a user by making me wait too long.

What I do not get is why I should want to use declarations for this purpose. I think that OWL already has everything included to allow a parser or whatever tool to do such a check. Let's see:

1) Classes:

AFAICS from OWL's abstract syntax, it can always be determined, from each single axiom in an ontology containing a name N, if N denotes a class or not. A few examples:

    * ... complementOf(N)                ==> N is a class
    * ... unionOf(... N ...)             ==> N is a class
    * ... restriction(p allValuesFrom(N))==> N is a class
    * ... oneOf(... N ...)               ==> N is NOT a class

Did I overlook a situation, where this is not always possible? If not, a parser can contain a map including all possible axiom types of OWL-1.1, where some pattern matching is applied. Not hard to implement for the parser's author, and probably pretty efficient (I will happily wait up to two seconds on my 2GHz machine :-)).

2) Individuals:

Again, I cannot see a case where one cannot easily decide from each single axiom containing a name N, if N denotes some individual or not.

3) Properties:

Ok, here it is at least not always possible to determine, from a single axiom, if a property is an object or a datatype property. Examples:

    * ... restriction(N cardinality(1)) ...
    * FunctionalProperty(N)

But then, there is still hope that the more concrete information (object or datatype property) is deducible from the rest of the ontology. So we have to look at the complete ontology. Maybe, somewhere else is some explicit typing axiom like 'ObjectProperty(N)', or some other axiom like

    ... restriction(N someValuesFrom(C)) ...

from which we can deduce that N is indeed an object property. So, again, no need for additional (and redundant in this case) declarations in this case!

The only problematic situation I see is when it is /not/ possible to deduce from the ontology, what kind of resource N stands for. This would, for instance, be the case, if the only axiom, where the name N occurs, would be the above cardinality restriction. In this case, I would like to see my parser to spit out a warning:

    "Hey, while your ontology is formally correct, it is probably not in the state you wish to have it. So explicitly say what N is meant to be!"

If the parser stops parsing in this case, or if it tries to apply some heuristics, or perhaps in the concrete case it is not affected at all... I can live with all of these cases, as long as I get warned in some way.

The only question which I have (and which you, Boris, probably are able to answer much better than me) is: Is it always efficiently decidable, if a Name N denotes a class, individual, object or datatype property, or if its kind cannot be determined? Only if this question is answered with "NO!", than I can see at least a /theoretical/ point for declaredAs statements. But of course, this question can always be answered /pragmatically/, by trying to find out the kind for a name N for a second or so, and than surrender by saying

    "Cannot deduce the kind of N, so please tell it explicitly!".

The important point, when such a situation occurs, is: I would then add a typing axiom, NOT a 'declaredAs' statement, because adding a declaration wouldn't fix my problem at all: After adding such a declaration statement, my ontology is still in the same under-determined state as before! Adding a declaredAs statement wouldn't be more than just adding a comment to the OWL file of the form: "N is an object property". Of course, a declaredAs statement is a very specific kind of comment, which is machine processable, and which is dedicated to the special purpose of telling parsers, what kind of resource a named entity within an axiom has. But IMO, it simply misses the point for what it is intended. If it is not possible to determine from the ontology axioms itself, what kind of information a name denotes, than somewhat is wrong with my ontology - not really formally wrong (an ontology is just a set of axioms, and if this set is semantically consistent, than the ontology is formally ok), but the modeling is probbly quirky. Modeling quirks can only be fixed by adding, changing or removing axioms, not by adding comments!

So, instead of inventing a new feature, why not try to find out how far we come with the things we already have? I think that one of the worst problems in this discussion is that the problem of "structural consistency" hasn't yet been largely discussed and understood within the OWL community. Wouldn't it therefore be a good idea to try out to create some kind of "best practice" document, which tells people what to do to create "structural consistent" ontologies, and how to work with them? Such a document would have to provide the following:

    * Giving a complete list of how to deduce from the axioms in an ontology, if a given name N stand for a class, individual, object property, or datatype property, or if N's kind of resource cannot be deduced. This would be in the line of my discussion above, but much more thoroughly analyzed then my three minute check.

    * Giving parser implementors useful hints, how to implement the list of deduction rules above.

    * Giving modeling tool implementors a set of guidelines, which tell them what information they should insert into an ontology in order to make it always possible to check for structural consistency.

    * Giving OWL authors a (small!) number of easy to follow guidelines for keeping their ontologies in a state, which is supposed to pass each parser, which does structural consistency checking in the way outlined in the document. These guidelines would only be needed, if the ontology author wants to do manual edits, or if he likes to do an eye-check over an ontology to see, if it is probably in a good state.

Additionally, one could build a reference implementation, which works in the way discussed in the document. The reference implementation would not do more than just the things outlined in the document (ok, one should additionally be able to decide to either do single file parsing or to regard the whole import closure :)). The tool implementors could use this reference implementation for studying it, and as a base for their own product, and they could then add all kinds of additional things, which make their product more interesting than those of their competitors, like heuristics, providing assistance on ontology "repair", etc. Users could use the reference implementation as a simple stand-alone tool to check their OWL files, if they do not have any of those much cooler products from the tool implementors.

This approach might fail, or at least turn out to be not a good idea. And /then/, this might bring back declarations into play. /Then/ the problem has at least been discussed and understood by a few more people, and the declaredAs fans can /then/ point to the document and say: "Look, we really need declarations, because the "more obvious" approach working with OWL axioms alone does not work." /Then/ one can think about adding such a declaration mechanism into the OWL standard. /Then/, but it is much to early to do so now!

[BTW: I wonder if Jena Eyeball already contains some functionality in the direction proposed above? Or Pellet within its ontology repair functionality? I Will have a look, if I find the time.]


Michael Schneider <m_schnei@gmx.de>
Received on Tuesday, 7 August 2007 15:12:33 UTC

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