Are action description languages, such as cal-A or cal-E the best way to go, or is it better to work directly in classical (more or less) logic? In today's issue, Rob Miller and Tony Kakas make their point of view precise, as an answer to a message by Tom Costello late last week, and Patrick Doherty questions the use of ADL's while reminding us of results that have been obtained without resort to such languages.

This debate emerged from the discussion about the paper by Kakas and Miller, but it has now shifted into the area of the ontologies panel. Doherty's contribution as well as forthcoming ones on the same topic will therefore be placed in that panel session.

Additional debate contributions have been received for the following article(s). Please click the title of the article to link to the interaction page, containing both new and old contributions to the discussion.

Antonis Kakas and Rob Miller
Reasoning about Actions, Narratives and Ramification

Patrick Doherty:

After following the discussion between Rob Miller and Tom Costello, I'd like to point out another approach that my group has been using in our research in the area of action and change. It is based on a distinction between surface and base languages made by Sandewall in Features and Fluents. The family of logics we use is called TAL (Temporal Action Logics) and the newer versions are generalizations of an entailment policy called PMON, first described in F&F.

Clearly, high-level narrative description languages are not only useful, but will obviously be necessary when dealing with scenarios more complex than those we see in the literature today. On the other hand, general purpose logics such as classical logic have great advantage when doing comparative analyses, debugging and incremental extension of formalisms.

In our approach, we combine the advantages of each. Our narrative descriptions are represented in terms of a high-level language which allows for straightforward description of observations, action instances and types, casual rules, and explicit temporal constraints. The high-level language may simply be viewed as a set of macros where each has a modular translation into formulas in the base language, 1st-order classical logic. The language is always extensible in an incremental manner. So far, we've extended the language for causal rules and concurrency simply by adding new macros and translation functions.

The logic TAL 1.0, and the approach using surface and base languages is implemented and accessible as an applet or a Marimba Castanet Channel. The visualization tool allows for the construction of narratives in the high-level language, their automatic translation into a 2nd-order theory, and that theories automatic translation into a first-order theory. One also has the possibility of viewing models as time-lines and a query mechanism is provided. The system and related references are accessible via the following URL:

    http://anton.ida.liu.se/vital/vital.html

The danger we find with the trend in using A language approaches is that it often appears to be the case that one is taking a relatively simple surface language and translating into what turns out to be something along the lines of classical logic, but in a rather indirect and complex manner. It is difficult to see how the guarantee of semantic continuity in the base language or incrementality in the surface language will be met as scenarios or narratives become increasingly more complex. On the other hand, if provided with well-understood and modular translations into classical logic, it is much easier to evaluate progress and simplify comparisons. One sign that there is a problem is that the A-type languages are generally only compared relative to other A-type languages. Of course, translations of formalisms to classical logic and ensuing comparisons are not all that simple when comparing widely differing ontologies, but we have a rich infrastructure of well-established technical tools to help us along.

I'm certainly all for the flourishing of alternative approaches to modeling action and change, but I really think it is time to clean up our methodology, do more comparative analyses across paradigms regarding strengths, weaknesses, assessments of use, and to apply the formalisms to some "real" problems in the area of DES, process control, etc. I'd like to see a library of tools and implementations which allow each of the different groups to actually test the representational capabilities of the perspective approaches and a number of realistic modeling challenges similar to those one finds in the "Hybrid Systems" area. This appears to be a common and useful aspect of methodology in other areas. Why is this lacking in our area and what can we do about it?

I hope the tool we have developed and placed on-line might serve as a starting point for discussion or for developing healthier methodological tools and coherence in the area. Perhaps Murray Shannahan's and Ray Reiter's interests in controlling robots with logics could also lead to another set of testbed's for comparative analysis of formalisms.