Chitta Baral and Son Cao Tran

Relating Theories of Actions and Reactive Control.

[full text]
[msg to moderator]
[debate procedure]

Overview of interactions

N:o Question Answer(s) Continued discussion
1 3.10  Erik Sandewall

Q1. Erik Sandewall (3.10):

Dear Chitta and Son Cao,

Your article addresses a very interesting topic for our group. I have some specific questions about the contents of the article, and also some comments about how it relates to other current approaches to the same or similar problems. I'll write them as two separate contributions. With respect to the article itself, I have the following questions.

1. For maintenance control modules, which are treated in section 3.3, you write:
  But in the presence of exogenous actions the robot with no control over those actions can only strive to make the maintenance goal true if they are made false by some exogenous action. In other words a maintenance control module can not guarantee that the maintenance goal will never be false...
This seems right if the maintenance goal is characterized by one single state in the state space. However, many applications do require controllers to keep the controlled system in an acceptable state at all times, which is possible if there is a number of such acceptable states, and the state transitions due to exogenous "actions" sometimes only move within the acceptable set. In such cases the controller strives to keep the system in those states among the acceptable ones that are "at a safe distance" from the unacceptable ones. More precisely, even if the system is in an acceptable state, the controller may perform actions as a precaution against future exogenous "actions", thereby shifting the system to a state where exogenous actions can not be harmful.

Possibly one can reduce this case to the one you consider by identifying the maintenance goal with a subset of the acceptable states, namely those acceptable states where that are at a safe distance from a transition to the unacceptable ones. Is this the way you intend maintenance goals to be defined, and if so, do you claim that the reduction is always valid? (Concurrency and the use of combined achievement and maintenance modules may introduce complications, I believe).

2. A second question concerns to what extent this work relates to current, logic-based theories of actions and change. In section 9.1 you write:
  ... in this paper we do not use any specific action theory... rather we use an abstract "entailment relation". Because of this, our formulation need not change with more sophisticated action theories.
However, as I read the paper I wonder whether it depends on any of the concepts or results of current action theories at all? (This is not a criticism, of course, I just want to understand the character of your definitions and results). For example, definition 6.1 on page 28 begins
  Let ...  A  be an action theory with a transition function  Phi 
and then the rest of the definition is expressed only in terms of  Phi . Could it be the case, then, that everything in this article could as well be expressed only in terms of states, state spaces, and transition functions? In this case, action languages and other logicist devices ought to be seen simply as a convenient notation for specifying transition functions, but otherwise they'd not play any role in this particular development.

3. Here is a standing question that I now ask regularly to authors in our area. Given that your paper proclaims a number of theorems, as do many of our publications, (a) do your results rely on any theorem previously published in this literature, (b) do you foresee any of your theorems later being used in a forthcoming publication by yourself or someone else? If the answers to both questions are "no", what do you think is the purpose of including theorems in our research articles?

4. There seems to be a close connection between your  Closure  concept and the most commonly studied cases of ramification. In both cases, one first specifies the effects of an action "in itself", and then one adds the possibility of additional changes due to phenomena that are characterized separately from that action. Is this a connection that you have also considered?

The relation to ramification is particularly transparent by comparison with my KR-96 paper, [c-kr-96-99]. That article uses two types of state transitions: one result function that maps an action and a state to a set of possible result states, and a binary successor relation that characterizes further transitions. The "closure" of an action, in that case, is the set of all states that can be reached via one member of the set of action result states, followed by an arbitrary number of steps using the successor relation, and that do not in turn have any successor. For ramification, the successor relation characterizes spontaneous and "caused" transitions in the world; in your work they would instead characterize the results of exogenous "actions". These two cases seem to be very close to each other.

The KR article is in fact formulated entirely in terms of states and state transitions, and analyses ramification methods without introducing any logic language at all. Its way of looking at actions is therefore quite close to what you use in your paper. (The main results in my article are assessments that characterize the range of sound applicability for a number of previously published approaches to ramification). Relating back to my question 2, this adds to the impression that ramification and exogenous events are related phenomena.

5. I put quotes around "actions" because I can not get used to the idea of calling natural events `actions'. Although I understand the background of this terminology, which comes from the situation calculus, I do think it makes communication with neighboring disciplines a lot more difficult than it ought to be.

Best regards, Erik


c-kr-96-99Erik Sandewall.
Assessment of ramification methods that use static domain constraints. [entry]
Proc. International Conf on Knowledge Representation and Reasoning, 1996, pp. 99-110.

Additional questions and answers will be added here.
To contribute, please click [send contribution] above and send your question or comment as an E-mail message.
For additional details, please click [debate procedure] above.
This debate is moderated by Erik Sandewall.

13-Jun-98 20:28