An Extended Query Language for Action Languages (and its Application to Aggregates and Preferences)

This paper continues our earlier work in representing arbitrary preferences in causal reasoning and planning systems, albeit in an oblique fashion. Previously, we defined a very general query language relative to histories; from this we specified a second language in which preferences on histories are defined. This in turn allowed us to define the notion of a most preferred history in a set of histories. In this paper, we extend these languages in two directions. First, we add a conditional construct that allows one to select between terms. Second, we add a capability for defining macros. With these two added constructs, one can now define aggregate quantities, such as the total cost of actions in a history, the maximum value of a fluent in a history, or a count of the number of times a fluent goes to zero in a history. Via the preference language, one can then express a preference for histories (or, plans) with minimum action cost, maximum value of a fluent, or in which a fluent is most often zero. We argue that this substantially increases the range of concepts about which one can express preferences.