2 COMPONENTS OF AN ACTION LANGUAGE

Some of the recent work on representing action makes use of high-level action languages. In this paper we show that an action language can b e represented as the sum of two distinct parts: an ''action descriptio n language'' and an ''action query language.'' A set of propositions i n an action description language describes the effects of actions on s tates. Mathematically, it defines a transition system of the kind fami liar from the theory of finite automata. An action query language serv es for expressing properties of paths in a given transition system. We define the general concepts of a transition system, of an action desc ription language and of an action query language, give a series of exa mples of languages of both kinds, and show how to combine a descriptio n language and a query language into one. This construction makes it p ossible to design the two components of an action language independent ly, which leads to the simplification and clarification of the theory of actions.