Execution mechanisms for the action language C+

The action language C+ of Giunchiglia, Lee, Lifschitz, McCain and Turner, is the most recent and expressive member of a family of knowledge representation formalisms for reasoning about action and change over time. However, frequently the reasoning proceeds very inefficiently, for large domains collapsing altogether; most often the reason for this is that in determining the truth of a fluent at a given time, one must determine the truth of all fluents at all times: the most irrelevant information is always calculated. This thesis is addressed to two strands of investigation: the questions of efficiency, and of expressivity. An alternative paradigm is proposed, inspired by the Event Calculus, which considers only the information relevant to a given fluent’s value. Current algorithms for working with C+ employ propositional satisfaction solvers; that described in this thesis uses logic-programming throughout. Proofs of correctness are provided, and the correspondence with a variant of the event calculus is systematically investigated. (A consequence of this work is the provision of a transition system semantics for this variant of the Event Calculus.) We also compare the performance of our system, with the current implementation. We would often like to be able constrain the behaviour of systems we represent depending on what has occurred in the distant past, yet in C+ there is no convenient way of doing this. Accordingly, we make use of relations between C+ and the formalism of ‘non-monotonic causal theories’ to broaden the expressivity of C+ action descriptions, and present an updated semantics based on an interesting generalization of the standard transition systems. A proof of correctness for this semantics is given. Finally, the standard query language for C+ suffers in its expressivity, with many interesting statements about domains unable to be made; for instance, one cannot ask whether some fluent eventually holds. Accordingly, we investigate the possibility of using the technique of model-checking on transition systems defined using C+, and show how these techniques can be used to verify that statements of standard temporal logics hold. Several different implementation routes are examined, executed and compared, including an interface with one representative, state-of-the-art model checker.