A Situation-Calculus Semantics for an Expressive Fragment of PDDL

The Planning Domain Definition Language (PDDL) has become a common language to specify planning problems, facilitating the formulation of benchmarks and a direct comparison of planners. Over the years PDDL has been extended beyond STRIPS and ADL in various directions, for example, by adding time and concurrent actions. The current semantics of PDDL is purely meta-theoretic and quite complex, which makes an analysis difficult. Moreover, relating the language to other action formalisms is also nontrivial. We propose an alternative semantics for an expressive fragment of PDDL within the situation calculus. This yields at least two advantages. For one, the new semantics is purely declarative, making it amenable to an analysis in terms of logical entailments. For another, it facilitates the comparison with and mapping to other formalisms that are defined on top of the same logic, such as the agent control language Golog. In particular we obtain the semantical foundation for embedding efficient PDDL-based planners into the more expressive, yet computationally expensive Golog, thus combining the benefits of both. Other by-products of our investigations are a simpler account of durative actions in the situation calculus and a new notion of compulsory actions.