On the Notion of Plan Quality for PDDL+

PDDL+ is a planning formalism designed to model mixed continuous-discrete problems. Despite its expressiveness, the absence of a well-established framework for evaluating plan quality makes it challenging to use PDDL+ in applications where plan shape and quality are crucial. This paper addresses this issue by introducing a comprehensive set of plan cost functions tailored for discrete-time PDDL+, along with a cost-preserving translation for generating cost-aware PDDL2.1 planning tasks. The plan cost functions provide a theoretical ground for assessing plan quality, whereas the translation shows their practicability by leveraging the connection between PDDL+ and PDDL2.1.