PDDL2.1 - The Art of the Possible? Commentary on Fox and Long

TL;DR

PDDL2.1 extended planning language capabilities with durative actions and quality metrics, but its semantics are flawed, suggesting future work should focus on clearer semantics and flexible implementation.

Contribution

The paper critically analyzes PDDL2.1's features, highlighting issues with durative actions and advocating for improved semantic clarity in future language developments.

Findings

Durative actions have flawed semantics.

PDDL2.1 successfully extends planning expressiveness.

Future language design should prioritize clean semantics.

Abstract

PDDL2.1 was designed to push the envelope of what planning algorithms can do, and it has succeeded. It adds two important features: durative actions,which take time (and may have continuous effects); and objective functions for measuring the quality of plans. The concept of durative actions is flawed; and the treatment of their semantics reveals too strong an attachment to the way many contemporary planners work. Future PDDL innovators should focus on producing a clean semantics for additions to the language, and let planner implementers worry about coupling their algorithms to problems expressed in the latest version of the language.