Define formal semantics for method effects in HPDDL/SHOP2-style HTN methods

Develop a formal, precise semantics for method effects (modeled after SHOP2's assert and retract) in HPDDL-style hierarchical task network methods, including a rigorous specification of when these effects apply relative to method preconditions and subtask execution.

Background

The paper discusses method preconditions and method effects used in some HTN languages (e.g., SHOP2 and HPDDL). While method preconditions are widely used, the authors point out that method effects—akin to SHOP2’s assert/retract—lack a formal semantic treatment in the literature.

Because of this gap, the authors recommend dropping method effects from the core HDDL specification; nonetheless, the absence of a published formal semantics remains an unresolved issue.

References

Method effects are executed in the state in which the method preconditions are evaluated. As far as we know, their formal semantics is not defined in any publication.

— HDDL -- A Language to Describe Hierarchical Planning Problems  (1911.05499 - Höller et al., 2019) in Section 3 (HDDL by Example)