A Formally Verified Validator for Classical Planning Problems and Solutions

In this paper we present a formally verified validator for planning problems and their solutions. We formalise the semantics of a fragment of PDDL (V, ¬, →, = in the preconditions, typing and constants) in the Higher-Order Logic theorem prover Isabelle/HOL. We then construct an efficient plan validator and mechanically prove it correct w.r.t. our semantics. We argue that our approach provides a superior compromise in constructing validators where one can have the best of two worlds: (i) clear and concise semantics w.r.t. which the validator is built thus helping to avoid bugs (unlike existing validators, which we show have bugs) and (ii) an optimised implementation whose performance is competitive with mainstream unverified validators.