Rewriting Logic Semantics of a Plan Execution Language

TL;DR

This paper formalizes the semantics of NASA's PLEXIL language using rewriting logic in Maude, enabling formal analysis, verification, and identifying design issues through executable specification.

Contribution

It introduces a rewriting logic semantics for PLEXIL in Maude, facilitating formal analysis and revealing design issues.

Findings

A formal rewriting logic semantics for PLEXIL was developed.

The semantics serves as a formal interpreter and benchmark.

Two design issues in PLEXIL were identified using the Maude-based specification.

Abstract

The Plan Execution Interchange Language (PLEXIL) is a synchronous language developed by NASA to support autonomous spacecraft operations. In this paper, we propose a rewriting logic semantics of PLEXIL in Maude, a high-performance logical engine. The rewriting logic semantics is by itself a formal interpreter of the language and can be used as a semantic benchmark for the implementation of PLEXIL executives. The implementation in Maude has the additional benefit of making available to PLEXIL designers and developers all the formal analysis and verification tools provided by Maude. The formalization of the PLEXIL semantics in rewriting logic poses an interesting challenge due to the synchronous nature of the language and the prioritized rules defining its semantics. To overcome this difficulty, we propose a general procedure for simulating synchronous set relations in rewriting logic that is sound and, for deterministic relations, complete. We also report on two issues at the design level of the original PLEXIL semantics that were identified with the help of the executable specification in Maude.