Programmed Strategies for Program Verification

Plover is an automated property-verifier for Haskell programs that has been under development for the past three years as a component of the Programatica project. In Programatica, predicate definitions and property assertions written in P-logic, a programming logic for Haskell, can be embedded in the text of a Haskell program module. Properties refine the type system of Haskell but cannot be verified by type-checking alone; a more powerful logical verifier is needed. Plover codes the proof rules of P-logic, and additionally, embeds strategies and decision procedures for their application and discharge. It integrates a reduction system that implements a rewriting semantics for Haskell terms with a congruence-closure algorithm that supports reasoning with equality. It employs strategies such as structure splitting and case analysis to explore alternative valuations of expressions of type Bool or other finite data types, but these strategies can lead to exponential growth of terms and must be employed cautiously. Plover itself is written in Stratego, which has proven to be a powerful language tool for implementating a verifier. We discuss the design and implementation of some strategies that enable Plover to comprehend Haskell and verify many valid property assertions.