Building Verification Condition Generators by Compositional Extension

AbstractCurrent mechanizations of programming logics are often in the form of verification condition generators. These front ends to a prover translate a program and assertions into conditions that state that the program fulfills its assertions. Traditional verification condition generators are monolithic encapsulations of a programming language's semantics. This makes it hard to build such verification generators when designing a new language, or when extending a language.We propose a more compositional method of building verification condition generators, using ideas from monadic denotational semantics and from generic programming. Our technique allows us to extend an existing verification condition generator to handle new language constructs, but also to add extensions at another level, such as the ability to generate validation traces. We explain the technique through an example, extending a simple while language with a construct for exception handling. This construct not only needs an extension to the logic, but also a change of its structure