A Framework for the Verified Transformation of Functional Programs

The compilation of functional programs relies on transformations that simplify their structure while ostensibly preserving their meanings. We argue that the combination of the λProlog language and the Abella interactive theorem-prover provide a natural framework for the verified implementation of such transformations. Underlying this argument is the fact that the transformations are syntax-directed and rule-based, with the important proviso that they pay attention to and also modify the binding structure of programs. The logic of higher-order hereditary Harrop formulas, the HoHH logic for short, is well-suited to formalizing such descriptions especially because of the support it provides for the higher-order representation of syntax. By virtue of the computational interpretation of the HoHH logic embodied in λProlog, these formalizations become implementations of the corresponding transformations. The logic that underlies Abella embeds the HoHH logic and provides a complementary capability for reasoning flexibly and succinctly about the properties of specifications written in the HoHH logic. In this presentation, we will consider typical functional program transformations and show how these twin capabilities can be exploited in their verified implementation; we will especially focus on demonstrating the benefits of a higher-order representation of syntax in both specification and reasoning. We will also discuss an extension to the logic underlying Abella for treating logical relations, a notion that is often needed in semantics preservation arguments.