Modular Effects in Haskell Through Effect Polymorphism and Explicit Dictionary Applications: A New Approach and the μVeriFast Verifier As a Case Study

In applications with a complex structure of side-effects, effects should be dealt with modularly: components should be programmed against abstract effect interfaces that other components can instantiate as required, and reusable effect patterns should be factored out from the rest of the application. In this paper, we study a new, general approach to achieve this in Haskell by combining effect polymorphism and the recently proposed coherent explicit dictionary applications. We demonstrate the elegance and generality of our approach in μVeriFast: a Haskell-based reimplementation of the semi-automatic separation-logic-based verification tool VeriFast. This implementation features a complex interplay of advanced side-effects: a backtracking search of program paths with angelic and demonic non-determinism, interaction with an underlying off-the-shelf SMT solver, and mutable state that is either backtracked or not during the search. Our use of effect polymorphism improves over the current non-modular implementation of VeriFast, allows us to nicely factor out the backtracking search pattern as a new AssumeAssert monad, and enables advanced features involving effects, such as the non-intrusive addition of a graphical symbolic debugger based on delimited continuations.