On Higher-Order Model Checking of Effectful Answer-Type-Polymorphic Programs

Applying higher-order model checking techniques to programs that use effect handlers is a major challenge, given the recent undecidability result obtained by Dal Lago and Ghyselen. This challenge has been addressed by using answer-type modifications, the use of a monomorphic version of which allows to recover decidability. However, the absence of polymorphism leads to a loss of modularity, reusability, and even expressivity. In this work, we study the problem of defining a calculus that on the one hand supports answer-type polymorphism and subtyping but on the other hand ensures the underlying model checking problem to remain decidable. The solution proposed in this paper is based on the introduction of the polymorphic answer-type whose role is to provide a good compromise between expressiveness and decidability, the latter demonstrated through the construction of a selective type-directed CPS transformation targeting a calculus without effect handlers and any form of polymorphism. Noticeably, the introduced calculus HEPCFATM allows the answer types of effects implemented by tail-resumptive effect handlers to be polymorphic. We also implemented a proof-of-concept model checker for HEPCFATM programs.