Dynamic Wind for Effect Handlers

Effect handlers offer an attractive way of abstracting over effectful computation. Moreover, languages with effect handlers usually statically track effects, which ensures the user is aware of all side effects different parts of a program might have. Similarly to exception handlers, effect handlers discharge effects by locally defining their behavior. In contrast to exception handlers, they allow for resuming computation, possibly later and possibly multiple times. In this paper we present a design, formalization, and implementation for a variant of dynamic wind that integrates well with lexical effect handlers. It has well-defined semantics in the presence of arbitrary control effects in arbitrary places. Specifically, the behavior of capturing and resuming continuations in the pre- or postlude is well-defined and respects resource bracketing. We demonstrate how these features can be used to express backtracking of external state and finalization of external resources.