On Complete Categorical Semantics for Effect Handlers

Abstract: Soundness and completeness with respect to equational theories for programming languages are fundamental properties in the study of categorical semantics. However, completeness results have not been established for programming languages with algebraic effects and handlers, which raises a question of whether the commonly used models in the literature, i.e., free model monads generated from algebraic theories, are the only valid semantic models for effect handlers. In this paper, we show that this is not the case. We identify the precise characterizations of categorical models of effect handlers that allow us to establish soundness and completeness results with respect to a certain equational theory for effect handling constructs. Notably, this allows us to capture not only free monad models but also the CPS semantics for effect handlers as models of the calculus.