An Effect System for Algebraic Effects and Handlers

We present an effect system for algebraic effects and handlers. Because handlers may transform an effectful computation into a pure one, the effect system is non-monotone in the sense that effects do not just accumulate, but may also be deleted from types or generally transformed. We also provide denotational semantics for the effect system, based on a domain-theoretic model with partial equivalence relations. The semantics validates equational reasoning about effectful computations.

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