Asymptotic Speedup via Effect Handlers

We study the fundamental efficiency of delimited control. Specifically, we show that effect handlers enable an asymptotic improvement in runtime complexity for a certain class of functions. We consider the generic count problem using a pure PCF-like base language λb and its extension with effect handlers λh. We show that λh admits an asymptotically more efficient implementation of generic count than any λb implementation. We also show that this efficiency gap remains when λb is extended with mutable state. To our knowledge this result is the first of its kind for control operators.