Execution levels for aspect-oriented programming

Abstract

In aspect-oriented programming languages, advice evaluation is usually considered as part of the base program evaluation. This is also the case for certain pointcuts, such as if pointcuts in AspectJ, or simply all pointcuts in higher-order aspect languages like AspectScheme. While viewing aspects as part of base level computation clearly distinguishes AOP from reflection, it also comes at a price: because aspects observe base level computation, evaluating pointcuts and advice at the base level can trigger infinite regression. To avoid these pitfalls, aspect languages propose ad-hoc mechanisms, which increase the complexity for programmers while being insufficient in many cases. After shedding light on the many facets of the issue, this paper proposes to clarify the situation by introducing levels of execution in the programming language, thereby allowing aspects to observe and run at specific, possibly different, levels. We adopt a defensive default that avoids infinite regression in all cases, and give advanced programmers the means to override this default using level shifting operators. We formalize the semantics of our proposal, and provide an implementation. This work recognizes that different aspects differ in their intended nature, and shows that structuring execution contexts helps tame the power of aspects and metaprogramming.