Higher-Order Aspects in Order

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 pointcuts and advice as 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 (sometimes insufficient) ad-hoc mechanisms, which make aspect-oriented programming more complex. This paper proposes to clarify the situation by introducing explicit 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, and give programmers the means to override this default through explicit level shifting expressions. We implement our proposal as an extension of AspectScheme, and formalize its semantics. 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.