Complex behavior often resists clean modularization in object-oriented languages. Aspect-oriented programming tackles this problem by providing flexible module boundaries that can span and partition classes. However, in order to achieve this flexibility, valuable modularity mechanisms, such as encapsulation and external composition, are lost. The ability to separately compile or reason about a modular unit is also compromised. We propose that this tradeoff is not necessary and that by expressing aspects as part of our modules, we can restore the lost modular properties while maintaining aspectual features. As a concrete demonstration, we present the main features of Aspectual Collaborations and show how these interact to combine modularity with aspectual behavior. The expressiveness of Aspectual Collaborations, AspectJ and Hyper/J are compared using a challenge problem, allowing us to estimate the effectiveness of the approach.

We present a domain-specific language for specifying recursive traversals of object structures, for use with the visitor pattern. Traversals are traditionally specified as iterations, forcing the programmer to adopt an imperative style, or are hard-coded into the program or visitor. Our proposal allows a number of problems best approached by recursive means to be tackled with the visitor pattern, while retaining the benefits of a separate traversal specification.