The separation of concerns, as a conceptual tool, enables us to manage the complexity of the software systems that we develop. There have been a number of approaches aimed at modularizing software around the natural boundaries of the various concerns, including subject-oriented programming, composition filters, aspect-oriented programming, and our own view-oriented programming. The growing body of experiences in using these approaches has identified a number of fundamental issues such as what is a concern, what is an aspect, which concerns are inherently separable, and which aspects are composable. To address these issues, we need to focus on the semantics of separation of concerns, as opposed to the mechanics (and semantics) of aspect-oriented software development methods. We propose a conceptual framework based on a transformational view of software development. Our framework affords us a unified view of the different aspect-oriented development techniques which enables us a simple expression for the separability issue.
