Essential retroactive weaving

Abstract

To help analyze unexpected behaviour, programming language environments and tools are beginning to support high-fidelity recordings of program executions. Such recordings are typically low-level and difficult to work with directly. Debugging and analyzing these recordings is easier and more powerful if it is possible to simulate executing additional code in the past context of the recording. In prior work we proposed retroactive weaving, the process of evaluating aspects as if they were present during a past execution. This concept is intended as a general framework for introducing additional code and defining the semantics of executing it post-hoc. In this paper we express retroactive weaving as a transformation on aspect-oriented programming languages and their semantics. We demonstrate this transformation by applying it to a simple core aspect-oriented language, and through a definitional interpreter illustrate its interactions with first-class function values, mutable state, and external input and output. In particular a key concern of retroactive weavers is maintaining soundness: behaving consistently with the context of the past execution, and failing if missing information makes this impossible. Retroactive weavers may need to include extra isolation or runtime checks to meet this requirement.