Reflection and Open Implementations

We review the state-of-the-art of reflection and metaprogramming, prior to our work on partial behavioral reflection and Reflex, and open implementations. The first four sections are dedicated to reflection. Section 1 introduces the concept of reflection and its application to programming languages. Section 2 discusses reflection in the particular context of object-oriented programming languages. Then, since we are interested in addressing issues in the concrete applicability of reflection, we dedicate Section 3 to the structuring and engineering of metalevel architectures, while implementation considerations are dealt with in Section 4. After this comprehensive review of reflection, the last section discusses the related area of open implementations (Section 5). 1 Reflection in Programming Languages This section introduces the concept of reflection and its application to programming languages. What reflection actually means is pretty well embodied in the following explanation of the word reflect: “One meaning of the word reflect is to consider some subject matter. Another is to turn back something (e.g. light or sound). Punning on these two meanings, we get the notion of turning one’s consideration or considering one’s own activities as a subject matter.” [82] This section is structured as follows. Section 1.1 gives a brief historical introduction to the difference between programs and data and concludes with the appealing idea of conceiving programs as data for other programs . Then, the notions of metaprogramming and reflection are defined (Section 1.2). Section 1.3 exposes the seminal experiments in reflection in programming languages, based on the idea of reflective towers. Finally, Section 1.4 discusses characteristics of reflective languages as well as some of these languages. 1 Credits for this historical introduction to the distinction between programs and data go to Julien Vayssière [111].