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Generic protocol-based OO metaprogramming

(Joint work by Rémi Douence and Mario Südholt.)

Virtually all protocol-based OO metaprogramming systems are ad hoc in the sense that they reify only a smaller-or-bigger subset of the underlying run-time system. This is in contrast to a basic premise of a general notion of computational reflection: the meta-level reifies the (i.e. all of the) base system. More general reflective systems - such as Smalltalk's reified run-time system - give up the protocol structure: developping well-structured meta-level applications is much harder in this case.

The goal of the MetaJ project is the development of a generic protocol-based self-applicative interpreter for Java. The basic idea is to develop a generic reification procedure which can be used to reify any class of the interpreter. We expect such an interpreter to provide the following benefits:

• Well-defined semantics. The semantics of meta-level entities can be derived directly from the semantics of the corresponding base-level entity.
• Well-defined metaprogramming protocol. Since a uniform reification procedure is used, the base-level entity can be interpreted as defining implicitly the protocol used to access it from the corresponding meta-level entity.
• Well-defined multi-level meta system. A uniform treatment of multi-level meta-systems similar to Lisp-like systems, such as 3-Lisp, for OO languages.
• Performance optimization. A uniform reification procedure should be amenable particularly well to performance optimization using program specialization techniques.

Currently, we have defined a reification scheme and we have built a reflective interpreter from a non-reflective interpreter for an essential subset of Java.

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A definition of the generic scheme and a brief discussion of the (current state of) the implementation can be found in the following articles:

• R. Douence, M. Südholt: "A generic reification technique for object-oriented reflective languages" in: "Higher-Order and Symbolic Computation", 14(1), Kluwer Academic Publishers, to appear, 2001
  preprint version

• R. Douence, M. Südholt: "Une technique générique de réification pour les langages à objets", 6th International Maghrebian Conference on Computer Science, November 2000

• R. Douence, M. Südholt: "The next 700 reflective object-oriented languages", École des mines de Nantes, technical report, no. 99-1-INFO, 1999