A Lightweight Approach to Customizable Composition Operators for Java-like Classes

We propose a formal framework for extending a class-based language, equipped with a given class composition mechanism, to allow programmers to define their own derived composition operators. These definitions can exploit the full expressive power of the underlying computational language.The extension is obtained by adding meta-expressions, that is, (expressions denoting) class expressions, to conventional expressions. Such meta-expressions can appear as class definitions in the class table.Extended class tables are reduced to conventional ones by a process that we call compile-time execution, which evaluates these meta-expressions.This mechanism poses the non-trivial problem of guaranteeing soundness, that is, ensuring that the conventional class table, obtained by compile-time execution, is well-typed in the conventional sense.This problem can be tackled in many ways. In this paper, we illustrate a lightweight solution which enriches compile-time execution by partial typechecking steps. Conventional typechecking of class expressions only takes place when they appear as class definitions in the class table. With this approach, it suffices to introduce a unique common type code for meta-expressions, at the price of a later error detection.