MetaFJig: a meta-circular composition language for Java-like classes

We propose a Java-like language where class definitions are first class values and new classes can be derived from existing ones by exploiting the full power of the language itself, used on top of a small set of primitive composition operators, instead of using a fixed mechanism like inheritance. Hence, compilation requires to perform (meta-)reduction steps, by a process that we call compile-time execution. This approach differs from meta-programming techniques available in mainstream languages since it is meta-circular, hence programmers are not required to learn new syntax and idioms. Compile-time execution is guaranteed to be sound (not to get stuck) by a lightweight technique, where class composition errors are detected dynamically, and conventional typing errors are detected by interleaving typechecking with meta-reduction steps. This allows for a modular approach, that is, compile-time execution is defined, and can be implemented, on top of typechecking and execution of the underlying language. Moreover, programmers can handle errors due to composition operators. Besides soundness, our technique ensures an additional important property called meta-level soundness, that is, typing errors never originate from (meta-)code in already compiled programs.