Very valid point. I keep fond memories of Modula-2, which has DEFINITION modules and IMPLEMENTATION modules, such that you can compile the former and the latter

Very valid point. I keep fond memories of Modula-2, which has DEFINITION modules and IMPLEMENTATION modules, such that you can compile the former and the latter seprately. In Modula-2, I can specify an API in its DEFINITION module, and after compiling it, client applications can use such an API without the implementation being ready yet, and still I can compile the client and check if it is free of syntax errors.