A much better approach is outside-in. Start with the protocol a client (command line/browser/desktop) will use to communicate with your system and then implemen

A much better approach is outside-in. Start with the protocol a client (command line/browser/desktop) will use to communicate with your system and then implement each req/rpy in logical usage order. And use auto tests to verify functionality. Having a test program running a full end-to-end req/rpy session checking all functionality makes the bug count approach zero. I am maintaining large scale mission critical C++ applications using this approach. With zero production bugs for 5+ years. And yes of course split the implementation into reusable parts as much possible. That is basic Software/System Engineering. Claims that it can't be done in other languages than Lisp is absolutely BS. Linux (for example) is mostly a huge collection of reusable components written in C. It works for any language/environment and is therefore a more fundamental technique.