I think the test infrastructure for the book described in that post is just so smart. For those who haven't read the post: the test infrastructure compiles (an

I think the test infrastructure for the book described in that post is just so smart. For those who haven't read the post: the test infrastructure compiles (and runs?) the interpreter code as it exists at the end of each chapter in the book. So not only is the code as a whole guaranteed to compile, but if you follow along correctly, at the end of each chapter you are guaranteed to get something runnable.