To master a lot of the most "hard core" programming concepts, dive into parsers and compilers. It will give you a perspective and generality that will permeate

To master a lot of the most "hard core" programming concepts, dive into parsers and compilers. It will give you a perspective and generality that will permeate the rest of your career. It gives you arguably the most high-level perspective on all things related to programming and the paradigms therein.