Common Lisp Object System (CLOS)
I’ve encountered Object-Oriented Programming when I was learning Python (which is my first language). I’ve struggled with it because the paradigm is not as obvious as using functions and structures in C. It finally clicked for me when I was doing Android development in Java. Instead of instructions like in imperative programming (and modularizing it with functions), your program is a set of entities talking to each other and collectively achieving the required goal.
I’ve been amazed by the CLOS. When doing OOP with Java, it always feels overly complicated. An IDE is almost necessary to implement your design speedily. CLOS have the leanness of C structures while implementing inheritance and polymorphism so effectively it became transparent. I don’t know the exact implementation details of CLOS, but I feel that it can be easily replicated in C by adding the needed metadata in the structure and doing the needed type checking in the functions.
The elegance of the CLOS has one surprising consequence: After learning it, it is immediately obvious that in most cases, you don’t need it. The primitive types like lists, hash maps, arrays, … are sufficient for most code, but it’s there when OOP is the way to realize a neater implementation of your specs.