Rust Is Beyond Object-Oriented, Part 3: Inheritance (2023)

Summary

In this next post of my series explaining how Rust is better off without Object-Oriented Programming, I discuss the last and (in my opinion) the weirdest of OOP’s 3 traditional pillars. It’s not encapsulation, a great idea which exists in some form in every modern programming language, just OOP does it oddly. It’s not polymorphism, also a great idea that OOP puts too many restrictions on, and that Rust borrows a better design for from Haskell (with syntax from C++). No, it’s that third pillar, inheritance, that I am discussing today, that concept that only shows up in OOP circles, causing no end of problems for your code. Unlike encapsulation and polymorphism, Rust does not have any direct analogue. Side note: In this series in general, but especially in this post, I am primarily discussing static OOP languages, like C++ and Java, where interfaces have to be explicit and where classes correspond to different static types. Much of what I write would have to be adapted to apply to more dynamic “duck-typing” styles of OOP like in Python or JavaScript (or Smalltalk), and won’t apply as directly. This series is about why Rust isn’t OOP, and Rust is closer to C++ or Java than to a dynamic language, so this bias makes sense in context. Why do people like inheritance?# I can see why inheritance is so compelling. The entire system of education encourages us to categorize things into neat little hierarchies. Rectangles are a type of shape, and squares are a type of rectangle. Humans are a type of animal, and men and women are types of humans. Inheritance allows us to take this “X is a Y” and express it to a computer. This “is a” relationship is seen as intuitive. As the entire point of OOP is to make programming more intuitive, more like reasoning about the real world, inheritance is a perfect match for it. Just like we reason about the real world with categories and subcategories, we can reason about the world of our program in a similar way. And this allows us to feel smart...