Functional programming, the idea of state machines and finally how Rust extends structs with functionality. Also experiencing an overengineered internal system that was microserviced, really made me reconsider the whole "microservices are epic" mantra. 'Cause, oh boy was that annoying and difficult to debug.

When I learned functional programming, in my head, classes suddenly became slightly worse versions of partially applied functions.

When I learned about state machines classes also now seems like a failed attempt at achieving the same thing.