In object-oriented programming, we write code by paying attention to encapsulation and polymorphism. To make the code easier to maintain, we follow SOLID principles, we isolate the infrastructure code from that of the domain, and we apply tactical patterns from Domain Driven Design (DDD). Yet, if we take a closer look, we can enhance our code's maintainability by incorporating basic functional programming concepts like pure functions, immutability, and idempotence. Let's see how functional programming concepts can be applied with object-oriented languages.