A Practical Guide to Applying Functional Programming in Kotlin

When introducing functional programming (FP) into practice, abstract theories such as functors and monads can actually become a barrier to entry. This is because side effects are unavoidable in real-world work, and code needs to be at a level that all team members can understand and maintain.

For effective application, 4 practical utilization strategies are needed that gradually increase in difficulty starting from the basics. The first principles to follow are utilizing referential transparency, immutability, and pure functions.

However, in actual backend environments, dependence on external data such as databases or global variables makes referential opacity easy to occur. Also, the approach of directly modifying an object's state carries a high risk of causing unexpected side effects as business logic becomes more complex.

For example, code that directly changes the values of a payment request object can cause data consistency problems as the logic expands. Therefore, in practice, functional techniques should be introduced gradually to prevent these kinds of problems.