Kotlin is Multi-Paradigm

No program is 100% pure functional
Push state out to the ‘edges’.
Place application’s core data and logic in immutable constructs (value types).
Represent state as objects with mutable references to immutable core constructs.
Shell: OOP, state, IO, Side effects, GUI
Core: Pure functions, stateless, logic
Use functional parameters to inject functions with side effects
Use functional parameters on the boundary of your shell/core