> In banking, telecom, and payments, reliability is not a nice to have. It is table stakes. The most reliable systems I have worked on reduce entire classes of bugs before the code even runs. Functional programming and Algebraic Data Types (ADTs) let you push correctness into the type system, so illegal states cannot be constructed in the first place. **What you will learn** - How invalid states show up in real systems and why they cause costly incidents - How ADTs encode business rules so the compiler enfo...