Why Reliability Demands Functional Programming

Summary

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 enforces them How pattern matching and exhaustiveness checks turn refactors into safe edits Practical modeling patterns for banking and telecom domains in TypeScript and OCaml A migration playbook that juniors and mid-levels can apply today References 1) Reliability starts at the type system Most production incidents are not due to complex algorithms. They are due to the code entering a state that should never have been possible. If you have been on call, you have seen variants of these: Magic strings: "paypal" sneaks into a system that only supports Cash, Card, Pix Nulls: a function expects an email and receives null in a path you forgot to guard Conflicting booleans: an account is both isActive = true and isSuspended = true Incomplete lifecycles: a transaction is marked Pending and then jumps to Reversed without an associated Settled record Functional programming helps by modeling the domain with types that make invalid states unrepresentable. Pure functions and immutability keep behavior predictable and testable. 2) ADTs in practice: sums and products Product types combine fields, think "and". Sum types choose one of several cases, think "or". Together they model your domain rules. Product type example (* OCaml *) type user = { id : int; name : string; email : string option; (* Some "[email protected]" or None *) } // TypeScript type User = Readonly<{ id: number; name: string; email?: string; // we will improve this with Option next }>; Sum type example (* OCaml *) type paym...