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:
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