functional-typescript-ts-patternlisted

# Functional TypeScript With ts-pattern ## Overview `ts-pattern` supplies the one construct TypeScript lacks: pattern matching over the shape of a value, with an exhaustiveness check the compiler enforces. Everything the core skills say about choice types and state machines becomes easier to express, because matching on nested structure no longer needs a chain of narrowing conditions. It changes how code is written, not what the design is. The types come from [functional-typescript](../functional-typescript/SKILL.md); this pack covers how to consume them. **Check the installed version before writing code.** The shapes below target ts-pattern 5. ## When to use - A TypeScript project that already depends on ts-pattern - Dispatching over a discriminated union, especially on nested fields - Implementing a state machine's transitions - Handling a `Result` or a union of errors Not for: designing the union itself, which is [functional-modeling-with-algebraic-types](../functional-modeling-with-algebraic-types/SKILL.md). ## Core rules 1. Rule. **End every match with `.exhaustive()`.** That is the whole point; `.otherwise()` throws the guarantee away. 2. Default. **Match on the discriminant first**, then on nested structure, so the patterns read in the order a person would ask the questions. 3. Default. **Use `P.when` for guards**, not for business rules that deserve a name. A guard should fit on a line. 4. Default. **Select what you use.** `P.select()` names the part