principle-type-system-discipline

# Type System Discipline The type checker is a proof assistant. Use it to eliminate impossible states, mismatched primitives, and unhandled variants at compile time. A case the types let you ignore becomes a runtime failure the compiler could have stopped. Applies to any typed language. Skills like `typescript-best-practices` ground it in specific syntax. **The patterns:** - **Make illegal states unrepresentable.** Model variants as sum types: discriminated unions in TypeScript, enums with payloads in Rust/Swift/Kotlin, sealed classes in Scala, ADTs in Haskell/OCaml. Don't model state as a bag of optional fields where contradictory combinations compile. A subtle anti-pattern worth naming: `{ completed: boolean; completedAt?: Date }` admits `completed: true; completedAt: undefined`, which is meaningless. Derive the boolean from a single source like `completedAt !== null`, or model the variants explicitly as `{ kind: 'open' } | { kind: 'done'; at: Date }`. If a bug forces the question "wait, can this combination actually happen?", the type is too loose. - **Types are constructions, not restrictions.** Build the type up from the values you want instead of carving them out of a looser type with checks. The invariant that seems to need a refinement type is usually a construction away. A non-empty list is a head plus a rest, not a list with a length check. A valid time range is a start plus a duration, not two timestamps you must keep ordered. No representation is privileged. A...