principle-type-system-disciplinelisted

# Type system discipline Use the type checker as a proof assistant. Model invariants so invalid states cannot be constructed, rather than spreading runtime guards through the system. ## Patterns - **Model variants explicitly.** Prefer sum types, discriminated unions, sealed classes, or enums with payloads over bags of booleans and optional fields. - **Construct valid values.** A non-empty list is a head plus a tail. A valid range is a start plus a validated non-negative duration. Use an opaque or branded duration so raw negative numbers cannot construct the illegal case. - **Distinguish semantic primitives.** Use branded, opaque, newtype, value-class, or phantom types when two strings or numbers must not be interchangeable. - **Parse at boundaries.** JSON, RPC, IPC, CLI args, configuration, environment values, and database rows are untyped until validated into the domain model. - **Do not lie to the compiler.** Replace casts and unchecked assertions with validation, narrowing, or a better model. - **Exhaust every variant.** Configure matching so a new case fails compilation at each incomplete consumer. - **Derive from the authority.** Generate or infer types from protocol, OpenAPI, GraphQL, database, and design-token schemas instead of duplicating shapes. - **Strengthen only where partiality appears.** Extra precision is valuable when it removes a crash or assertion, not when it only adds ceremony. ## Review questions - Can contradictory field combinati