Type System Discipline

Make the compiler reject impossible states, mismatched primitives, and unhandled variants. Prefer defining errors and special cases out of existence over proliferating handlers. For TypeScript syntax, see the typescript-best-practices skill.

• Make illegal states unrepresentable. Model variants as sum types (discriminated unions, enums with payloads, sealed classes), not as a bag of optional fields where contradictory combinations compile. { completed: boolean; completedAt?: Date } admits completed: true with no date; model { kind: 'open' } | { kind: 'done'; at: Date } instead. If you can write a comment explaining when a combination of fields is valid, split it into a sum type. Match on it exhaustively, so the compiler fails when a new variant is added without handling.
• Brand semantic primitives. UserId and OrderId are strings underneath but should not be interchangeable. Give each its own type, such as a branded intersection in TypeScript or a newtype elsewhere. Validate once at creation, trust the type downstream. If two arguments share a primitive type but mean different things, brand them.
• External data is untyped until parsed. RPC payloads, JSON, IPC messages, CLI args, config files, environment variables, database rows. Have a parse function at every boundary that turns unstructured input into the typed model. The boundary-discipline principle skill says where to put validation.
• Derive types from authoritative schemas. When a protocol buffer, OpenAPI spec, GraphQL schema, database migration, design-token file, or another file owns a shape, derive from it instead of hand-rolling a parallel type. See the encode-lessons-in-structure principle skill.
• Strengthen a type only where partiality appears. A runtime assertion, null check, or "this should never happen" throw marks the place a type is too weak. Push that check up into the type, then stop. head of an empty list has no answer, so it takes a non-empty list; sum of an empty list is 0, so it takes the plain one. If nothing would otherwise panic, keep the plain type.