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, if $typescript-best-practices is available, apply it; otherwise use the language's native sum types, narrowing, branding, parsing, and exhaustiveness checks.

• 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. If $principle-boundary-discipline is available, apply it for where to put validation; otherwise validate once where data enters and trust the parsed type inside.
• 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. If $principle-encode-lessons-in-structure is available, apply it; otherwise prefer generated types, lint rules, or canonical helpers over repeated textual instructions.