---
name: principle-type-system-discipline
description: "Designs types that make illegal states unrepresentable, brand primitives, and parse external data at boundaries."
disable-model-invocation: true
---

# 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.

- **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.
- **One value for alternatives.** Model exclusive states as one enum or union, not flags or nested optionals. Encode required order and valid inputs in the signature.
- **Make errors branchable.** Return errors callers can branch on. Use the language's optional or result type, not sentinel values.
- **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.