principle-type-system-discipline

---
name: principle-type-system-discipline
description: "Apply when designing types, reviewing a function signature, or writing code in any statically-typed language. Make illegal states unrepresentable, brand semantic primitives, parse external data at boundaries, exhaust variants, derive from authoritative schemas."
---

# 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 examples, invoke /typescript-best-practices.

- **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 /principle-boundary-discipline 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 /principle-encode-lessons-in-structure.
- **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. The type system's job is to track the cases each use site must handle, not to describe the data as precisely as possible. Prefer total functions. `sum` of an empty list is 0, so it takes the plain list. `head` of an empty list has no answer, so it demands the non-empty one. If nothing would otherwise panic, keep the plain type.