Type-Driven Design

A type is a promise about which states exist. This skill changes what the types allow — it is the only skill in the Rust bucket that restructures a domain model, and it does so with a stopping rule: encode an invariant in a type when violating it is a real bug class in this codebase, not when it is merely expressible. How a rejection is reported is /rust-errors; what a change to a published type costs is /rust-api-design.

The principle

A type that cannot represent the bad state removes the runtime check, the test for it, and the bug report about it. Ask of every struct: how many field combinations are constructible, and how many are valid? The gap between those two numbers is the surface where bugs live.

Enums over flag soup