principle-type-system-discipline

Model variants as discriminated unions instead of objects with optional fields. For example, replace `{ completed: boolean; completedAt?: Date }` with `{ kind: 'open' } | { kind: 'done'; at: Date }` so contradictory combinations cannot compile.

Brand semantic primitives so UserId and OrderId are not interchangeable even though both are strings. Use newtypes in Rust, opaque types in Swift, value classes in Kotlin, or branded intersections in TypeScript, validating once at creation.

Yes, the patterns apply to any statically-typed language. It covers enums with payloads in Rust, Swift, and Kotlin, sealed classes in Scala, and ADTs in Haskell and OCaml, with language-specific idioms for exhaustive matching.

Strengthen a type only where partiality appears, such as a runtime assertion, null check, or a throw marking a case the type is too weak to track. If nothing would otherwise panic, keep the plain type rather than pursuing precision for its own sake.

Casts, unsafe coercions, and assertion functions bypass the compiler's proof, creating latent runtime crashes. Instead, validate and narrow at the boundary where untyped data enters, or refine the model so the compiler can prove the fact.