principle-type-system-discipline

What problem does it solve? Codebases in statically-typed languages often model state as bags of optional fields, rely on unchecked casts, and duplicate hand-written types that drift from authoritative schemas, producing runtime failures the compiler could have caught. ## Core Features & Use Cases - Illegal-state elimination: Model variants as sum types (discriminated unions, enums with payloads, sealed classes) so contradictory field combinations cannot compile. - Boundary parsing and branding: Parse external data (JSON, RPC payloads, env vars) into typed models at boundaries and brand semantic primitives like UserId versus OrderId. - Exhaustiveness and schema derivation: Enforce compiler-checked exhaustive matching and derive types from protobuf, OpenAPI, GraphQL, or migration schemas instead of hand-rolling parallel types. - Use Case: While reviewing a TypeScript function signature with { completed: boolean; completedAt?: Date }, apply the skill to remodel it as { kind: 'open' } | { kind: 'done'; at: Date } so invalid states stop compiling. ## Quick Start Ask the agent to review this function signature and apply type system discipline to make illegal states unrepresentable.