20 — Advanced Patterns

Core Concepts

• Branded/opaque types — prevent mixing structurally identical types
• Builder pattern — type-safe fluent API construction
• Type-safe event emitter — events with typed payloads
• Phantom types — type params that exist only at compile time
• Variance — covariance, contravariance, invariance
• Recursive types — self-referencing type definitions
• HKT simulation — higher-kinded types in TypeScript

Why These Patterns

• Branded types prevent userId from being used as orderId
• Builder pattern ensures required fields are set before building
• Phantom types add compile-time safety without runtime cost
• These patterns push TypeScript's type system to its limits