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1. Strict Discriminated Union with Exhaustive Pattern Matching

This pattern guarantees that every possible state is handled at compile time, eliminating unhandled branch runtime exceptions:

type ApiResponse<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

// Exhaustive compile-time checker
function assertNever(x: never): never {
  throw new Error(`Unhandled union case: ${JSON.stringify(x)}`);
}

function renderState(state: ApiResponse<{ id: number; name: string }>): string {
  switch (state.status) {
    case 'idle':
      return 'Waiting for request trigger...';
    case 'loading':
      return 'Fetching remote resources...';
    case 'success':
      return `Welcome, ${state.data.name} (ID: ${state.data.id})`;
    case 'error':
      return `Failed with error: ${state.error.message}`;
    default:
      // If a new status is added to ApiResponse, this causes a compile-time type error!
      return assertNever(state);
  }
}

2. Deep Readonly Generic Transformation

type DeepReadonly<T> = T extends Function | boolean | number | string
  ? T
  : T extends Array<infer U>
  ? ReadonlyArray<DeepReadonly<U>>
  : { readonly [K in keyof T]: DeepReadonly<T[K]> };

interface AppConfig {
  server: {
    host: string;
    port: number;
    ssl: { enabled: boolean };
  };
  features: string[];
}

const config: DeepReadonly<AppConfig> = {
  server: { host: 'api.domain.com', port: 443, ssl: { enabled: true } },
  features: ['telemetry', 'caching']
};

// config.server.port = 8080; // Compile Error: Cannot assign to 'port' because it is read-only
// config.features.push('auth'); // Compile Error: Property 'push' does not exist on ReadonlyArray