typing-generic-responses

---

name: typing-generic-responses description: Defines generic types for cross-boundary data: ApiResponse, Paginated, Result<T,E>, Repository<T,ID>. Use when designing HTTP responses, use-case return types, or persistence ports. license: MIT

Typing Generic Responses

When to use

• Designing HTTP response shapes
• Defining use-case return types
• Creating persistence ports (Repository interfaces)

Core rules

1. ApiResponse<T> for all HTTP responses (success + error envelope)
2. Result<T,E> for use-case outputs (typed success/error without throwing)
3. Paginated<T> for list endpoint responses
4. Repository<T,ID> generic interface for persistence ports
5. All cross-boundary types are explicit, no any or object

Reference shape (TypeScript)

ApiResponse

export interface ApiResponse<T = unknown> {
  readonly success: boolean;
  readonly data?: T;
  readonly error?: { code: string; message: string; details?: Record<string, unknown> };
  readonly metadata?: { timestamp: string; traceId: string };
}

Result<T,E>

export type Result<T, E extends AppError = AppError> =
  | { readonly success: true; readonly value: T }
  | { readonly success: false; readonly error: E };

export const ok = <T>(value: T): Result<T, never> => ({ success: true, value });
export const err = <E extends AppError>(error: E): Result<never, E> => ({ success: false, error });

Repository<T,ID>

export interface Repository<T, ID> {
  findById(id: ID): Promise<T | null>;
  save(entity: T): Promise<T>;
  deleteById(id: ID): Promise<boolean>;
}

Examples — Do

// Use-case returns Result
async execute(id: string): Promise<Result<User, NotFoundError>> {
  const user = await this.repo.findById(id);
  return user ? ok(user) : err(new NotFoundError('User', id));
}

Examples — Don't

// ❌ No typing
async findUser(id: string): Promise<any> { ... }
// ❌ Direct HTTP response from use-case
return res.status(200).json({ data: user });

Checklist

• ApiResponse<T> used for all HTTP responses
• Result<T,E> used for use-case returns
• Repository<T,ID> generic interface defined
• No any in cross-boundary types

See reference/generic-patterns.md for full patterns.