parse-boundary

The Principle

Parse, don't validate.

External data (API requests, file reads, env vars) is unknown. Parse it ONCE at the boundary into typed data. Internal code trusts the types completely.

Boundary Examples

// HTTP request boundary
const handleRequest = (req: Request) =>
  Effect.gen(function* () {
    const body = yield* Effect.tryPromise(() => req.json());
    const data = yield* Schema.decodeUnknown(CreateUserSchema)(body);
    return yield* createUser(data); // data is typed
  });

// Environment boundary
const Config = Effect.gen(function* () {
  const raw = { apiKey: process.env.API_KEY, port: process.env.PORT };
  return yield* Schema.decodeUnknown(ConfigSchema)(raw);
});

// File read boundary
const loadConfig = (path: string) =>
  Effect.gen(function* () {
    const content = yield* readFile(path);
    const json = yield* Effect.try(() => JSON.parse(content));
    return yield* Schema.decodeUnknown(ConfigSchema)(json);
  });

Internal Code

// Internal function - trusts types, no parsing
const createUser = (data: CreateUserData) =>
  Effect.gen(function* () {
    const repo = yield* UserRepository;
    const id = yield* generateId();
    return yield* repo.save({ ...data, id });
  });

No Schema.decode inside business logic - types are trusted.