better-result Adoption

Migrate existing error handling (try/catch, Promise rejections, thrown exceptions) to typed Result-based error handling with better-result.

When to Use

• Adopting better-result in existing codebase
• Converting try/catch blocks to Result types
• Replacing thrown exceptions with typed errors
• Migrating Promise-based code to Result.tryPromise
• Introducing railway-oriented programming patterns

Migration Strategy

1. Start at Boundaries

Begin migration at I/O boundaries (API calls, DB queries, file ops) and work inward. Don't attempt full-codebase migration at once.

2. Identify Error Categories

Before migrating, categorize errors in target code:

3. Migration Order

1. Define TaggedError classes for domain errors
2. Wrap throwing functions with Result.try/tryPromise
3. Convert imperative error checks to Result chains
4. Refactor callbacks to generator composition

Pattern Transformations

Try/Catch to Result.try

// BEFORE
function parseConfig(json: string): Config {
  try {
    return JSON.parse(json);
  } catch (e) {
    throw new ParseError(e);
  }
}

// AFTER
function parseConfig(json: string): Result<Config, ParseError> {
  return Result.try({
    try: () => JSON.parse(json) as Config,
    catch: (e) => new ParseError({ cause: e, message: `Parse failed: ${e}` }),
  });
}

Async/Await to Result.tryPromise

// BEFORE
async function fetchUser(id: string): Promise<User> {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new ApiError(res.status);
  return res.json();
}

// AFTER
async function fetchUser(id: string): Promise<Result<User, ApiError | UnhandledException>> {
  return Result.tryPromise({
    try: async () => {
      const res = await fetch(`/api/users/${id}`);
      if (!res.ok) throw new ApiError({ status: res.status, message: `API ${res.status}` });
      return res.json() as Promise<User>;
    },
    catch: (e) => (e instanceof ApiError ? e : new UnhandledException({ cause: e })),
  });
}

Null Checks to Result

// BEFORE
function findUser(id: string): User | null {
  return users.find((u) => u.id === id) ?? null;
}
// Caller must check: if (user === null) ...

// AFTER
function findUser(id: string): Result<User, NotFoundError> {
  const user = users.find((u) => u.id === id);
  return user
    ? Result.ok(user)
    : Result.err(new NotFoundError({ id, message: `User ${id} not found` }));
}
// Caller: yield* findUser(id) in Result.gen, or .match()

Callback Hell to Generator

// BEFORE
async function processOrder(orderId: string) {
  try {
    const order = await fetchOrder(orderId);
    if (!order) throw new NotFoundError(orderId);
    const validated = validateOrder(order);
    if (!validated.ok) throw new ValidationError(validated.errors);
    const result = await submitOrder(validated.data);
    return result;
  } catch (e) {
    if (e instanceof NotFoundError) return { error: "not_found" };
    if (e instanceof ValidationError) return { error: "invalid" };
    throw e;
  }
}

// AFTER
async function processOrder(orderId: string): Promise<Result<OrderResult, OrderError>> {
  return Result.gen(async function* () {
    const order = yield* Result.await(fetchOrder(orderId));
    const validated = yield* validateOrder(order);
    const result = yield* Result.await(submitOrder(validated));
    return Result.ok(result);
  });
}
// Error type is union of all yielded errors

Defining TaggedErrors

See references/tagged-errors.md for TaggedError patterns.

Workflow

1. Check for source reference: Look for opensrc/ directory - if present, read the better-result source code for implementation details and patterns
2. Audit: Find try/catch, Promise.catch, thrown errors in target module
3. Define errors: Create TaggedError classes for domain errors
4. Wrap boundaries: Use Result.try/tryPromise at I/O points
5. Chain operations: Convert if/else error checks to .andThen or Result.gen
6. Update signatures: Change return types to Result<T, E>
7. Update callers: Propagate Result handling up call stack
8. Test: Verify error paths with .match or type narrowing

Common Pitfalls

• Over-wrapping: Don't wrap every function. Start at boundaries, propagate inward.
• Losing error info: Always include cause/context in TaggedError constructors.
• Mixing paradigms: Once a module returns Result, callers should too (or explicitly .unwrap).
• Ignoring Panic: Callbacks that throw become Panic. Fix the bug, don't catch Panic.

References

• TaggedError Patterns - Defining and matching typed errors
• opensrc/ directory (if present) - Full better-result source code for deeper context