Swift Academy · Lesson

Custom Error Types with LocalizedError

Designing rich error enums that conform to Error and LocalizedError.

Custom Error Types with LocalizedError is a free Swift Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome

Well-designed error types carry enough context to diagnose problems and display user-friendly messages. `LocalizedError` provides the hooks for that.

Basic Error Enum

```swift enum AuthError: Error { case invalidCredentials case accountLocked(until: Date) case networkUnavailable } ```

LocalizedError Protocol

```swift extension AuthError: LocalizedError { var errorDescription: String? { switch self { case .invalidCredentials: return "Username or password is incorrect." case .accountLocked(let date): return "Account locked until \(date.formatted())." case .networkUnavailable: return "No internet connection." } } } print(AuthError.invalidCredentials.localizedDescription) ```

failureReason and recoverySuggestion

```swift extension AuthError: LocalizedError { var failureReason: String? { "Authentication service rejected the request." } var recoverySuggestion: String? { "Reset your password or contact support." } } ```

Nested Error Context

```swift enum APIError: Error { case server(statusCode: Int, body: String) case decode(underlying: DecodingError) case timeout } ``` Carry the underlying error so it can be logged or shown in debug builds.

Throwing Custom Errors

```swift func login(user: String, pass: String) throws -> Token { guard !user.isEmpty && !pass.isEmpty else { throw AuthError.invalidCredentials } // ... } ```

Catching Specific Cases

```swift do { let token = try login(user: u, pass: p) } catch AuthError.invalidCredentials { showAlert("Bad credentials") } catch AuthError.accountLocked(let date) { showAlert("Locked until \(date)") } catch { showAlert(error.localizedDescription) } ```

CustomDebugStringConvertible

```swift extension AuthError: CustomDebugStringConvertible { var debugDescription: String { "AuthError.\(self)" } } ```

Error Wrapping Pattern

```swift enum AppError: Error { case auth(AuthError) case api(APIError) case unknown(Error) } func process() throws { do { try login() } catch let e as AuthError { throw AppError.auth(e) } catch { throw AppError.unknown(error) } } ```

Equatable Errors for Testing

```swift enum FetchError: Error, Equatable { case notFound case serverError(code: Int) } XCTAssertEqual(error, FetchError.notFound) ```

Quick Check

Which protocol provides `errorDescription` for user-facing error messages?

Recap

Key takeaways: • Conform errors to `LocalizedError` for user-facing messages • Use associated values to carry context (status codes, dates) • Wrap lower-level errors in domain-specific enums • `Equatable` errors simplify unit testing • `CustomDebugStringConvertible` aids debugging Next: bridging Result to async/await.

Frequently asked questions

Is the “Custom Error Types with LocalizedError” lesson free?

Yes — the full text of “Custom Error Types with LocalizedError” is free to read here on the web, and the Swift Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Swift Academy course, upgrade to CoddyKit PRO.

What will I learn in “Custom Error Types with LocalizedError”?

Designing rich error enums that conform to Error and LocalizedError. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Custom Error Types with LocalizedError” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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