Error / Defining Errors with enum

// Define a custom error
enum ErrorName: Error {
    case case1
    case case2(detail: String)
}

// LocalizedError: provides user-facing messages
enum ErrorName: LocalizedError {
    case case1

    var errorDescription: String? {
        switch self {
        case .case1: return "Description of the error"
        }
    }
}

// Define a custom error
enum NetworkError: Error {
    case notFound
    case unauthorized
    case serverError(statusCode: Int)
    case timeout(after: Double)
}

// Add user-facing messages via LocalizedError
extension NetworkError: LocalizedError {
    var errorDescription: String? {
        switch self {
        case .notFound:
            return "Page not found (404)"
        case .unauthorized:
            return "Authentication required (401)"
        case .serverError(let code):
            return "A server error occurred (\(code))"
        case .timeout(let seconds):
            return "Request timed out after \(seconds) seconds"
        }
    }
}

// Use the error
func fetchData(url: String) throws -> String {
    guard url.starts(with: "https") else {
        throw NetworkError.unauthorized
    }
    guard url != "https://example.com/missing" else {
        throw NetworkError.notFound
    }
    return "Data fetched successfully"
}

// Handle errors with do-catch
do {
    let data = try fetchData(url: "https://example.com/missing")
    print(data)
} catch let error as NetworkError {
    print("Network error: \(error.localizedDescription)")
} catch {
    print("Other error: \(error)")
}

// Convert to Optional using try?
let result = try? fetchData(url: "https://example.com/api")
print("Result: \(result ?? "Failed")")

Swift error handling uses throws, try, and do-catch together. Errors can be defined as any type conforming to the Error protocol, but enums are the most common choice.

Conforming to LocalizedError lets you retrieve user-facing messages via error.localizedDescription. Swift error handling is not exception-based — it is "error propagation." When an error is thrown, the function exits immediately and the caller is responsible for handling it.

For more on throws / try / do-catch, see throws / try / do-catch.