Swift — Error Handling

Defining errors

enum NetworkError: Error {
    case badURL
    case noData
    case decodingFailed
    case serverError(statusCode: Int)
}

Throwing functions

func fetchData(from urlString: String) throws -> String {
    guard let url = URL(string: urlString) else {
        throw NetworkError.badURL
    }
    // Simulate network request
    return "Data from \(url)"
}

try and catch

do {
    let data = try fetchData(from: "https://example.com")
    print(data)
} catch NetworkError.badURL {
    print("Invalid URL")
} catch NetworkError.serverError(let code) {
    print("Server error: \(code)")
} catch {
    print("Unexpected error: \(error)")
}

try?

let data = try? fetchData(from: "https://example.com")
print(data ?? "No data")

try!

// Use only when you're sure it won't fail
let data = try! fetchData(from: "https://example.com")

Result type

enum Result<Success, Failure: Error> {
    case success(Success)
    case failure(Failure)
}

func divide(_ a: Int, by b: Int) -> Result<Int, Error> {
    guard b != 0 else {
        return .failure(NetworkError.badURL)
    }
    return .success(a / b)
}

switch divide(10, by: 3) {
case .success(let result):
    print("Result: \(result)")
case .failure(let error):
    print("Error: \(error)")
}

Mini Practice

Write Swift code that:

1. Defines a custom error enum
2. Creates a throwing function
3. Uses do-try-catch for error handling
4. Uses try? and Result type

Up Next

In the next lesson, you'll learn about Generics — writing reusable code.