Error handling is a fundamental aspect of programming and Swift has continuously improved its approach since its inception. With Swift 6, one of the most notable advancements is typed throws, which allows developers to specify the exact error types that a function can throw. This addition provides better clarity, safer error handling, and improved compiler checks. In this guide, we’ll see how typed throws work, why they matter, and how you can use them effectively in your Swift code.

What is Typed Throws?

Before Swift 6, functions that threw errors had a generic throws keyword, meaning they could potentially throw any error conforming to Error. This approach had limitations:

• Callers had no compile-time guarantee about the types of errors thrown.
• Error handling often requires manual type-checking or casting.
• It was easy to introduce unexpected errors into a function’s output.

Swift 6 introduces typed throws, allowing functions to specify the exact type of error they may throw. The syntax looks like this:

enum NetworkError: Error {
    case badURL
    case timeout
    case noInternet
}

func fetchData(from url: String) throws(NetworkError) -> String {
    guard url.starts(with: "https") else {
        throw NetworkError.badURL
    }
    
    // A timeout error
    throw NetworkError.timeout
}

Here, the fetchData(from:) function explicitly states that it can only throw errors of type NetworkError.

Why Typed Throws is a Big Deal

1. Stronger Type Safety

Previously, throws provided no guarantees about the kind of errors that could be thrown. Developers often relied on documentation or runtime type-checking, which could lead to unexpected issues. Typed throws enforces strict type constraints, reducing ambiguity and improving maintainability.

2. Better Code Documentation

Typed throws make it clear at a glance what kind of errors a function can throw, serving as implicit documentation. This helps other developers understand potential failure cases without diving into function implementations.

3. More Predictable Error Handling

Since the compiler knows exactly what error types a function may throw, catch blocks can be written more predictably:

do {
     let result = try fetchData(from: "http://example.com")
     print("Data received: \(result)")
 } catch let error as NetworkError {
     switch error {
     case .badURL:
         print("Invalid URL")
     case .timeout:
         print("Request timed out.")
     case .noInternet:
         print("No internet connection.")
     }
 }

Here, we no longer need to use generic catch blocks or manually cast the error. The compiler enforces that only NetworkError cases need to be handled.

How Typed Throws Works with Multiple Error Types

Sometimes, a function might need to throw multiple types of errors. Swift 6 allows throwing a union of error types using an or (|) separator:

enum APIError: Error {
    case unauthorized
    case serverError
}

func fetchSecureData(from url: String) throws(NetworkError | APIError) -> String {
    if url == "unauthorized" {
        throw APIError.unauthorized
    } else if url == "timeout" {
        throw NetworkError.timeout
    }
    return "Secure Data"
}

Now, the caller must handle both NetworkError and APIError, making error handling explicit and robust.

When to Use Typed Throws

Typed throws is particularly useful when:

• You want explicit control over the error types that a function can propagate.
• You are designing a well-defined API where predictable error handling is crucial.
• Your codebase follows a strict type-safety policy.

However, for cases where multiple unrelated errors need to be handled, using a generic throws or a common error type might still be preferable.

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Swift 6’s typed throws is a powerful addition to the language, bringing stronger type safety, better error documentation, and more predictable error handling. By enforcing explicit error types, Swift reduces runtime surprises and improves code clarity. As Swift continues to evolve, typed throws is set to become a standard best practice for robust error management.

If you’re migrating to Swift 6, consider refactoring your error-handling logic to leverage typed throws. It’s a small change that can bring significant long-term benefits to your Swift applications.