Type-safe and user-friendly error handling in Swift 6

Summary

Learn how to implement user-friendly, type-safe error handling in Swift 6 with structured diagnostics and a hierarchical error model. Swift 6 brings an exciting new feature to the language: typed throws. This change makes error handling in Swift much more type-safe, allowing us to define exactly what kinds of errors a function can throw. It’s a small change on the surface, but it opens the door to writing cleaner, more reliable code.Now, you might be wondering — how do we actually use this in practice? The idea I’m going to share with you came up during a conversation with my wife. She came up with this user-friendly layered error message model, and I turned it into a technique we even ended up using in Toucan, our Swift-based static site generator at Binary Birds.In this post, I’ll show you how this approach works and how you can use it to improve your own Swift projects.A custom error protocolFirst of all, I was never satisfied with the built-in LocalizedError protocol. Sure, its errorDescription property could be used as a user-facing error message, but sometimes it’s more like a riddle, alongside NSError’s domains and codes. Additionally, if we stick to the base Error protocol, it can cause naming convention issues, or you have to write Swift.Error, which is very inconvenient.That’s the reason why I started to define my own protocol, using Error as a base protocol, but with some additional functionalities:public protocol SystemError: Error { // 1. var logMessage: String { get } // 2. var userFriendlyMessage: String { get } // 3. var underlyingErrors: [Error] { get } // 4. func logMessageStack() -> String // 5. func lookup<T: Error>(_ errorType: T.Type) -> T? // 6. func lookup<T: Error, V>(_ t: (T) -> V?) -> V? } Let me explain each point:The log message used by developers to display the error.The user-facing error message for your end users.An array of underlying errors for structured error diagnostics.The backtrace for developers to easily access the error hier...