The Swift Programming Language book writes, “In Swift, errors are represented by values of types conforming to the ErrorType protocol…Swift enumerations that adopt ErrorType automatically have the implementation of their conformance synthesized by the compiler.”

The entire documentation currently in the Standard Library for ErrorType outside the book is this:

Which, you know, right?

I did finally get non-enum error working. Thought I’d share.

public struct Error: ErrorType {
    // Required but currently undocumented
    public var _domain: String {return "com.sadun"}
    public var _code: Int {return 0}

    // My custom bits
    var reason: String
    var source: String

    // Initializer only requires a reason
    // The working inline macros are courtesy of Mike Ash
    public init(_ reason: String, 
        source: String = __FUNCTION__, 
        file: String = __FILE__, 
        line: Int = __LINE__) {
            self.reason = reason
            self.source = "Thrown in \(source) (File: \(file) Line: \(line))"
    }
}

You implement the _domain and _code properties. Otherwise the rest of the ErrorType is handled for you by default protocol implementations.

Here’s an example of what this error looks like:

You get more file information (beyond <EXPR>) outside of playgrounds.  Somewhat sadly, in an OCD way, I’m weighing whether to adjust the casing and punctuation to match the results from fatalError():

I’ve also recently written about asynchronous errors.