Extending the Swift Result type

Jun 3, 2020 · Follow on Twitter and Mastodon swift extensions

In this post, we’ll extend the Swift native Result type with extensions that make it easier to use it in certain situations.

The basics

Result is a basic enum that consists of a Success and a Failure type. When you have a result instance, you can switch over it like this:

let result = Result<Bool, Error>.success(true)
switch result {
    case .failure(let error): print(error)
    case .success(let result): print(result)
}

While this is easy and the encouraged way to use Result, I’d prefer more convenient tools for working with result values.

Extending Result

I think it would be convenient to quickly check if a Result is a failure or success, without having to switch over it. This is easily implemented with two extensions:

public extension Result {
    
    var isFailure: Bool { !isSuccess }
    
    var isSuccess: Bool {
        switch self {
        case .failure: false
        case .success: true
        }
    } 

We can now use .isSuccess and .isFailure to get this base information about the result.

I also like to have ways to quickly access the result error or success value. Let’s implement this with two additional extensions:

public extension Result {
    
    var failureError: Failure? {
        switch self {
        case .failure(let error): error
        case .success: nil
        }
    }
    
    var successResult: Success? {
        switch self {
        case .failure: nil
        case .success(let value): value
        }
    }
}

We can now call .failureError and .successResult to get the generic, optional success value, or the result error, without having to switch over the result.

Conclusion

This extensions in this post changes how you can work with Result. Since it’s not the way that Swift as a language seems to think you should handle results, use them with caution.

Source Code

I have added these extensions to my SwiftUIKit library. You can find the source code here. Feel free to try it out and let me know what you think!