Extending the Swift Result type

Jun 3, 2020 · Follow on Twitter and Mastodon swiftextensions

In this post, we’ll extend Result with extensions that make using it easier in certain situations.

The basics

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

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

Extending Result

First, 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: return false
        case .success: return true

We can now call .isSuccess and .isFailure to get information about the nature of the result.

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

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

We can now call .failureError and .successResult to get the generic, optional values without having to switch over the result.


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 SwiftKit library. You can find the source code here. Feel free to try it out and let me know what you think!

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