Is there a better way to compare errors in Swift?

This suffices for your .expired check, without needing to define ==:

let error: Error = ...
if case AuthenticationError.expired = error {
    print("it's expired")
}

If you want to extract associated data (as in the .webRequestFailed case), you can do this:

if case AuthenticationError.webRequestFailed(error: let innerError) = error {
    print("web request failed due to \(innerError.localizedDescription)")
}

Here's my macOS playground, created in Xcode 9.2:

import Foundation

public enum AuthenticationError: Error {
    case unknownError
    case canceledByUser
    case userOrPasswordMismatch
    case unableToExtractOneTimeCode
    case unableToExchangeOneTimeCodeForToken
    case credentialsUnavailable
    case expired
    case webRequestFailed(error: Error)
}

func test(_ error: Error) {
    if case AuthenticationError.expired = error {
        print("it's expired; error = \(error)")
    } else if case AuthenticationError.webRequestFailed(error: let innerError) = error {
        print("web request failed due to \(innerError.localizedDescription); error = \(error)")
    } else {
        print("no match; error = \(error)")
    }
}

test(AuthenticationError.expired)
test(AuthenticationError.webRequestFailed(error: AuthenticationError.credentialsUnavailable))
test(NSError(domain: NSPOSIXErrorDomain, code: Int(ENOENT), userInfo: [:]))

Details

• Swift 5.1
• Xcode 11.6 (11E708)

Solution

public func == (lhs: Error, rhs: Error) -> Bool {
    guard type(of: lhs) == type(of: rhs) else { return false }
    let error1 = lhs as NSError
    let error2 = rhs as NSError
    return error1.domain == error2.domain && error1.code == error2.code && "\(lhs)" == "\(rhs)"
}

extension Equatable where Self : Error {
    public static func == (lhs: Self, rhs: Self) -> Bool {
        lhs as Error == rhs as Error
    }
}

Usage

enum MyError: Error { case problem, bigProblem, catastrophicException }
enum MyError2: Error, Equatable { case oops, RUUUUUN(where: String) }

var errors = [Error]()
errors.append(MyError.problem)
errors.append(MyError.catastrophicException)
errors.append(MyError2.oops)
errors.append(MyError2.RUUUUUN(where: "I don't know"))
errors.append(MyError2.RUUUUUN(where: "No matter!!!"))
errors.append(NSError(domain: "domain", code: 234, userInfo: nil))
errors.append(NSError(domain: "domain2", code: 345, userInfo: nil))

for i in 0..<errors.count {
    for j in i..<errors.count {
        print("==============================================")
        print("Error1: \(errors[i])")
        print("Error2: \(errors[j])")
        let result = errors[i] == errors[j]
        print("(Error1 == Error2) = \(result ? "✅" : "❌" ) ")
    }
}

Console

==============================================
Error1: problem
Error2: problem
(Error1 == Error2) = ✅ 
==============================================
Error1: problem
Error2: catastrophicException
(Error1 == Error2) = ❌ 
==============================================
Error1: problem
Error2: oops
(Error1 == Error2) = ❌ 
==============================================
Error1: problem
Error2: RUUUUUN(where: "I don\'t know")
(Error1 == Error2) = ❌ 
==============================================
Error1: problem
Error2: RUUUUUN(where: "No matter!!!")
(Error1 == Error2) = ❌ 
==============================================
Error1: problem
Error2: Error Domain=domain Code=234 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: problem
Error2: Error Domain=domain2 Code=345 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: catastrophicException
Error2: catastrophicException
(Error1 == Error2) = ✅ 
==============================================
Error1: catastrophicException
Error2: oops
(Error1 == Error2) = ❌ 
==============================================
Error1: catastrophicException
Error2: RUUUUUN(where: "I don\'t know")
(Error1 == Error2) = ❌ 
==============================================
Error1: catastrophicException
Error2: RUUUUUN(where: "No matter!!!")
(Error1 == Error2) = ❌ 
==============================================
Error1: catastrophicException
Error2: Error Domain=domain Code=234 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: catastrophicException
Error2: Error Domain=domain2 Code=345 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: oops
Error2: oops
(Error1 == Error2) = ✅ 
==============================================
Error1: oops
Error2: RUUUUUN(where: "I don\'t know")
(Error1 == Error2) = ❌ 
==============================================
Error1: oops
Error2: RUUUUUN(where: "No matter!!!")
(Error1 == Error2) = ❌ 
==============================================
Error1: oops
Error2: Error Domain=domain Code=234 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: oops
Error2: Error Domain=domain2 Code=345 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: RUUUUUN(where: "I don\'t know")
Error2: RUUUUUN(where: "I don\'t know")
(Error1 == Error2) = ✅ 
==============================================
Error1: RUUUUUN(where: "I don\'t know")
Error2: RUUUUUN(where: "No matter!!!")
(Error1 == Error2) = ❌ 
==============================================
Error1: RUUUUUN(where: "I don\'t know")
Error2: Error Domain=domain Code=234 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: RUUUUUN(where: "I don\'t know")
Error2: Error Domain=domain2 Code=345 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: RUUUUUN(where: "No matter!!!")
Error2: RUUUUUN(where: "No matter!!!")
(Error1 == Error2) = ✅ 
==============================================
Error1: RUUUUUN(where: "No matter!!!")
Error2: Error Domain=domain Code=234 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: RUUUUUN(where: "No matter!!!")
Error2: Error Domain=domain2 Code=345 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: Error Domain=domain Code=234 "(null)"
Error2: Error Domain=domain Code=234 "(null)"
(Error1 == Error2) = ✅ 
==============================================
Error1: Error Domain=domain Code=234 "(null)"
Error2: Error Domain=domain2 Code=345 "(null)"
(Error1 == Error2) = ❌ 
==============================================
Error1: Error Domain=domain2 Code=345 "(null)"
Error2: Error Domain=domain2 Code=345 "(null)"
(Error1 == Error2) = ✅