Is there a better way to compare errors in Swift?

Question

I have this error:

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

I'd like to overload == to compare it but I find I need to repeat the code three ways:

public func == (lhs: Error, rhs: AuthenticationError) -> Bool {    return lhs._code == rhs._code && lhs._domain == rhs._domain }  public func == (lhs: AuthenticationError, rhs: Error) -> Bool public func == (lhs: AuthenticationError, rhs: AuthenticationError) -> Bool 

An usage example is:

if let error = error, error == AuthenticationError.expired {    // do something } 

Is there a better way to do this (with generics) so I don't need to repeat similar code?

Answer 1

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: [:]))