The right way to use the outcome kind to deal with errors in Swift 5?


Error dealing with fundamentals in Swift

The best way of dealing with errors modified so much because the first model of Swift. The primary large milestone occurred in Swift 2, the place Apple fully revamped error administration. These days you should use the do, strive, catch, throw, throws, rethrows key phrases as a substitute of coping with nasty NSError pointers, so this was a warmly welcomed addition for the language. Now in Swift 5 we take one other big leap ahead by introducing the Consequence kind as a built-in generic. First, let me present you all the very best practices of error dealing with within the Swift programming language, subsequent I am going to present you some cool stuff by utilizing outcomes to take care of errors. 🚧

Optionals as error indicators

For easy eventualities you’ll be able to at all times use optionally available values, to point that one thing dangerous occurred. Additionally the guard assertion is extraordinarily useful for conditions like this.

let zeroValue = Int("0")! 
let nilValue = Int("not a quantity") 

guard let quantity = Int("6") else {
    fatalError("Ooops... this could at all times work, so we crash.")
}
print(quantity)

In case you do not actually care concerning the underlying kind of the error, this method is okay, however generally issues can get extra difficult, so that you may want some particulars about the issue. Anyway, you’ll be able to at all times cease the execution by calling the fatalError methodology, however for those who achieve this, effectively… your app will crash. 💥

There are additionally a pair different methods of cease execution course of, however this may very well be a subject of a standalone put up, so right here is only a fast cheat sheet of accessible strategies:

precondition(false, "ouch")
preconditionFailure("ouch")
assert(false, "ouch")
assertionFailure("ouch")
fatalError("ouch")
exit(-1)

The important thing distinction between precondition and assertion is that assert will work solely in debug builds, however precondition is evaluated at all times (even in launch builds). Each strategies will set off a deadly error if the situation fails aka. is fake. ⚠️

Throwing errors by utilizing the Error protocol

You may outline your individual error varieties by merely confirming to the built-in Error protocol. Often most builders use an enum so as to outline completely different causes. It’s also possible to have a customized error message for those who conform to the LocalizedError protocol. Now you are able to throw customized errors, simply use the throw key phrase if you would like to lift an error of your kind, however for those who achieve this in a perform, you need to mark that perform as a throwing perform with the throws key phrases. 🤮

enum DivisionError: Error {
    case zeroDivisor
}

extension DivisionError: LocalizedError {
    public var errorDescription: String? {
        change self {
        case .zeroDivisor:
            return "Division by zero is kind of problematic. " +
                   "(https://en.wikipedia.org/wiki/Division_by_zero)"
        }
    }
}

func divide(_ x: Int, by y: Int) throws -> Int {
    guard y != 0 else {
        throw DivisionError.zeroDivisor
    }
    return x / y
}

Nice, so the divide perform above can generate a customized error message. If the divisor is zero it will throw the zeroDivision error case. Now think about the next state of affairs: you are attempting to learn the contents of a file from the disk. There may very well be a number of kinds of errors associated to permission or file existence, and so on.

Rethrowing Capabilities and Strategies A perform or methodology might be declared with the rethrows key phrase to point that it throws an error provided that one in every of it’s perform parameters throws an error. These features and strategies are often called rethrowing features and rethrowing strategies. Rethrowing features and strategies will need to have no less than one throwing perform parameter.

Okay, so a throwing perform can emit completely different error varieties, additionally it will probably propagate all of the parameter errors, however how can we deal with (or ought to I say: catch) these errors?

The do-try-catch syntax

You simply merely must attempt to execute do a throwing perform. So do not belief the grasp, there may be undoubtedly room for attempting out issues! Dangerous joke, proper? 😅

do {
    let quantity = strive divide(10, by: 0)
    print(quantity)
}
catch let error as DivisionError {
    print("Division error handler block")
    print(error.localizedDescription)
}
catch {
    print("Generic error handler block")
    print(error.localizedDescription)
}

As you’ll be able to see the syntax is fairly easy, you could have a do block, the place you’ll be able to attempt to execute your throwing features, if one thing goes flawed, you’ll be able to deal with the errors in several catch blocks. By default an error property is offered inside each catch block, so you do not have to outline one your self by hand. You may nonetheless have catch blocks for particular error varieties by casting them utilizing the let error as MyType sytnax proper subsequent to the catch key phrase. So at all times strive first, do not simply do! 🤪

Variations between strive, strive? and check out!

As we have seen earlier than you’ll be able to merely attempt to name a perform that throws an error inside a do-catch block. If the perform triggers some type of error, you’ll be able to put your error dealing with logic contained in the catch block. That is quite simple & easy.

Generally for those who do not actually care concerning the underlying error, you’ll be able to merely convert your throwing perform outcome into an optionally available by utilizing strive?. With this method you will get a zero outcome if one thing dangerous occurs, in any other case you will get again your common worth as it’s anticipated. Right here is the instance from above by utilizing strive?:

guard let quantity = strive? divide(10, by: 2) else {
    fatalError("This could work!")
}
print(quantity) 

One other approach is to stop error propagation by utilizing strive!, however you need to be extraordinarily cautious with this method, as a result of if the execution of the “tried perform” fails, your software will merely crash. So use provided that you are completely positive that the perform will not throw an error. ⚠️

let quantity = strive! divide(10, by: 2) 
print(quantity)

There are just a few locations the place it is accepted to make use of power strive, however in many of the circumstances you must go on an alternate path with correct error handlers.

Swift errors are usually not exceptions

The Swift compiler at all times requires you to catch all thrown errors, so a state of affairs of unhandled error won’t ever happen. I am not speaking about empty catch blocks, however unhandled throwing features, so you’ll be able to’t strive with out the do-catch companions. That is one key distinction when evaluating to exceptions. Additionally when an error is raised, the execution will simply exit the present scope. Exceptions will normally unwind the stack, that may result in reminiscence leaks, however that is not the case with Swift errors. 👍

Introducing the outcome kind

Swift 5 introduces a long-awaited generic outcome kind. Because of this error dealing with might be much more easy, with out including your individual outcome implementation. Let me present you our earlier divide perform by utilizing Consequence.

func divide(_ x: Int, by y: Int) -> Consequence<Int, DivisionError> {
    guard y != 0 else {
        return .failure(.zeroDivisor)
    }
    return .success(x / y)
}

let outcome = divide(10, by: 2)
change outcome {
case .success(let quantity):
    print(quantity)
case .failure(let error):
    print(error.localizedDescription)
}

The outcome kind in Swift is mainly a generic enum with a .success and a .failure case. You may go a generic worth in case your name succeeds or an Error if it fails.

One main benefit right here is that the error given again by result’s kind protected. Throwing features can throw any type of errors, however right here you’ll be able to see from the implementation {that a} DivisionError is coming again if one thing dangerous occurs. One other profit is that you should use exhaustive change blocks to “iterate via” all of the doable error circumstances, even and not using a default case. So the compiler can hold you protected, e.g. if you’ll introduce a brand new error kind inside your enum declaration.

So by utilizing the Consequence kind it is clear that we’re getting again both outcome knowledge or a strongly typed error. It isn’t doable to get each or neither of them, however is that this higher than utilizing throwing features? Effectively, let’s get asynchrounous!

func divide(_ x: Int, by y: Int, completion: ((() throws -> Int) -> Void)) {
    guard y != 0 else {
        completion { throw DivisionError.zeroDivisor }
        return
    }
    completion { return x / y }
}

divide(10, by: 0) { calculate in
    do {
        let quantity = strive calculate()
        print(quantity)
    }
    catch {
        print(error.localizedDescription)
    }
}

Oh, my expensive… an interior closure! A completion handler that accepts a throwing perform, so we are able to propagate the error thrown to the outer handler? I am out! 🤬

Another choice is that we get rid of the throwing error fully and use an optionally available consequently, however on this case we’re again to sq. one. No underlying error kind.

func divide(_ x: Int, by y: Int, completion: (Int?) -> Void) {
    guard y != 0 else {
        return completion(nil)
    }
    completion(x / y)
}

divide(10, by: 0) { outcome in
    guard let quantity = outcome else {
        fatalError("nil")
    }
    print(quantity)
}

Lastly we’re getting someplace right here, however this time let’s add our error as a closure parameter as effectively. You need to notice that each parameters must be optionals.

func divide(_ x: Int, by y: Int, completion: (Int?, Error?) -> Void) {
    guard y != 0 else {
        return completion(nil, DivisionError.zeroDivisor)
    }
    completion(x / y, nil)
}

divide(10, by: 0) { outcome, error in
    guard error == nil else {
        fatalError(error!.localizedDescription)
    }
    guard let quantity = outcome else {
        fatalError("Empty outcome.")
    }
    print(quantity)
}

Lastly let’s introduce outcome, so we are able to get rid of optionals from our earlier code.

func divide(_ x: Int, by y: Int, completion: (Consequence<Int, DivisionError>) -> Void) {
    guard y != 0 else {
        return completion(.failure(.zeroDivisor))
    }
    completion(.success(x / y))
}

divide(10, by: 0) { outcome in
    change outcome {
    case .success(let quantity):
        print(quantity)
    case .failure(let error):
        print(error.localizedDescription)
    }
}

See? Strongly typed errors, with out optionals. Dealing with errors in asynchronous perform is approach higher by utilizing the Consequence kind. In case you contemplate that many of the apps are performing some type of networking, and the result’s normally a JSON response, there you have already got to work with optionals (response, knowledge, error) plus you could have a throwing JSONDecoder methodology… cannot wait the brand new APIs! ❤️

Working with the Consequence kind in Swift 5

We already know that the outcome kind is mainly an enum with a generic .succes(T) and a .failure(Error) circumstances, however there may be extra that I would like to point out you right here. For instance you’ll be able to create a outcome kind with a throwing perform like this:

let outcome = Consequence {
    return strive divide(10, by: 2)
}

It is usually doable to transform again the outcome worth by invoking the get perform.

do {
    let quantity = strive outcome.get()
    print(quantity)
}
catch {
    print(error.localizedDescription)
}

Additionally there are map, flatMap for remodeling success values plus you may also use the mapError or flatMapError strategies if you would like to remodel failures. 😎


let outcome = divide(10, by: 2) 


let mapSuccess = outcome.map { divide($0, by: 2) } 


let flatMapSuccess = outcome.flatMap { divide($0, by: 2) } 
let mapFailure = outcome.mapError { 
    NSError(area: $0.localizedDescription, code: 0, userInfo: nil)
}

let flatMapFailure = outcome.flatMapError { 
    .failure(NSError(area: $0.localizedDescription, code: 0, userInfo: nil)) 
}

That is it concerning the Consequence kind in Swift 5. As you’ll be able to see it is extraordinarily highly effective to have a generic implementation constructed straight into the language. Now that we now have outcome, I simply want for larger kinded varieties or an async / await implementation. 👍

Related Articles

LEAVE A REPLY

Please enter your comment!
Please enter your name here

Latest Articles