Easy methods to use the outcome sort to deal with errors in Swift 5?

Error dealing with fundamentals in Swift

The way in which of dealing with errors modified rather a lot for the reason that first model of Swift. The primary massive milestone occurred in Swift 2, the place Apple utterly revamped error administration. These days you need to 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 End result sort as a built-in generic. First, let me present you all one of the best practices of error dealing with within the Swift programming language, subsequent I am going to present you some cool stuff through the use of outcomes to cope with errors. 🚧

Optionals as error indicators

For easy eventualities you’ll be able to at all times use non-obligatory 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 the event you do not actually care in regards to the underlying sort of the error, this method is okay, however typically issues can get extra sophisticated, 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 if you happen to achieve this, properly… your app will crash. 💥

There are additionally a pair different methods of cease execution course of, however this could possibly be a subject of a standalone submit, 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 through the use of the Error protocol

You may outline your personal error varieties by merely confirming to the built-in Error protocol. Normally most builders use an enum with a view to outline completely different causes. You can too have a customized error message if you happen to 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 sort, however if you happen to achieve this in a operate, you must mark that operate as a throwing operate 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 sort 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 operate above can generate a customized error message. If the divisor is zero it’s going to 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 could possibly be a number of forms of errors associated to permission or file existence, and many others.

Rethrowing Features and Strategies A operate or methodology may be declared with the rethrows key phrase to point that it throws an error provided that considered one of it’s operate parameters throws an error. These features and strategies are often known as rethrowing features and rethrowing strategies. Rethrowing features and strategies should have not less than one throwing operate parameter.

Okay, so a throwing operate can emit completely different error varieties, additionally it might probably propagate all of the parameter errors, however how will 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 operate. 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’ve gotten a do block, the place you’ll be able to attempt to execute your throwing features, if one thing goes fallacious, you’ll be able to deal with the errors in numerous catch blocks. By default an error property is on the market 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 operate that throws an error inside a do-catch block. If the operate triggers some form of error, you’ll be able to put your error dealing with logic contained in the catch block. That is quite simple & easy.

Generally if you happen to do not actually care in regards to the underlying error, you’ll be able to merely convert your throwing operate outcome into an non-obligatory through the use of strive?. With this method you may get a zero outcome if one thing dangerous occurs, in any other case you may get again your common worth as it’s anticipated. Right here is the instance from above through the use of strive?:

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

One other approach is to forestall error propagation through the use of strive!, however you must be extraordinarily cautious with this method, as a result of if the execution of the “tried operate” fails, your software will merely crash. So use provided that you are completely positive that the operate 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 drive strive, however in a lot of the instances it’s best to go on an alternate path with correct error handlers.

Swift errors usually are not exceptions

The Swift compiler at all times requires you to catch all thrown errors, so a scenario 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 often unwind the stack, that may result in reminiscence leaks, however that is not the case with Swift errors. 👍

Introducing the outcome sort

Swift 5 introduces a long-awaited generic outcome sort. Which means error dealing with may be much more easy, with out including your personal outcome implementation. Let me present you our earlier divide operate through the use of End result.

func divide(_ x: Int, by y: Int) -> End result<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 sort in Swift is mainly a generic enum with a .success and a .failure case. You may move 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 sort protected. Throwing features can throw any form 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 need to use exhaustive change blocks to “iterate by” all of the attainable error instances, even with out a default case. So the compiler can maintain you protected, e.g. if you will introduce a brand new error sort inside your enum declaration.

So through the use of the End result sort it is clear that we’re getting again both outcome information or a strongly typed error. It isn’t attainable 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 pricey… an interior closure! A completion handler that accepts a throwing operate, 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 utterly and use an non-obligatory because of this, however on this case we’re again to sq. one. No underlying error sort.

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 properly. It’s best to be aware that each parameters should 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: (End result<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 operate is method higher through the use of the End result sort. In the event you contemplate that a lot of the apps are performing some form of networking, and the result’s often a JSON response, there you have already got to work with optionals (response, information, error) plus you’ve gotten a throwing JSONDecoder methodology… cannot wait the brand new APIs! ❤️

Working with the End result sort in Swift 5

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

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

It is usually attainable to transform again the outcome worth by invoking the get operate.

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 in regards to the End result sort 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’ve got outcome, I simply want for greater kinded varieties or an async / await implementation. 👍