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WWDC 2016: Increased Safety in Swift 3.0

Matt Mathias

Since its release, Swift has emphasized safety, and optionals are an important part of Swift’s approach. They provide a mechanism for representing nil, and require a definite syntax for working with an instance that may be nil.

Optionals come in two forms:

  1. Optional
  2. ImplicitlyUnwrappedOptional

The first is the safe kind: it requires the developer to unwrap the optional in order to access the underlying value. The second is the unsafe kind: developers can directly access the underlying value without unwrapping the optional. Thus, working with ImplicitlyUnwrappedOptionals can result in a bad access if the underlying value is nil.

This problem can be summarized like so:

let x: Int! = nil
print(x) // Crash! `x` is nil!

Swift 3.0 changes the semantics of ImplicitlyUnwrappedOptional, and is even safer than its previous incarnations. What has changed in Swift 3.0 for this type that improves the language’s safety? The answer has to do with improvements in how the compiler infers type information for ImplicitlyUnwrappedOptional.

The Old Way: Swift 2.x

Let’s take a look at an example to understand the change.

struct Person {
    let firstName: String
    let lastName: String

    init!(firstName: String, lastName: String) {
        guard !firstName.isEmpty && !lastName.isEmpty else {
            return nil
        }
        self.firstName = firstName
        self.lastName = lastName
    }
}

We have a struct Person with a failable initializer. If we do not supply a person with a first and last name, then the initializer fails.

The initializer is declared with a ! and not a ?: init!(firstName: String, lastName: String).
We use init! here simply to make a point of how ImplicitlyUnwrappedOptionals work in Swift 2.x vs. Swift 3.0. init! should be used sparingly regardless of the version of Swift you are using. Generally speaking, you’ll want to use init! when you want accesses to the resulting instance to lead to a crash if the instance is nil.

In Swift 2.x, this initializer would yield an ImplicitlyUnwrappedOptional<Person>. If the initializer failed, then accessing the underlying Person instance would generate a crash.

For example, in Swift 2.x the following would crash:

// Swift 2.x

let nilPerson = Person(firstName: "", lastName: "Mathias")
nilPerson.firstName // Crash!

Notice that we don’t have to use optional binding or chaining to try to access a value on nilPerson because it was implicitly unwrapped by the initializer.

The New Way: Swift 3.0

Things are different with Swift 3.0. The ! in init! indicates that the initialization process can fail, and if it doesn’t, that the resulting instance may be forced (i.e., implicitly unwrapped). Unlike in Swift 2.x, instances resulting from init! are Optionals and not ImplicitlyUnwrappedOptionals. That means you will have to employ optional binding or chaining to access the underlying value.

// Swift 3.0

let nilPerson = Person(firstName: "", lastName: "Mathias")
nilPerson?.firstName

In this reprise of the example above, nilPerson is now an Optional<Person>. Thus, nilPerson needs to be unwrapped if we want to access its value. The usual machinery for optional unwrapping is appropriate here.

Safety and Type Inference

This change may feel unintuitive. Why is the initializer, which was declared with init!, creating a regular Optional? Doesn’t that ! at the end of init mean that it should create an ImplicitlyUnwrappedOptional?

The answer depends upon the relationship between being safe and being declarative. Remember that the code above (namely: let nilPerson = Person(firstName: "", lastName: "Mathias")) relied upon the compiler to infer the type of nilPerson.

In Swift 2.x, the compiler would infer nilPerson to be ImplicitlyUnwrappedOptional<Person>.
We got used to this, and it made some sense. After all, the initializer at play above was declared with init!.

Nonetheless, this isn’t quite safe. We never explicitly declared that nilPerson should be an ImplicitlyUnwrappedOptional. And it’s not great that the compiler inferred unsafe type information.

Swift 3.0 solves this problem by treating ImplicitlyUnwrappedOptionals as Optional unless we explicitly declare that we want an ImplicitlyUnwrappedOptional.

Curtailing the Propagation of ImplicitlyUnwrappedOptional

The beauty of this change is that it curtails the propagation of implicitly unwrapped optionals. Given our definition for Person above, consider what we would expect in Swift 2.x code:

// Swift 2.x

let matt = Person(firstName: "Matt", lastName: "Mathias")
matt.firstName // `matt` is `ImplicitlyUnwrappedOptional<Person>`; we can access `firstName` directly
let anotherMatt = matt // `anotherMatt` is also `ImplicitlyUnwrappedOptional<Person>`

anotherMatt is created with the exact same type information as matt. You may have expected that to be the case, but it isn’t necessarily desirable. The ImplicitlyUnwrappedOptional has propagated to another instance in our code base. There is yet another line of potentially unsafe code that we have to be careful with.

For example, what if there were something asynchronous about the code above (a stretch, I know…)?

// Swift 2.x

let matt = Person(firstName: "Matt", lastName: "Mathias")
matt.firstName // `matt` is `ImplicitlyUnwrappedOptional<Person>`, and so we can access `firstName` directly
... // Stuff happens; time passes; code executes; `matt` is set to nil
let anotherMatt = matt // `anotherMatt` has the same type: `ImplicitlyUnwrappedOptional<Person>`

In this contrived example, anotherMatt is nil, which means any direct access to its underlying value will lead to a crash. But this sort of access is exactly what ImplicitlyUnwrappedOptional encourages. Wouldn’t it be better if anotherMatt’s type was Optional<Person>?

That is exactly the case in Swift 3.0!

// Swift 3.0

let matt = Person(firstName: "Matt", lastName: "Mathias")
matt?.firstName // `matt` is `Optional<Person>`
let anotherMatt = matt // `anotherMatt` is also `Optional<Person>`

Since we do not explicitly declare that we want an ImplicitlyUnwrappedOptional, the compiler infers the safer Optional type.

Type Inference Should be Safe

The main benefit in the change is that type inference no longer automatically makes our code less safe. If we choose to not be safe, then we should have to be explicit about it. The compiler should not make that choice for us.

If we want to use an ImplicitlyUnwrappedOptional for whatever reason, then we still can. We just have to explicitly declare that we want to be unsafe.

// Swift 3.0

let runningWithScissors: Person! = Person(firstName: "Edward", lastName: "") // Must explicitly declare Person!
let safeAgain = runningWithScissors // What's the type here?

runningWithScissors is nil because the initializer was given an empty String for lastName.

Notice that we declared runningWithScissors to be an ImplicitlyUnwrappedOptional<Person>. If we want to run through the house with scissors in both hands, then Swift will let us. But Swift wants us to be explicit about it; we must specifically declare that we know we want an ImplicitlyUnwrappedOptional.

Thankfully, however, the compiler does not infer safeAgain to be an ImplicitlyUnwrappedOptional. Instead, the compiler dutifully hands us a very safe Optional<Person>. Swift 3.0 seeks to curtail the unwitting propagation of unsafe code by default.

The Future

ImplicitlyUnwrappedOptional now behaves more similarly to the intent behind its purpose: it exists to facilitate how we interact with APIs for whom nullability is meaningful or the return type cannot be known. Both of these are often true in Objective-C, which Swift needs in order to do work on macOS or iOS.

But pure Swift seeks to eschew these problems. Thanks to the changes to ImplicitlyUnwrappedOptional, we are now in an even better position to transcend them. Imagine: a future without ImplicitlyUnwrappedOptionals.

Further Information

If you’re curious to read more, then see this proposal for further information. You’ll see how the proposal’s authors thought through this issue, and get a little more detail on the change. There is also a link to a thread where the community discussed the proposal.

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