Type Placeholders: New Swift 5.6 Feature

Type placeholders were recently introduced in Swift 5.6. And yes, they are a nice add-on to powerful Swift type inference system. If you are familiar with C++, you must know about an auto keyword. Type placeholders are almost the same.

Generics and type placeholder

let number: _ = 42 // Type placeholder
let anotherNumber = 42

Yes, Swift can infer variable's type, but type placeholders mean to be used for a type with multiple types in it. Generics. That's where they really shine.

Consider regular Result enum

enum Result<Success, Failure> where Failure : Error {
    case success(Success)
    case failure(Failure)
}

And what if we have some kind of complex object

var ohMy = [1: [3: (1, 2, 3, "That's a long tuple")]]

If you will try to create a Result  from ohMy, you'll see compilation error.

let result = Result.success(ohMy)

Bruh. So I need to write...

let result = Result<[Int : [Int : (Int, Int, Int, String)]], Error>.success(ohMy)

Thanks to type placeholders, no. Swift can infer object's type by itself. So, we need to provide Failure type only.

let result = Result<_, Error>.success(ohMy) // Nice

Collections and type placeholder

This feature also useful with collections. What if we need a dictionary with enum keys?

enum Foo {
	case bizz
	case bonk
}

let results = [
	.bizz: ohMy,
	.bonk: ohMy
]

So, let's provide this contextual type, but you remember how ohMy's type is bad-looking? Let's use type placeholder.

// 🚫
let results:[Foo: [Int : [Int : (Int, Int, Int, String)]]] = [
	.bizz: ohMy,
	.bonk: ohMy
]

// ✅
let results:[Foo: _] = [
	.bizz: ohMy,
	.bonk: ohMy
]

More examples

Examples of types containing placeholders are:

Array<_> // array with placeholder element type
[Int: _] // dictionary with placeholder value type
(_) -> Int // function type accepting a single type placeholder argument and returning 'Int'
(_, Double) // tuple type of placeholder and 'Double'
_? // optional wrapping a type placeholder

Final notes

That's a great feature and broadens Swift’s type inference capabilities. For now, it's some kind of less-known, but I think it will be more used in the future.

You can check out other less-known Swift features in my previous post:

Top 7 Subtle Swift Features | Alex Dremov

Here, I collected Swift features that are less known and can be useful when you prepare for interviews or want to deepen your Swift knowledge.

Alex DremovAlex Dremov

References

swift-evolution/0315-placeholder-types.md at main · apple/swift-evolution

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - swift-evolution/0315-placeholder-types.md at main · apple/swift-evolution

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