Why does `@Namespace` not need a type annotation?

andtie · 2020-07-07T10:13:41.027Z

I have a question regarding the new @Namespace property wrapper from SwiftUI. In the example code, it is shown without a type-annotation. E.g.:

@Namespace var namespace

How is this possible in Swift? How can this be used for your own types / property wrappers?

cukr · 2020-07-07T10:32:54.002Z

there's only one possible type that wrappedValue can be, which is Namespace.ID so there's no ambiguity what type namespace is

@propertyWrapper struct Foo {
    var wrappedValue: Int = 0
}
struct Bar {
    @Foo var test1
    @Foo var test2: Int
    // @Foo var test3: String // error: Property type 'String' does not match that of the 'wrappedValue' property of its wrapper type 'Foo'
}
print(Bar().test1) // prints 0

andtie · 2020-07-07T11:08:18.836Z

Thank you. So this is simply an improvement in the type inference algorithm in Swift 5.3?
With Swift 5.2.4 (Xcode 11.5) I get the error-message Type annotation missing in pattern.

cukr · 2020-07-07T11:35:01.007Z

andtie:

So this is simply an improvement in the type inference algorithm in Swift 5.3?

Yep!

Val · 2020-07-22T18:42:43.227Z

I believe this has been the case from the start (when property wrappers were first introduced).

cukr · 2020-07-22T19:12:58.448Z

Nope!

godbolt.org

Compiler Explorer - Swift (x86-64 swiftc 5.2)

@propertyWrapper struct Foo { var wrappedValue: Int = 0 } struct Bar { @Foo var test1 @Foo var test2: Int }

hborla · 2020-07-22T20:49:37.380Z

What you're seeing here is default initialization of a property wrapper, which allows the wrapped property type to be inferred when possible. The ability to drop the type annotation from a default-initialized property wrapper was introduced in 5.3 (https://github.com/apple/swift/pull/30663), but this was previously supported by adding empty parentheses after the property wrapper attribute (e.g. @Namespace() var namespace). You can support this in your own property wrappers by providing a default initializer.