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Expanding Swift Self to class members and value types

Introduction

Within a class scope, Self means "the dynamic class of self". This proposal extends that courtesy to value types and to the bodies of class members by renaming dynamicType to Self. This establishes a universal and consistent way to refer to the dynamic type of the current receiver.

This proposal was discussed on the Swift Evolution list in the [Pitch] Adding a Self type name shortcut for static member access thread.

Motivation

It is common in Swift to reference an instance's type, whether accessing a static member or passing types for unsafe bitcasts, among other uses. You can either specify a type by its full name or use self.dynamicType to access an instance's dynamic runtime type as a value.

struct MyStruct {
    static func staticMethod() { ... }
    func instanceMethod() {
        MyStruct.staticMethod()
        self.dynamicType.staticMethod()
    }
}

Introducing Self addresses the following issues:

  • dynamicType remains an exception to Swift's lowercased keywords rule. This change eliminates a special case that's out of step with Swift's new standards.
  • Self is shorter and clearer in its intent. It mirrors self, which refers to the current instance.
  • It provides an easier way to access static members. As type names grow large, readability suffers. MyExtremelyLargeTypeName.staticMember is unwieldy to type and read.
  • Code using hardwired type names is less portable than code that automatically knows its type.
  • Renaming a type means updating any TypeName references in code.
  • Using self.dynamicType fights against Swift's goals of concision and clarity in that it is both noisy and esoteric.

Note that self.dynamicType.classMember and TypeName.classMember may not be synonyms in class types with non-final members.

Detail Design

This proposal introduces Self, which equates to and replaces self.dynamicType. You will continue to specify full type names for any other use. Joe Groff writes, "I don't think it's all that onerous to have to write ClassName.foo if that's really what you specifically mean."

Alternatives Considered

Not at this time

Acknowledgements

Thanks Sean Heber, Lily Ballard, Joe Groff, Timothy Wood, Becca Royal-Gordon, Andrey Tarantsov, Austin Zheng

Rationale

On April 27, 2016, the core team decided to accept a subset of this proposal.

This proposal had light discussion in the community review process, but the core team heavily debated it. It includes two pieces:

  1. Expanding the existing support for Self to work in value types, and in the bodies of classes.

  2. Replacing the x.dynamicType expression with x.Self, a purely syntactic change that eliminates the “dynamicType” keyword.

The core team has accepted the first half for this proposal. This allows the use of “Self” as shorthand for referring to the containing type (in the case of structs, enums, and final class) or the dynamic type (in the case of non-final classes). Most of the discussion in the core team centered around whether people familiar with the former behavior would be surprised by the (more general) behavior when using it in a class, but they came to agree that this is actually a simple and general model, and a helpful point of consistency.

In contrast, there are still a number of concerns with rebranding x.dynamicType as x.Self. This may (or may not) be the right ultimate direction to go, but it should be split out of this proposal. There is another outstanding proposal that would eliminate the “Type.self” syntax as being necessary, and the core team would like to resolve that discussion before tackling x.dynamicType.