SE-0040: Replacing Equal Signs with Colons For Attribute Arguments

Introduction

Attribute arguments are unlike other Swift language arguments. At the call site, they use = instead of colons to distinguish argument names from passed values. This proposal brings attributes into compliance with Swift standard practices by replacing the use of "=" with ":" in this one-off case.

Discussion took place on the Swift Evolution mailing list in the [\[Discussion\] Replacing Equal Signs with Colons For Attribute Arguments](https://forums.swift.org/t/discussion-replacing-equal-signs-with-colons-for-attribute-arguments/1459) thread. Thanks to Doug Gregor for suggesting this enhancement.

Motivation

Attributes enable developers to annotate declarations and types with keywords that constrain behavior. Recognizable by their at-sign "@" prefix, attributes communicate features, characteristics, restrictions, and expectations of types and declarations to the Swift compiler. Common attributes include @noescape for parameters that cannot outlive the lifetime of a call, @convention, to indicates whether a type's calling conventions follows a Swift, C, or (Objective-C) block model, and @available to enumerate a declaration's compatibility with platform and OS versions. Swift currently offers about a dozen distinct attributes, and is likely to expand this vocabulary in future language updates.

Some attributes accept arguments: @attribute-name(attribute-arguments) including, at this time, @available, @warn_unused_result and @swift3_migration. In the current grammar, an equal sign separates attribute argument keywords from values.

introduced=version-number
deprecated=version-number
obsoleted=version-number
message=message
renamed=new-name
mutable_variant=method-name

Using = is out of step with other Swift parameterization call-site patterns. Although the scope of this proposal is quite small, tweaking the grammar to match the rest of Swift introduces a small change that adds consistency across the language.

parameter name: parameter value

Detail Design

This proposal replaces the use of = with : in the balanced tokens used to compose an attribute argument clause along the following lines:

attribute → @ attribute-name attribute-argument-clause<sub>opt</sub>
attribute-name → identifier
attribute-argument-clause → ( balanced-tokens<sub>opt<opt> )
balanced-tokens → balanced-token
balanced-tokens → balanced-token, balanced-tokens
balanced-token → attribute-argument-label : attribute argument-value

This design can be summarized as "wherever current Swift attributes use =, use : instead", for example:

@available(*, unavailable, renamed: "MyRenamedProtocol")
typealias MyProtocol = MyRenamedProtocol

@warn_unused_result(mutable_variant: "sortInPlace")
public func sort() -> [Self.Generator.Element]

@available(*, deprecated, message: "it will be removed in Swift 3.  Use the 'generate()' method on the collection.")
public init(_ bounds: Range<Element>)

Alternatives Considered

There are no alternatives to put forth other than not accepting this proposal.