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Replacing Equal Signs with Colons For Attribute Arguments


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 thread. Thanks to Doug Gregor for suggesting this enhancement.


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.


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.