[swift-evolution] [Discussion] Replacing Equal Signs with Colons For Attribute Arguments

Erica Sadun erica at ericasadun.com
Tue Feb 16 16:42:11 CST 2016

Thoughts and feedback appreciated. Thank you, -- Erica

Replacing Equal Signs with Colons For Attribute Arguments

Proposal: TBD
Author(s): Erica Sadun <http://github.com/erica>
Status: TBD
Review manager: TBD

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 <https://github.com/DougGregor> for suggesting this enhancement.


Attributes enable developers to annotate declarations and types with keywords that constrain behavior. Recognizable by their at-sign <http://foldoc.org/strudel> "@" 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 @available and @warn_unused_result. 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. Tweaking the grammar to match the rest of Swift introduces a small change that adds consistency across the language.

parameter name: parameter value
 <https://gist.github.com/erica/d96011a5c22d9b995b91#detail-design>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".

 <https://gist.github.com/erica/d96011a5c22d9b995b91#alternatives-considered>Alternatives Considered

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