---
title: "SE-0040: Replacing Equal Signs with Colons For Attribute Arguments"
framework: swift-evolution
role: article
path: swift-evolution/0040-attributecolons
---

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

* Proposal: [SE-0040](0040-attributecolons.md)
* Author: [Erica Sadun](https://github.com/erica)
* Review Manager: [Chris Lattner](https://github.com/lattner)
* Status: **Implemented (Swift 3.0)**
* Decision Notes: [Rationale](https://forums.swift.org/t/accepted-se-0040-replacing-equal-signs-with-colons-for-attribute-arguments/1719)
* Implementation: [apple/swift#1537](https://github.com/apple/swift/pull/1537)

## 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](https://github.com/DougGregor) for suggesting this enhancement.*

## Motivation

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, at this time, `@available`, `@warn_unused_result` and `@swift3_migration`. In the current grammar, an equal sign separates attribute  argument keywords from values.

```swift 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.

```swift 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:

```swift 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:

```swift @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.