---
title: "SE-0031: Adjusting `inout` Declarations for Type Decoration"
framework: swift-evolution
role: article
path: swift-evolution/0031-adjusting-inout-declarations
---

# SE-0031: Adjusting `inout` Declarations for Type Decoration

* Proposal: [SE-0031](0031-adjusting-inout-declarations.md)
* Authors: [Joe Groff](https://github.com/jckarter), [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-0031-adjusting-inout-declarations-for-type-decoration/1478)
* Implementation: [apple/swift#1333](https://github.com/apple/swift/pull/1333)

## Introduction

The `inout` keyword indicates copy-in/copy-out argument behavior. In its current implementation the keyword prepends argument names. We propose to move the `inout` keyword to the right side of the colon to decorate the type instead of the parameter label.

*The initial Swift-Evolution discussion of this topic took place in the "[Replace 'inout' with &](https://forums.swift.org/t/pitch-replace-inout-with/652/29)" thread.*

[Thread to Proposal](https://forums.swift.org/t/proposal-adjusting-inout-declarations-for-type-decoration/1239), [Review](https://forums.swift.org/t/review-se-0031-adjusting-inout-declarations-for-type-decoration/1399)

## Motivation

In Swift 2, the `inout` parameter lives on the label side rather than the type side of the colon although the keyword isn't modifying the label but its type. Decorating types instead of labels offers identifiable advantages:

* It enables the `inout` keyword to properly integrate into full type syntax, for example:

```swift     (x: inout T) -> U // => (inout T) -> U     ```

* It avoids notational similarity with arguments labeled `inout`, for example:

```swift     func foo(inOut x: T) // foo(inOut:), type (T) -> Void     func foo(inout x: T) // foo(_:), type (inout T) -> Void     ```

* Moving it would allow `inout` to be used as a parameter label.  While this   isn't a particularly strong motivation by itself, currently `inout` is the    *only* keyword not allowed as a parameter label in Swift 3.  Removing this   restriction would simplify the language.

* It better matches similar patterns in other languages such as borrowing in Rust, that may be later introduced back to Swift

## Detailed design

``` parameter → external-parameter-name optlocal-parameter-name : type-annotation type-annotation → inout type-annotation ```

## Alternatives Considered

Decorations using `@inout` (either `@inout(T)` or `@inout T`) were considered and discarded