[swift-evolution] [Proposal] Adjusting `inout` Declarations for Type Decoration

Haravikk swift-evolution at haravikk.me
Fri Jan 29 13:55:21 CST 2016

Surely inout isn’t a modifier of the type either, but rather a modifier of the parameter as a whole, not the label specifically. Couldn’t we just allow the label to be omitted for closures such as in your (inout T) -> U example?

I don’t suppose it really matters if we’re considering getting rid of var on parameters (so they’re all immutable by default if not inout), so it could go anywhere, I just think that it’s a bit strange to associate with the type specifically when it’s not really specific to that either IMO.

> On 29 Jan 2016, at 19:40, Dave via swift-evolution <swift-evolution at swift.org> wrote:
> +1
>> On Jan 29, 2016, at 11:37, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> https://github.com/apple/swift-evolution/pull/127 <https://github.com/apple/swift-evolution/pull/127>
>> Adjusting inout Declarations for Type Decoration
>> Proposal: TBD
>> Author(s): Joe Groff <https://github.com/jckarter>, Erica Sadun <http://github.com/erica>
>> Status: TBD
>> Review manager: TBD
>> Introduction
>> The inout keyword indicates copy-in/copy-out argument behavior. In its current implementation
>> the keyword prepands 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 &” thread.
>> 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: 
>> (x: inout T) -> U // => (inout T) -> U
>> It avoids notational similarity with arguments labeled inout, for example:
>> func foo(inOut x: T) // foo(inOut:), type (T) -> Void
>> func foo(inout x: T) // foo(_:), type (inout T) -> Void
>> 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
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