[swift-evolution] [Proposal] Use inout at function call sites

[Trent Nadeau]

C# uses its `ref` keyword in both function declarations and call sites (see
https://msdn.microsoft.com/en-us/library/14akc2c7.aspx), and I don't think
people consider that syntax to be penalized or an edge case.

On Fri, Jan 29, 2016 at 8:32 PM, Kevin Ballard via swift-evolution <
swift-evolution at swift.org> wrote:

> -1
>
> I feel like the people who are voting +1 probably don't actually *use*
> inout parameters very often, because it seems very obvious that requiring
> the label "inout" at the function call site is extremely unwieldy. inout
> parameters aren't some weird edge case that we want to penalize, they're a
> perfectly legitimate feature of the language, and they should be relatively
> easy to call.
>
> -Kevin Ballard
>
> On Fri, Jan 29, 2016, at 02:44 PM, Trent Nadeau via swift-evolution wrote:
>
>
> https://github.com/tanadeau/swift-evolution/blob/master/proposals/00xx-use-inout-at-func-call-site.md
>
>
> # Use `inout` at Function Call Sites
>
> * Proposal: TBD
> * Author(s): [Trent Nadeau](http://github.com/tanadeau)
> * Status: TBD
> * Review manager: TBD
>
> ## Introduction
>
> Currently when a function has `inout` parameters, the arguments are passed with the `&` prefix operator. For example:
>
> ```swift
> func add1(inout num: Int) {
>     num += 1
> }
>
> var n = 5
> add1(&n) // n is now 6
> ```
>
> This operator does not fit with the rest of the language nor how the parameter is written at the function declaration. It should be replaced so that `inout` is used in both locations so that the call site above would instead be written as:
>
> ```swift
> add1(inout n) // symmetric and now obvious that n can change
> ```
>
> *Discussion thread TBD*
>
> ## Motivation
>
> The `&` prefix operator is a holdover from C where it is usually read as "address of" and creates a pointer. While very useful in C due to its pervasive use of pointers, its meaning is not the same and introduces an unnecessary syntactic stumbling block from users coming from C. Removing this operator and using `inout` removes this stumbling block due to the semantic change.
>
> This operator is also disconnected from how the function declaration is written and does not imply that the argument may (and likely will) change. Using `inout` stands out, making it clear on first read that the variable may change.
>
> It is also possible that Swift may add Rust-like borrowing in the future. In that case, the `&` symbol would be better used for a borrowed reference. Note that Rust uses the same symbol for declaring a borrowed reference and creating one, creating a nice symmetry in that respect of the language. I think Swift would want to have such symmetry as well.
>
> ## Detailed design
>
> ```
> in-out-expression → inout identifier
> ```
>
> ## Alternatives Considered
>
> Keeping the syntax as it currently is.
>
>
> --
> Trent Nadeau
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>
>
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-- 
Trent Nadeau
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