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

[Jordan Rose]

+1 from me, but I suggest finding some real-world examples of inout (and UnsafePointer) and showing those, rather than a made-up add1. The fact that we currently require '&' for Unsafe-(immutable)-Pointer might make this a bit less nice than it otherwise would be.

(We've talked about allowing pass-by-value for UnsafePointer, but that has its own pros and cons, and should be discussed separately.)

Jordan


> On Jan 29, 2016, at 14:44, Trent Nadeau via swift-evolution <swift-evolution at swift.org> wrote:
> 
> https://github.com/tanadeau/swift-evolution/blob/master/proposals/00xx-use-inout-at-func-call-site.md <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 <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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