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

Dany St-Amant dsa.mls at icloud.com
Fri Jan 29 21:57:59 CST 2016

> Le 29 janv. 2016 à 17:44, Trent Nadeau via swift-evolution <swift-evolution at swift.org> a écrit :
> 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

inout vs & doesn’t look that ugly in a simple single argument function, but what if you have many:

getUserData(userid, &username, &groupid, &shell) // Current syntax
getUserData(userid, inout username, inout groupid, inout shell) // Proposal

Yes, for the above one should use something better ( userData=getUserData(userid) ). But, I’m sure there are valid scenario where one wants multiple inout parameters. And such an example must be provided to visualize the impact of moving from & to inout.

Just realizing that the above syntax is without label, even the proposal doesn’t show the use of the inout with labels…
So the current proposal changes:
add(number: &n)
add(inout number: n) // Perfect symmetry
add(number: inout n) // Matching token location

So with my bad example from above changing:
getUserData(id: userid, name: &username, gid: &groupid, shell: &shell)
getUserData(id: userid, inout name: username, inout gid: groupid, inout shell: shell)
getUserData(id: userid, name: inout username, gid: inout groupid, shell: inout shell)

That’s a lot of word, syntax highlighting does help a bit but I do not want to rely on it.

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