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

Kevin Ballard kevin at sb.org
Fri Jan 29 19:32:42 CST 2016


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
> _________________________________________________
> swift-evolution mailing list swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160129/831c34b3/attachment.html>

More information about the swift-evolution mailing list