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

Zach Waldowski zach at waldowski.me
Fri Jan 29 17:14:08 CST 2016

-1. I understand the semantic reasons, but I find it really hard to read
at the call-site. I don't see "&" as exclusively a holdover from C; if
we didn't have it, it'd be an unused sigil that'd probably be used by
something like addressing due to its prevalence in other languages.

Cheers. Zachary Waldowski zach at waldowski.me

On Fri, Jan 29, 2016, at 05:44 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
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