<div dir="ltr">-1<div><br></div><div>If we agree with the Swift API Guidelines: "Clarity at the point of use is your most important goal", it seems to me that adding trailing commas to a method/function signature reduces clarity significantly, with others on this thread and the previous articulating the supporting reasons.</div><div><br></div><div>As an aside: I also eschew trailing commas in array declarations, but recognized that this convention is well established in other languages, and am resigned to letting the linter handle that case. I would look to do that in this case as well.</div><div><br></div><div>--T</div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Mar 8, 2016 at 2:07 PM, Grant Paul via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">## Introduction<br>
<br>
Right now, Swift argument lists are not permitted to contain trailing commas. To make multi-line calls easier, we propose allowing trailing commas in argument (and tuple) syntax:<br>
<br>
let person = Person(<br>
id: json['id'],<br>
name: json['name'],<br>
picture: Im2age(picture),<br>
friends: friends,<br>
)<br>
<br>
<br>
## Motivation<br>
<br>
It’s common for functions to take a number of arguments. In some languages, this can make it difficult to figure out what a function does, leading to patterns like fluent interfaces and configuration objects.<br>
<br>
Swift, by contrast, handles this very well. Argument labels make sure parameters aren’t confused even when they’re of the same type. And compared to Objective-C, it’s much easier to write a multi-line list of arguments in Swift.<br>
<br>
However, with a parentheses placement style placing the closing parentheses for a multi-line call on a new line, Swift does not support a trailing comma. Trailing commas have a number of benefits:<br>
<br>
- It is easier to re-arrange lines (especially in certain text editors) when all lines have commas.<br>
- Line-based diffs (as used by most source control systems) only show added lines when adding a new parameter to the end, rather than two lines where one just adds the comma.<br>
- It’s more consistent with other Swift lists which do support trailing commas.<br>
<br>
<br>
## Proposed Solution<br>
<br>
The proposed solution is to allow and ignore trailing commas in argument lists and tuples:<br>
<br>
let person = Person(<br>
id: json['id'],<br>
name: json['name'],<br>
picture: Image(picture),<br>
friends: friends,<br>
)<br>
<br>
let tuple = (<br>
color,<br>
32,<br>
)<br>
<br>
<br>
## Detailed Design<br>
<br>
Support for trailing commas in argument lists and tuples would make them consistent with Swift’s handling of array literals, which do support trailing commas:<br>
<br>
let array = [<br>
2,<br>
4,<br>
8,<br>
]<br>
<br>
There should not be any impact to existing code from this proposal.<br>
<br>
Support for this syntax is also found in other programming languages like Python, D, and Hack. It’s been proposed for JavaScript (positive response) and PHP (rejected):<br>
<br>
- JavaScript: <a href="https://jeffmo.github.io/es-trailing-function-commas/" rel="noreferrer" target="_blank">https://jeffmo.github.io/es-trailing-function-commas/</a><br>
- PHP: <a href="https://wiki.php.net/rfc/trailing-comma-function-args" rel="noreferrer" target="_blank">https://wiki.php.net/rfc/trailing-comma-function-args</a><br>
<br>
<br>
## Alternatives Considered<br>
<br>
The main alternative is the existing behavior. This has the benefit of standardizing Swift code on a particular style. However, many people will in practice continue to use a style regardless of support trailing commas, especially for cross-language consistency. It could also lead to JavaScript-inspired parameter ordering:<br>
<br>
let person =<br>
Person(id: json['id']<br>
, name: json['name']<br>
, picture: Image(picture)<br>
, friends: friends)<br>
<br>
Another alternative would be to support this syntax for function parameters but not tuples. However, this would be an arbitrary inconsistency.<br>
<br>
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</blockquote></div><br></div>