[swift-evolution] [Draft] Allow trailing commas in argument lists

[Dave Abrahams]

on Wed Mar 09 2016, Radosław Pietruszewski <swift-evolution at swift.org> wrote:

> JavaScript’s common convention (and weird looking!) to use leading
> commas is exactly because it can’t do trailing commas.

The reason I've seen for leading commas is that to add a new element to
the end of a list, you only need to change one line instead of two.

> Swift already allows trailing commas in arrays. Even if not everyone
> will know or use this, I see zero harm in allowing trailing commas in
> argument lists and tuples.
>
> +1. Because Why Not™.
>
> — Radek
>
>> On 09 Mar 2016, at 13:54, Nisse Bergman via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> -1
>> Let’s not go down this path and enable the javascript flamewar of trailing or non-trailing commas. 
>> 
>> Nisse
>> 
>> 
>> 
>>> On 09 Mar 2016, at 01:23, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> Yes please.
>>> 
>>>> On Mar 8, 2016, at 2:07 PM, Grant Paul via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> ## Introduction
>>>> 
>>>> 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:
>>>> 
>>>>  let person = Person(
>>>>     id: json['id'],
>>>>     name: json['name'],
>>>>     picture: Im2age(picture),
>>>>     friends: friends,
>>>>  )
>>>> 
>>>> 
>>>> ## Motivation
>>>> 
>>>> 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.
>>>> 
>>>> 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.
>>>> 
>>>> 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:
>>>> 
>>>> - It is easier to re-arrange lines (especially in certain text editors) when all lines have commas. 
>>>> - 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.
>>>> - It’s more consistent with other Swift lists which do support trailing commas.
>>>> 
>>>> 
>>>> ## Proposed Solution
>>>> 
>>>> The proposed solution is to allow and ignore trailing commas in argument lists and tuples:
>>>> 
>>>>  let person = Person(
>>>>      id: json['id'],
>>>>      name: json['name'],
>>>>      picture: Image(picture),
>>>>      friends: friends,
>>>>  )
>>>> 
>>>>  let tuple = (
>>>>     color,
>>>>     32,
>>>>  )
>>>> 
>>>> 
>>>> ## Detailed Design
>>>> 
>>>> 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:
>>>> 
>>>>  let array = [
>>>>      2,
>>>>      4,
>>>>      8,
>>>>  ]
>>>> 
>>>> There should not be any impact to existing code from this proposal.
>>>> 
>>>> 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):
>>>> 
>>>> - JavaScript: https://jeffmo.github.io/es-trailing-function-commas/
>>>> - PHP: https://wiki.php.net/rfc/trailing-comma-function-args
>>>> 
>>>> 
>>>> ## Alternatives Considered
>>>> 
>>>> 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:
>>>> 
>>>>  let person =
>>>>      Person(id: json['id']
>>>>           , name: json['name']
>>>>           , picture: Image(picture)
>>>>           , friends: friends)
>>>> 
>>>> Another alternative would be to support this syntax for function parameters but not tuples. However, this would be an arbitrary inconsistency.
>>>> 
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> 
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-- 
-Dave