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

[Nisse Bergman]

-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.
>> 
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>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
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