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

[rintaro ishizaki]

I'm against this proposal.

In Swift, like many other programming languages,
there are several purposes for function calling.

* Definition
    Person(name: "Daniel", age: 28, address: "...")
* Query
    obj.contains(id: "ABC001", inBox: .Blue)
* Operation
    obj.tune(toChannel: "CBS", using: .RCU)

For me, trailing comma for definition is OK:

    Person(
      name: "Daniel",
      age: 28,
      countryCode: "US",
    )

But not for others:

    obj.contains(
      id: "ABC001",
      inBox: .Blue,
    )

    tv.tune(
      toChannel: "CBS",
      using: .RCU,
    )

As an alternative, I propose: "Allow omitting commas in function parameters"
Like Objective-C, but only for multi-line.

# OK
    fn(x: 1, y: 2, z: 3)
    fn(x: 1
        y: 2, z: 3
    )
    fn(x: 1, y: 2
        z: 3
    )
    fn(
        x: 1
        y: 2
        z: 3
    )
    fn(x: 1
        y: 2, z: 3)

# NG
    fn(x: 1 y: 2 z: 3)
    fn(x: 1, y: 2, z: 3,)
    fn(
        x: 1,
        y: 2,
        z: 3,
    )

 - It is easier to re-arrange lines
 - Line-based diffs
 - Easy to read

I don't think it's consistent with other parts in Swift,
but I also don't think function parameters are "list" or "dictionary".
It's more like "Who, what, when, where, why".

Could you please add this in your "Alternatives Considered" list? :)

2016-03-09 7:07 GMT+09:00 Grant Paul via swift-evolution
<swift-evolution at swift.org>:
> ## 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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