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

[Tim Schmelter]

-1

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.

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.

--T


On Tue, 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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