PHP: rfc:trailing_comma_in_parameter_list

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====== PHP RFC: Allow trailing comma in parameter list ======
  * Date: 2020-03-26
  * Author: Nikita Popov <nikic@php.net>
  * Status: Implemented
  * Target Version: PHP 8.0
  * Implementation: https://github.com/php/php-src/pull/5306

===== Introduction =====

This RFC proposes to allow an optional trailing comma in parameter lists. This is already supported for argument lists.

Consider the constructor of the [[https://github.com/thephpleague/uri/blob/70e1b5044dbf923146a81a0869a67c71548cc880/src/Uri.php#L283-L302|League\Uri\Uri class]]:

<PHP>
class Uri {
    private function __construct(
        ?string $scheme,
        ?string $user,
        ?string $pass,
        ?string $host,
        ?int $port,
        string $path,
        ?string $query,
        ?string $fragment // <-- ARGH!
    ) {
        ...
    }
}
</PHP>

This constructor has too many parameters to place all of them on one line, at least under conventional coding standards. In such cases [[https://www.php-fig.org/psr/psr-12/#45-method-and-function-arguments|PSR-12]] recommends to use the formatting above instead.

Unfortunately, it is currently not possible to place a trailing comma in the parameter list. This breaks uniformity, and results in larger diffs when a new optional parameter is added. Additionally, it is inconsistent with call-sites, which **do** allow a trailing comma:

<PHP>
new Uri(
    $scheme,
    $user,
    $pass,
    $host,
    $port,
    $path,
    $query,
    $fragment, // <-- Huh, this is allowed!
);
</PHP>

At this point, I'm used to //always// adding a trailing comma to multi-line lists, regardless of what kind of element they contain. I write the comma by default, and then have to go back to remove it upon receiving an error. We should avoid having this kind of arbitrary restriction.

===== Proposal =====

Allow a single optional trailing comma in parameter lists. This includes parameter lists for functions, methods and closures.

===== Prior Art =====

[[rfc:trailing-comma-function-calls|Allow a trailing comma in function calls]] added support for trailing commas in calls.

[[rfc:list-syntax-trailing-commas|Trailing Commas In List Syntax]] was a previous proposal to support optional trailing commas in all list-like structures.

Nothing has substantially changed since those RFCs in technical terms, but I think the aforementioned style for breaking up large parameter lists has become more accepted in the meantime, and features like [[rfc:constructor_promotion|Constructor Property Promotion]] are going to increase its use further.

===== Backward Incompatible Changes =====

None.

===== Vote =====

Voting opened 2020-04-14 and closes 2020-04-28.

<doodle title="Allow trailing comma in parameter lists?" auth="nikic" voteType="single" closed="true">
   * Yes
   * No
</doodle>
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