====== PHP RFC: Allow trailing comma in closure use lists ======
  * Version: 0.2
  * Date: 2020-07-01
  * Author: Tyson Andre <tandre@php.net>
  * Status: Implemented
  * Target Version: PHP 8.0
  * Implementation: https://github.com/php/php-src/pull/5793

===== Introduction =====

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

Consider this example:

<PHP>
$longArgs_longVars = function (
    $longArgument,
    $longerArgument,
    $muchLongerArgument,  // Trailing commas were allowed in parameter lists in PHP 8.0
) use (
    $longVar1,
    $longerVar2,
    $muchLongerVar3
) {
   // body
};
$longArgs_longVars(
    $longArgumentValue,
    $obj->longMethodCall(),
    $obj->longPropertyName ?? $longDefault,
);
</PHP>

This closure declaration has too many parameters and closure use variables 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/#7-closures|PSR-12]] recommends using the formatting above instead (PSR-12 predates support for trailing commas in parameter lists).

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

At this point, I'm used to //always// adding a trailing comma to multi-line lists, regardless of what kind of element they contain. We should avoid having this kind of arbitrary restriction.

===== Proposal =====

Allow a single optional trailing comma in closure use lists.

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

[[rfc:trailing_comma_in_parameter_list|Allow trailing comma in parameter list]] added support for trailing commas in parameter lists.

[[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 lists has become more accepted in the meantime.

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

None.

===== Vote =====

Yes/No, requiring 2/3 majority.

Voting starts 2020-07-15 and ends 2020-07-29.

<doodle title="Allow trailing comma in closure use lists" auth="tandre" voteType="single" closed="true">
   * Yes
   * No
</doodle>

===== Changelog =====

0.2: Fix implementation link.

===== Discussion ======

https://externals.io/message/110715 "Permitting trailing commas in closure use() declarations"

https://externals.io/message/110804 "[RFC] Allow trailing comma in closure use list"