PHP: rfc:trailing_comma_in_parameter_list

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 League\Uri\Uri class:

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

This constructor has too many parameters to place all of them on one line, at least under conventional coding standards. In such cases 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:

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

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.