====== PHP RFC: Null Coalesce Operator ======
  * Version: 0.2.3
  * Date: 2014-09-06, last updated 2014-09-16
  * Author: Andrea Faulds <ajf@ajf.me>
  * Contributor: Nikita Popov <nikic@php.net> (initial patch)
  * Status: Implemented (PHP 7)
  * First Published at: http://wiki.php.net/rfc/isset_ternary


===== Introduction =====

PHP is a web-focussed programming language, so processing user data is a frequent activity. In such processing it is common to check for something's existence, and if it doesn't exist, use a default value. Yet the simplest way to do this, something along the lines of ''<nowiki>isset($_GET['mykey']) ? $_GET['mykey'] : ""</nowiki>'', is unnecessarily cumbersome. The short ternary operator, ''?:'' provides a way to do this much more conveniently: ''<nowiki>$_GET['mykey'] ?: ""</nowiki>''. However, this is not good practice, as if the value does not exist it will raise an ''E_NOTICE''. Because of these issues, some sort of ifsetor() operator or a modification to ''?:'''s behaviour to make this common pattern easier has been a frequent request (See References).

===== Proposal =====

The coalesce, or ''??'', operator is added, which returns the result of its first operand if it exists and is not NULL, or else its second operand. This means the ''<nowiki>$_GET['mykey'] ?? ""</nowiki>'' is completely safe and will not raise an E_NOTICE. Some examples of its use:

<code php>
// Fetches the request parameter user and results in 'nobody' if it doesn't exist
$username = $_GET['user'] ?? 'nobody';
// equivalent to: $username = isset($_GET['user']) ? $_GET['user'] : 'nobody';

// Calls a hypothetical model-getting function, and uses the provided default if it fails
$model = Model::get($id) ?? $default_model;
// equivalent to: if (($model = Model::get($id)) === NULL) { $model = $default_model; }

// Parse JSON image metadata, and if the width is missing, assume 100
$imageData = json_decode(file_get_contents('php://input'));
$width = $imageData['width'] ?? 100;
// equivalent to: $width = isset($imageData['width']) ? $imageData['width'] : 100;
</code>

It can even be chained:

<code php>
$x = NULL;
$y = NULL;
$z = 3;
var_dump($x ?? $y ?? $z); // int(3)

$x = ["yarr" => "meaningful_value"];
var_dump($x["aharr"] ?? $x["waharr"] ?? $x["yarr"]); // string(16) "meaningful_value"
</code>

This example demonstrates the precedence relative to the ternary operator and the boolean or operator, which is the same as C#:

<code php>
var_dump(2 ?? 3 ? 4 : 5);      // (2 ?? 3) ? 4 : 5        => int(4)
var_dump(0 || 2 ?? 3 ? 4 : 5); // ((0 || 2) ?? 3) ? 4 : 5 => int(4)
</code>

This example demonstrates how it is a short-circuiting operator:

<code php>
function foo() {
    echo "executed!", PHP_EOL;
}
var_dump(true ?? foo()); // outputs bool(true), "executed!" does not appear as it short-circuited
</code>

===== Proposed PHP Version(s) =====

This proposed for the next PHP x, which at the time of this writing would be PHP 7.

===== Patches and Tests =====

A pull request with a working implementation and test, targeting master, is here: https://github.com/php/php-src/pull/824

The original patch was graciously provided by Nikita Popov.

===== Vote =====

As this is a language change, a 2/3 majority is required. A straight Yes/No vote is being held.

Voting started on 2014-09-20 and ended on 2014-09-27.

<doodle title="Approve Null Coalesce Operator RFC and merge patch into master?" auth="ajf" voteType="single" closed="true">
   * Yes
   * No
</doodle>

===== Implementation =====

Merged into master (which will be PHP 7): https://github.com/php/php-src/commit/2d069f640e6cccfa3ba8b1e4f375ade20fb33f64

Documented at: http://php.net/manual/en/language.operators.comparison.php

===== References =====

There have been several previous discussions and proposals about adding an ifsetor operator with similar behaviour, or changing the behaviour of ''?:'', indeed there has been at least one each year:

  * 2004: http://marc.info/?t=108214447100002&r=1&w=2 and http://marc.info/?t=108214551500002&r=1&w=2
  * 2005: http://marc.info/?t=111826997700001&r=1&w=2, http://marc.info/?t=111829868900001&r=1&w=2 and http://marc.info/?t=113069916200003&r=1&w=2
  * 2006: http://marc.info/?t=114669117400001&r=1&w=2
  * 2007: http://marc.info/?l=php-internals&m=118208519027980&w=2
  * 2008: [[rfc:ifsetor|RFC: ifsetor()]] - http://marc.info/?t=120136702900003&r=1&w=2
  * 2009: http://marc.info/?l=php-internals&m=124214617109310&w=2
  * 2011: http://marc.info/?l=php-internals&m=130158916712977&w=2
  * 2013: http://marc.info/?t=138547713500001&r=1&w=2

This list is quite probably incomplete.

Operator precedence in C#: http://msdn.microsoft.com/en-us/library/6a71f45d.aspx

===== Rejected Features =====
Keep this updated with features that were discussed on the mail lists.

===== Errata =====

The name of this RFC [[http://blog.ajf.me/2015-12-07-poorly-named-rfcs|ought to have been "null coalescing operator", not "null coalesce operator"]].

===== Changelog =====

  * (2016-03-13) Added Errata
  * v0.2.3 - Added short-circuit example
  * v0.2.2 - Added precedence example
  * v0.2.1 - Added chaining example
  * v0.2 - Overhauled proposal, proposing new operator ''??'' instead of an extension to ''?:''
  * v0.1 - Created