PHP: rfc:null-propagation

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====== PHP RFC: Null Propagation Operator ======
  * Version: 0.1
  * Date: 2018-01-23
  * Author: Chris Wright, daverandom@php.net
  * Status: Draft
  * First Published at: https://wiki.php.net/rfc/null-propagation

===== Introduction =====
A common requirement in object-oriented programming is the need to check whether an object is ''NULL'' before attempting access its members. At present in PHP this must be done by breaking the statement into multiple parts, typically assigning the result of an expression to a variable and then branching with an ''if'' statement. This RFC proposes an operator that would allow such checks to be written inline.

The following code sample demonstrates the pattern that must currently be used:

<code php>
class Foo {
    private $bar;
  
    public function __construct(?Bar $bar) {
        $this->bar = $bar;
    }
    
    public function getBar(): ?Bar {
        return $this->bar;
    }
}
  
$foo = new Foo(null);
$bar = $foo->getBar();

// We must check that $bar is not null before attempting access its members
if ($bar !== null) {
    $result = $bar->method();
} else {
    $result = null;
}
</code>

===== Proposal =====

This proposal allows a question mark ''?'' to precede any object instance member access ''->''. This would cause the engine to check that the expression on the left of the member access does not evaluate to ''NULL'', if it does then no error will be emitted, the remainder of the expression on the right will be short-circuited and the expression will evaluate to ''NULL''. Using the same definitions from the Introduction:

<code php>
$foo = new Foo(null);
$result = $foo->getBar()?->method();
var_dump($result); // NULL
</code>

This operator does not suppress any errors that are generated by the left hand side of the expression, it only short-circuits the right hand side.

===== Examples in other languages =====
  * [[https://docs.hhvm.com/hack/operators/null-safe|Hack]]
  * [[https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-conditional-operators|C# and Visual Basic]]
  * [[https://github.com/tc39/proposal-optional-chaining|ECMAScript]] (Stage 1 proposal)
  * [[http://mitrev.net/ruby/2015/11/13/the-operator-in-ruby/|Ruby]]

===== Backward Incompatible Changes =====
None

===== Proposed PHP Version(s) =====
PHP 7.3

===== RFC Impact =====
==== To Opcache ====
TODO

===== Open Issues =====
Make sure there are no open issues when the vote starts!

===== Proposed Voting Choices =====
As this is a language change, a 2/3 majority is required.

===== Patches and Tests =====
TODO: Patch required

===== Implementation =====
TODO

===== References =====
Links to external references, discussions or RFCs
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