Implement property hooks via virtual properties

[thekid]

This pull request adds support for property hooks to all PHP versions

Example

This example calculates and caches the full name from first and last names

class User {
  private string $full;
  private string $first { set { $field= $value; unset($this->full); } }
  private string $last { set { $field= $value; unset($this->full); } }
  public string $fullName => $this->full??= $this->first.' '.$this->last;

  public function __construct(string $first, string $last) {
    $this->first= $first;
    $this->last= $last;
  }
}

$u= new User('Timm', 'Friebe');
$u->fullName; // "Timm Friebe"

Implementation

The above results in the following being emitted:

class User {
  private string $full;
  private function __set_first($value) { $this->__virtual['first']= $value; unset($this->full); }
  private function __set_last($value) { $this->__virtual['last']= $value; unset($this->full); }
  public function __get_fullName() { return $this->full??= $this->first.' '.$this->last; }

  public function __construct(string $first, string $last) {
    $this->first= $first;
    $this->last= $last;
  }

  private $__virtual= ['first' => null, 'last' => null, 'fullName' =>  null];

  public function __set($name, $value) {
    switch ($name) {
       case 'first': /* Check omitted for brevity */ $this->__set_first($value); break;
       case 'last': /* Check omitted for brevity */ $this->__set_last($value); break; 
       default: throw new \Error('Unknown property '.$name);
    }
  }

  public function __get($name) {
    switch ($name) {
       case 'fullName': return $this->__get_fullName(); break;
       default: throw new \Error('Unknown property '.$name);
    }
  }
}

• Although the code could be inlined within __set and __get, exceptions raised would print incorrect line numbers in their stack trace!
• The check for private and protected properties uses debug_backtrace() to verify the calling scope.

Shortcomings

• Currently, property modifiers are unchecked, the __set and __get functions effectively make them public, we would need to use debug_backtrace() for this! addressed in ed5003f
• The syntax for invoking parent selectors parent::$x::get() is not implemented yet to be rewritten to parent::__get_x() as the former is ambiguous, see RFC addressed in f21a615, see comment below
• We always create a backing property as seen in the above case for fullName, where it wouldn't be necessary.
• Overwriting properties with properties with hooks will not work correctly due to the nature of this implementation with __set / __get, see Implement property hooks via virtual properties #166 (comment)

See also

• https://wiki.php.net/rfc/property-hooks
• https://github.com/Crell/php-rfcs/blob/master/property-hooks/examples.md
• Implement property hooks syntax ast#45
• Property hooks iluuu1994/php-src#82
• [RFC] Property hooks php/php-src#13455
• https://www.swiftbysundell.com/articles/property-wrappers-in-swift/
• Reusable property hooks #167
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get

[thekid]

On the ambiguity of parent::$propertyName::get()

In current PHP, this means the following:

On the parent class' static property $x, invoke a static method named get

Example

abstract class OS {
  public function exec($command, $arguments= []) { ... } }

class Command {
  public static $OS;

  static fucntion __static() {
    self::$OS= ...
  }
}

class Test extends Command {
  public function run($arguments= []) {
    return parent::$OS::exec('xp', ['test', ...$arguments]);
  }
}

Inside Test, however, self::$OS would suffice, so after carefully thinking about it I agree with the statement in the RFC: As the above code is very rare in the wild and rather contrived, and easily worked around, we feel this edge case is acceptable.

Possible alternative: parent::$propertyName (without ::[set|get]())

Kind	This member	Parent member
Constant	self::CONSTANT	parent::CONSTANT
Static method	self::method()	parent::method()
Static property	self::$property	parent::$property
Instance method	$this->method()	parent::method()
Constructor	never invoked directly	parent::__construct()
Instance property	$this->property	not possible or useful

The most consistent way would be to use parent::$property here. Determining whether to invoke get or set could be done by looking for assignment expression, and only invoking set in those situations.

Possible alternative: $this->propertyName

Inside hooks, we should use $field to reference the property rather than $this->propertyName, which the RFC states is "supported by discouraged". If we drop this translation, we could use it as follows:

class Base {
  public $test { get => 'Test'; }
}

class Child extends Base {
  public $test { get => $this->test.'!'; }
}

$test= (new Child())->test; // "Test!"

See https://gist.github.com/thekid/d83bac22f30e3d3e41425bb703ab1d0b

Possible alternative: parent->propertyName

Same as above, a bit more specific and in line with #164, but would be ambiguous with accessing the property propertyName of a constant named parent:

define('parent', new class() { public $propertyName= 'Test'; }); 

$name= parent->propertyName; // "Test"

This would be inconsistent with parent::<method>() syntax to invoke parent methods and the parent constructor.

Discussion in the original "Property accessors" RFC

An open question is how parent accessors may be invoked. A possible syntax is to use parent::$prop for the parent getter and parent::$prop = $value for the parent setter. As the “static-ness” of properties cannot change, this reference should be unambiguous.

This syntax can't extend to other accessor types though, so those would either have no way to invoke the parent accessor, or invoke it implicitly. For possible guard accessors, it would make sense to automatically chain the accessors (though the order is not entirely obvious). For lazy accessors this wouldn't be possible though.

Other syntax choices like parent::$prop::get() are somewhat ambiguous, for example this syntax looks like a static property access followed by a static method call.

In any case, adding support for parent accessors will be technically non-trivial, because we need to perform a modified-scope property access through a separate syntax.

Source: https://wiki.php.net/rfc/property_accessors

[thekid]

This can be written as return new Block([new Code($check), ...$nodes]); as of PHP 7.4.0, see https://wiki.php.net/rfc/spread_operator_for_array and xp-framework/rfc#343

[thekid]

After much on-again/off-again work, Ilija and I are back with a more polished property access hooks/interface properties RFC. It’s 99% unchanged from last summer; the PR is now essentially complete and more robust, and we were able to squish the last remaining edge cases.

Baring any major changes, we plan to bring this to a vote in mid-March.

https://externals.io/message/122445

[thekid]

More changes, see https://externals.io/message/122445#122583, especially:

Shorthands

The $foo => expression shorthand has been removed. The legal shorthands are now:

public string $foo {
  get => /* evaluates to a value; */
  set => /* assigns this value; */
}

The set shorthand (with =>) now means "write this value instead", see https://wiki.php.net/rfc/property-hooks#short-set

Type and name

On a set hook, the user may specify both a type and name, or neither. (That is, set {} or set (Foo $newFoo). If not specified, it defaults to the type of the property and $value, as before.

Parent

Clarified that the parent::$foo::get() syntax works on a parent property regardless of whether it has hooks

---

The rest of the changes seem to not have any effect on this implementation

[thekid]

Clarified that the parent::$foo::get() syntax works on a parent property regardless of whether it has hooks

This cannot work in this implementation due to us relying on __set and __get, as in the following, the virtual property would never be triggered:

class B {
    public mixed $x;
}
class C extends B {
    public mixed $x {
        set {
            $f = parent::$x::set(...);
            $f($value);
        }
    }
}

$c = new C();
$c->x = 0;

[thekid]

Status: Waiting for PHP RFC to successfully be voted on

[thekid]

The vote for the Property Hooks RFC is now open, see https://externals.io/message/123139

[thekid]

See php/php-src#13455

[thekid]

✅ Now works with both current PHP 8.4 and PHP 8.4 with php/php-src#13455 in place