Implement readonly properties

[thekid]

See https://wiki.php.net/rfc/readonly_properties_v2:

A readonly property can only be initialized once, and only from the scope where it has been declared. Any other assignment or modification of the property will result in an Error exception.

class Fixture {
  public readonly $prop;
  public function __construct($prop) { $this->prop= $prop; }
}

$f= new Fixture(1);

// Illegal, throws an error
$f->prop++;         

// Returns int(1)
var_dump($f->prop);

[thekid]

The following could be a head-start for an implementation:

class Fixture {
  /* public readonly $prop; */
  private $__readonly= ['prop' => null];

  public function __construct($prop) {
    $this->prop= $prop;
  }

  public function __get($member) {
    if (!array_key_exists($member, $this->__readonly)) {
      trigger_error('Undefined property '.__CLASS__.'::'.$member, E_USER_WARNING);
    }

    return $this->__readonly[$member][0] ?? null;
  }

  public function __set($member, $value) {

    // Illegal reassignment
    if (isset($this->__readonly[$member])) {
      throw new \Error('Cannot modify readonly property '.__CLASS__.'::'.$member);
    }

    // Illegal initialization outside of private scope
    $caller= debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 2)[1];
    $scope= $caller['class'] ?? null;
    if (__CLASS__ !== $scope) {
      throw new \Error('Cannot initialize readonly property '.__CLASS__.'::'.$member.' from '.($scope
        ? 'scope '.$scope
        : 'global scope'
      ));
    }

    // Legal initialization
    $this->__readonly[$member]= [$value];
  }
}

[thekid]

The following could be a head-start for an implementation:

While using this, we could also go ahead and implement typed properties properly in PHP 7.x /cc @Danon

[thekid]

PHP 8.1 support depends on https://github.com/xp-framework/core/pull/278 and https://github.com/xp-framework/ast/pull/30. The only change needed is the following (which is included in 6.8.3):

diff --git a/src/main/php/lang/ast/emit/PHP.class.php b/src/main/php/lang/ast/emit/PHP.class.php
index fb1693a..586a5d9 100755
--- a/src/main/php/lang/ast/emit/PHP.class.php
+++ b/src/main/php/lang/ast/emit/PHP.class.php
@@ -602,7 +602,7 @@ abstract class PHP extends Emitter {
       $meta[DETAIL_ARGUMENTS][]= $param->type ? $param->type->name() : 'var';

       if (isset($param->promote)) {
-        $promoted.= $param->promote.' $'.$param->name.';';
+        $promoted.= $param->promote.' '.$this->propertyType($param->type).' $'.$param->name.';';
         $result->locals[1]['$this->'.$param->name]= new Code(($param->reference ? '&$' : '$').$param->name);
         $result->meta[0][self::PROPERTY][$param->name]= [
           DETAIL_RETURNS     => $param->type ? $param->type->name() : 'var',

However, for full support in PHP 7.x and PHP 8.0, we will need support for virtual properties, see xp-framework/rfc#340

[thekid]

Released in https://github.com/xp-framework/compiler/releases/tag/v6.9.0

[thekid]

Documentation updated in https://github.com/xp-framework/compiler/wiki/Implementation-status