Mutable<T>

<?hh // sort-of

trait ImmutableHelper {
  private function makeMutableCopy(T $object): Mutable<T> {
    $mutable_t = clone $object;
    /* ... insert reflection magic here to make all properties mutable ... */
    return $mutable_t;
  }

  private function makeImmutable(Mutable<T> $object): T { /* similar to above */ }

  protected function modified((function(Mutable<T>)):mixed) $callback): T {
    $mutable = $this->makeMutableCopy($this);
    $callback($mutable);
    return $this->makeImmutable($mutable);
  }
}

class MyImmutableThing {
  use ImmutableHelper;
  
  public immutable ?string $foo = null;
  // ...

  public function withFoo(string $foo): MyImmutableThing {
    return $this->modified($mutable ==> $mutable->foo = $foo);
  }
  // ...
}

WithProperty

<?hh // sort-of

trait ImmutableHelper {
  protected function withProperty(string $property, mixed $value): this
    $modified = clone $this;
    $ro = new ReflectionObject($modified);
    $rp = $ro->getProperty($property);
    assert($rp->isImmutable());
    $rp->setIsImmutable(false);
    $rp->setValue($value);
    $rp->setIsImmutable(true);
    return $modified;
  }
}

class MyImmutableThing {
  use ImmutableHelper;
  
  public immutable ?string $foo = null;
  // ...

  public function withFoo(string $foo): MyImmutableThing {
    return $this->withProperty('foo', $foo);
  }
  // ...
}