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To: Michael Vostrikov <michael.vostrikov@gmail.com>,
 PHP Internals <internals@lists.php.net>
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Date: Sun, 12 Mar 2017 19:01:47 +0100
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Subject: Re: [PHP-DEV] Type variants
From: php@fleshgrinder.com (Fleshgrinder)

On 3/12/2017 6:16 PM, Michael Vostrikov wrote:
> No, thanks) I don't have real problem. I know some problems with usual
> inheritance and try to suggest the tool to solve them. Problems which are
> mostly kinds of Rectangle-Square problem, and where is some restrictions in
> derived types.
> 

As I said already, there is no problem if you just change the direction
of the inheritance.

    class Square {
        private $x;
        private $y;
        private $w;

        public function __construct(int $x, int $y, int $w) {
            $this->x = $x;
            $this->y = $y;
            $this->w = $w;
        }

        public function draw(Canvas $canvas): void { }
    }

    class Rectangle extends Square {
        private $h;

        public function __construct(int $x, int $y, int $w, int $h) {
            parent::__construct($x, $y, $w);
            $this->height = $h;
        }

        public function draw(Canvas $canvas): void { }
    }

If you want to ensure that you always get a Square if width and height
match, no problem either.

    class Square {
        private $x;
        private $y;
        private $w;
        private $h;

        final protected function __construct(
            int $x, int $y, int $w, int $h
        ) {
            $this->x = $x;
            $this->y = $y;
            $this->w = $w;
            $this->h = $h;
        }

        public static function new(int $x, int $y, int $w): self {
            return new self($x, $y, $w, $w);
        }

        public function draw(Canvas $canvas): void { }
    }

    class Rectangle extends Square {
        public static function new(
            int $x, int $y, int $w, ?int $h = null
        ): parent {
            if ($h === null || $w === $h) {
                return new parent($x, $y, $w, $w);
            }
            return new self($x, $y, $w, $h);
        }
    }

Added bonus here is the fact that we disabled the possibility for
multiple constructor invocations, and are enforcing constructor argument
invariants (which PHP does not).

-- 
Richard "Fleshgrinder" Fussenegger