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Date: Tue, 6 May 2008 16:21:59 +0200
Reply-To: Marcus Boerger <helly@php.net>
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To: "John Carter -X (johncart - PolicyApp Ltd at Cisco)" <johncart@cisco.com>, 
	"Derick Rethans" <derick@php.net>
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Subject: Re: [PHP-DEV] Class Properties in Interfaces?
From: helly@php.net (Marcus Boerger)

Hello John,

  the main reason really is storage. If you allow storage in interfaces
then you get multiple inheritance with its virtual inheritance (diamond
style inheritance) issue. That however onlly applies to plain attributes
and not properties which are usually understood as attributes that result
in function calls (getter & setter). That said, if PHP had properties, PHP
could also allow peroperties in interfaces. So the actual question shoul
dbe why we do not support properties. To give an idea how properties could
look like:

interface Coordinate {
  public $coord = __get => getCoord, __set => setCoord,
                 __isset => hasCoord, __unset => delCoord;
  public getCoord();
  public setCoord($val);
  public hasCoord();
  public delCoord();
}

class MyCoord implements Coordinate {
  private $_coord = array(0, 0);  // actual storage for $coord
  public __construct($x, $y) {
    $this->coord = array($x, $y); // calls setCoord
  }
  public getCoord() {
    return $this->_coord;
  }
  public setCoord($val) {
    if (!is_array($val) || count($val) != 2
    || !is_numeric($val[0]) || !is_numeric($val[1])) {
      throw new UnexpectedValueException('Array(x,y) of floats expected');
    }
    $this->_coord = $val;
  }
  public hasCoord() {
    return isset($this->_coord[0]) || isset($this->_coord[1]);
  }
  public delCoord() {
    $this->_coord = array(0, 0);
  }
}

This tells me two things:
a) Pretty damn complex and in that kind of the opposite of PHP's usual KISS
approach (Keep It Simple Safe).

b) Pretty sexy as it gives a hell lot of control and you can document
everything. Check out all the minor details and think of what that would
allow.

My conclusion is that even it looks sexy, it might be a bit tooooo complex.

A much shorter thing to do might be:

interface Coord {
  abstract $coord;
}

This would just somehow make sure the attribute $this->coord will be
accessible as a public attribute in the implementing class. The same rules
as for methods would apply.

Derick and me discussed solutions around abstract/virtual attributes a lot
in the passed. They were all refused however. But what we had in mind also
was more about specifying attributes in an abstract way in classes so that
they become documentable while being still handled through __get() and
friends.

marcus

Tuesday, April 29, 2008, 5:51:30 PM, you wrote:

> The article explicitly mentions OOP interfaces in a few languages. But
> the article, or for that matter any formal definition of an interface
> isn't really what I asked about:
>  
> My question was simply: why can't interfaces have properties?
>  
> John.

> ________________________________

> From: Nathan Nobbe [mailto:quickshiftin@gmail.com] 
> Sent: 29 April 2008 16:17
> To: John Carter -X (johncart - PolicyApp Ltd at Cisco)
> Cc: internals@lists.php.net
> Subject: Re: Re: [PHP-DEV] Class Properties in Interfaces?


> On Tue, Apr 29, 2008 at 6:28 AM, John Carter -X (johncart - PolicyApp
> Ltd at Cisco) <johncart@cisco.com> wrote:


>         
>         I think I must be missing something here, but this sounds a
> little
>         tautological - "we can't do it because it doesn't make sense.
> This is
>         because it doesn't make sense"
>         
>         Certainly most people (myself included) consider interfaces to
> be
>         methods only, but I can't find a reason why not, and a
>         search-on-wikipedia-instead-of-doing-proper-research appears to
> allow
>         it:
> http://en.wikipedia.org/wiki/Interface_%28computer_science%29


> the problem with that article, as it pertains to this conversation is
> that it is referring to interfaces as a general concept.  'interfaces'
> can be formed in many ways, via extension of an abstract class or even
> an expected set of a parameters in an HTTP request...
> getting to the more concrete term 'interface' as it pertains to a
> language keyword, interfaces define a contract; they do not specify
> implementation details.  member variables only facilitate the actions
> member functions expose and therefore, they are part of the
> implementation.

> additionally, one can look to java (and likely others) to see that
> public attributes are not supported on interfaces. here is a definition
> from a java5 cert book;
> "When you create an interface, you're defining a contract for *what* a
> class can do, without saying anything about how the class will do it.
> An interface is a contract."

> -nathan




Best regards,
 Marcus