On 09/02/2016 09:06 AM, Silvio Marijić wrote:
Well at the moment expection is thrown in case when you try to clone
immutable object. But you do seem to have valid point there regarding
__clone method. I'm definitely going to give it a thought.
Best,
Silvio.
2016-09-02 15:52 GMT+02:00 André Rømcke <andre.rom...@ez.no>:
On Sep 2, 2016, at 09:10 , Silvio Marijić <marijic.sil...@gmail.com>
wrote:
Hi Fleshgrinder,
Since Michal answered most of the questions, I'll just add some notes.
Initially I added restrictions to abstract classes, but I did think about
that over the last couple of days and couldn't find any concrete reason
for
that restriction, so I think I'm going to remove that. As far as cloning,
it is disabled for immutable objects, because you'll end up with the copy
of object that you can not modify. I did mention in Cons sections that
cloning is disabled, maybe it should be made more clear.
_If_ there are use-cases for it, wouldn’t it also be safe that the clone
is allowed to be modified during __clone() and afterwards sealed? Like in
__construct().
And if you don’t want to allow cloning, throw in __clone.
Best,
André
I'd have to agree here. I love the idea of "lockable" immutable
objects. However, the __clone() method has to be a modifiable area just
like __construct() or else it's effectively useless for anything more
than a trivial object.
This was one of the main concerns with immutability in the PSR-7
discussions. Consider this sample class, with 8 properties (entirely
reasonable for a complex value object):
immutable class Record {
public $a;
public $b;
public $c;
public $d;
public $e;
public $f;
public $g;
public $h;
public function __construct($a, $b, $c, $d, $e, $f, $g, $h) {
$this->a = $a;
$this->b = $b;
$this->c = $c;
$this->d = $d;
$this->e = $e;
$this->f = $f;
$this->g = $g;
$this->h = $h;
}
}
Now I want a new value object that is the same, except that $d is
incremented by 2. That is, I'm building up the value object over time
rather than knowing everything at construct time. (This is exactly the
use case of PSR-7.) I have to do this:
$r1 = new Record(1, 2, 3, 4, 5, 6, 7, 8);
$r2 - new Record($r1->a, $r1->b, $r1->c, $r1->d + 2, $1->e, $r1->f,
$r1->g, $r1->h);
That's crazy clunky, and makes immutable objects not very useful.
Imagine a money object where you had to dissect it to its primitives,
tweak one, and then repackage it just to add a dollar figure to it.
That's not worth the benefit of being immutable.
The way PSR-7 addressed that (using fake-immutability, basically), was this:
class Response {
// ...
protected $statusCode;
public function withStatusCode($code) {
$new = clone($this);
$new->statusCode = $code;
return $new;
}
}
That is, outside of the object there's no way to modify it in place, but
it becomes super easy to get a slightly-modified version of the object:
$r2 = $r1->withStatusCode(418);
And because of PHP's copy-on-write support, it's actually surprisingly
cheap.
For language-level immutable objects, we would need some equivalent of
that behavior. I'm not sure exactly what form it should take (explicit
lock/unlock commands is all I can think of off hand, which I dislike),
but that's the use case that would need to be addressed.
--Larry Garfield
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