On 07/09/2016 11:28, Silvio Marijić wrote:
Hi Stephen,
Cloning is disabled at the moment in implementation because you would end
up with a object that you can not change, so you have no use of that. I'll
change that as soon as we find some good solution for handling that. Your
example is not really clear to me. At what point we should unlock/lock
object based on your example?
what would happen if you tried to clone an immutable object, throw an
error ?
it means that you might have to use reflection to check if the object is
immutable before cloning it. otherwise making a class immutable would be
a BC
DateTimeImmutable does not prevent cloning because immutability is achieved
by encapsulation, and we want to get rid of the need of encapsulation in
our implementation of immutable objects.
Best,
Silvio.
2016-09-07 11:05 GMT+02:00 Stephen Reay <php-li...@koalephant.com>:
(Sorry for any dupes, sent from wrong address originally)
From a developer point of view, I would suggest that a feature should aim
to be as clear to understand with as little “magic" as possible.
If the goal of an immutable class is to allow public properties to be made
read-only, my expectation would be that:
- write access to any public property from outside class context, is an
error.
This seems to be pretty much accepted by everyone
- clone still works as expected
There has been some suggestion that clone $immutableObj should not be
allowed. Unless there is some specific language/engine gain by that, what
is the point of having this behaviour?
Existing built-in immutable classes (like DateTimeImmutable) do not
prevent cloning, so why should this?
- regular cloning from within class method(s) is the suggested way to
provide “create a copy of the object with a new value” functionality.
This example was given before, effectively:
public function withValue($val) {
$clone = clone $this;
$clone->val = $val;
return $clone;
}
On 7 Sep 2016, at 13:57, Michał Brzuchalski <mic...@brzuchalski.com>
wrote:
06.09.2016 9:13 PM "Fleshgrinder" <p...@fleshgrinder.com> napisał(a):
I understand the concerns of all of you very well and it's nice to see a
discussion around this topic. Fun fact, we are not the only ones with
these issues: https://github.com/dotnet/roslyn/issues/159
On 9/6/2016 6:01 PM, Larry Garfield wrote:
How big of a need is it to allow returning $this instead of $clone,
and/or can that be internalized somehow as well? With copy-on-write,
is that really an issue beyond a micro-optimization?
I asked the same question before because I am also unable to answer this
question regarding the engine.
However, for me it is more than micro-optimization, it is about
identity.
final class Immutable {
// ... the usual ...
public function withValue($value) {
$clone = clone $this;
$clone->value = $value;
return $clone;
}
}
$red = new Immutable('red');
$still_red = $red->withValue('red');
var_dump($red === $still_red); // bool(false)
This is a problem in terms of value objects and PHP still does not allow
us operator overloading. A circumstance that I definitely want to
address in the near future.
But the keyword copy-on-write leads me to yet another proposal, actually
your input led me to two new proposals.
# Copy-on-Write (CoW)
Why are we even bothering on finding ways on making it hard for
developers while the solution to our problem is directly in front of us:
PHP Strings!
AFAIK CoW in case of objects would be impossible to implement.
Every place in a PHP program refers to the same string if that string is
the same string. In the second someone mutates that string in any way
she gets her own mutated reference to that string.
That's exactly how we could deal with immutable objects. Developers do
not need to take care of anything, they just write totally normal
objects and the engine takes care of everything.
This approach also has the advantage that the return value of any method
is (as always) up to the developers.
(Cloning is disabled and results in an error as is because it makes no
sense at all.)
# Identity
This directly leads to the second part of my thoughts and I already
touched that topic: identity. If we have two strings their binary
representation is always the same:
var_dump('string' === 'string'); // bool(true)
This is the exact behavior one wants for value objects too. Hence,
immutable objects should have this behavior since they identify
themselves by their values and not through instances. If I create two
instances of Money with the amount 10 and the Currency EUR then they are
always the same, no matter what. This would also mean that no developer
ever needs to check if the new value is the same as the existing one,
nor does anyone ever has to implement the flyweight pattern for
immutable objects.
A last very important attribute is that it does not matter in which
thread an immutable value object is created because it always has the
same identity regardless of it.
This could easily be achieved by overwriting the object hashes
(spl_object_hash) with something that hashes based on the values, and
predictably across threads (UUIDs?).
# Full Example
<?php
final immutable class ValueObject {
public $value;
public function __construct($value) {
$this->value = $value;
}
public function withValue($value) {
$this->value = $value;
}
}
class A {
public $vo;
public function __construct(ValueObject $vo) {
$this->vo = $vo;
}
}
class B {
public $vo;
public function __construct(ValueObject $vo) {
$this->vo = $vo;
}
}
$vo = new ValueObject(1);
$a = new A($vo);
$b = new B($vo);
var_dump($a->vo === $b->vo); // bool(true)
$a->vo->withValue(2);
var_dump($a->vo === $b->vo); // bool(false)
$a->vo->withValue(1);
var_dump($a->vo === $b->vo); // bool(true)
// :)
?>
--
Richard "Fleshgrinder" Fussenegger
--
PHP Internals - PHP Runtime Development Mailing List
To unsubscribe, visit: http://www.php.net/unsub.php
--
PHP Internals - PHP Runtime Development Mailing List
To unsubscribe, visit: http://www.php.net/unsub.php
