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Date: Wed, 7 Sep 2016 08:57:16 +0200
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To: PHP Internals List <internals@lists.php.net>
Cc: Larry Garfield <larry@garfieldtech.com>, =?UTF-8?Q?Silvio_Mariji=C4=87?= <marijic.silvio@gmail.com>
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Subject: Re: [PHP-DEV] RFC - Immutable classes
From: michal@brzuchalski.com (=?UTF-8?Q?Micha=C5=82_Brzuchalski?=)

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06.09.2016 9:13 PM "Fleshgrinder" <php@fleshgrinder.com> napisa=C5=82(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 =3D clone $this;
>       $clone->value =3D $value;
>       return $clone;
>     }
>   }
>
>   $red =3D new Immutable('red');
>   $still_red =3D $red->withValue('red');
>
>   var_dump($red =3D=3D=3D $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' =3D=3D=3D '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 =3D $value;
>   }
>
>   public function withValue($value) {
>     $this->value =3D $value;
>   }
>
> }
>
> class A {
>
>   public $vo;
>
>   public function __construct(ValueObject $vo) {
>     $this->vo =3D $vo;
>   }
>
> }
>
> class B {
>
>   public $vo;
>
>   public function __construct(ValueObject $vo) {
>     $this->vo =3D $vo;
>   }
>
> }
>
> $vo =3D new ValueObject(1);
>
> $a =3D new A($vo);
> $b =3D new B($vo);
>
> var_dump($a->vo =3D=3D=3D $b->vo); // bool(true)
>
> $a->vo->withValue(2);
>
> var_dump($a->vo =3D=3D=3D $b->vo); // bool(false)
>
> $a->vo->withValue(1);
>
> var_dump($a->vo =3D=3D=3D $b->vo); // bool(true)
>
> // :)
>
> ?>
>
> --
> Richard "Fleshgrinder" Fussenegger
>

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