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Date: Mon, 19 May 2025 19:06:03 +0200
Message-ID: <CAB-BujbF5BACvaFMJUWoxikizBB1jDyy2F8ehjiuj-UoNKf14Q@mail.gmail.com>
Subject: Re: [PHP-DEV] [RFC] Clone with v2
To: Nicolas Grekas <nicolas.grekas+php@gmail.com>
Cc: Larry Garfield <larry@garfieldtech.com>, php internals <internals@lists.php.net>
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From: andreas@dqxtech.net (Andreas Hennings)

On Mon, 19 May 2025 at 17:13, Nicolas Grekas
<nicolas.grekas+php@gmail.com> wrote:
>
>
>
> Le lun. 19 mai 2025 =C3=A0 16:30, Andreas Hennings <andreas@dqxtech.net> =
a =C3=A9crit :
>>
>> On Fri, 16 May 2025 at 21:59, Nicolas Grekas
>> <nicolas.grekas+php@gmail.com> wrote:
>> >
>> >
>> >
>> > Le jeu. 15 mai 2025 =C3=A0 16:06, Larry Garfield <larry@garfieldtech.c=
om> a =C3=A9crit :
>> >>
>> >> On Thu, May 15, 2025, at 1:22 AM, Stephen Reay wrote:
>> >>
>> >> > I may be missing something here..
>> >> >
>> >> > So far the issues are "how do we deal with a parameter for the actu=
al
>> >> > object, vs new properties to apply",  "should __clone be called bef=
ore
>> >> > or after the changes" and "this won't allow regular readonly proper=
ties
>> >> > to be modified".
>> >> >
>> >> > Isn't the previous suggestion of passing the new property arguments
>> >> > directly to the __clone method the obvious solution to all three
>> >> > problems?
>> >> >
>> >> > There's no potential for a conflicting property name, the developer=
 can
>> >> > use the new property values in the order they see fit relative to t=
he
>> >> > logic in the __clone call, and it's inherently in scope to write to=
 any
>> >> > (unlocked during __clone) readonly properties.
>> >>
>> >> I did some exploratory design a few years ago on this front, looking =
at the implications of different possible syntaxes.
>> >>
>> >> https://peakd.com/hive-168588/@crell/object-properties-part-2-example=
s
>> >>
>> >> What that article calls "initonly" is essentially what became readonl=
y.  The second example is roughly what this RFC would look like if the extr=
a arguments were passed to __clone().  As noted in the article, the result =
is absolutely awful.
>> >>
>> >> Auto-setting the values while using the clone($object, ...$args) synt=
ax is the cleanest solution.  Given that experimentation, I would not suppo=
rt an implementation that passes args to __clone and makes the developer fi=
gure it out.  That just makes a mess.
>> >>
>> >> Rob makes a good point elsewhere in thread that running __clone() aft=
erward is a way to allow the object to re-inforce validation if necessary. =
 My concern is whether the method knows it needs to do the extra validation=
 or not, since it may be arbitrarily complex.  It would also leave no way t=
o reject the changes other than throwing an exception, though in fairness t=
he same is true of set hooks.  Which also begs the question of whether a se=
t hook would be sufficient that __clone() doesn't need to do extra validati=
on?  At least in the typical case?
>> >>
>> >> One possibility (just brainstorming) would be to update first, then c=
all __clone(), but give clone a new optional arg that just tells it what pr=
operties were modified by the clone call.  It can then recheck just those p=
roperties or ignore it entirely, as it prefers.  If that handles only compl=
ex cases (eg, firstName was updated so the computed fullName needs to be up=
dated) and set hooks handle the single-property ones, that would probably c=
over all bases reasonably well.
>> >
>> >
>> > I like where this is going but here is a variant that'd be even more c=
apable:
>> >
>> > we could pass the original object to __clone.
>>
>> My proposal earlier was to pass the original object _and_ the values
>> that were passed to the clone call, by reference.
>>
>> And this would happen before those values are assigned to the object.
>>
>> class MyClass {
>>   public function __construct(
>>     public readonly int $x,
>>     public readonly int $y,
>>     public readonly int $z,
>>   ) {}
>>   public function __clone(object $original, array &$values): void {
>>     // Set a value directly, and modify it.
>>     if (isset($values['x'])) {
>>       $this->x =3D $values['x'] * 10;
>>       // Prevent that the same property is assigned again.
>>       unset($values['x']);
>>     }
>>   }
>> }
>>
>> $obj =3D new C(5, 7, 9);
>> $clone =3D clone($obj, x: 2, y: 3);
>> assert($clone->x =3D=3D=3D 20);  // x was update in __clone().
>> assert($clone->y =3D=3D=3D 3);  // y was auto-updated after __clone().
>> assert($clone->z =3D=3D=3D 9);  // z was not touched at all.
>
>
> I'm not sure I understand, there might be missing bits to your idea, eg w=
here is visibility enforced? why is pass-by-ref needed at all?
> Pass-by-ref makes me think this is a bad idea already :)

Maybe we are looking at different problems to be solved.
To me, the main questions are:
- Could a __clone() method want to behave differently depending which
property values are passed with the clone call?
- Can there be conflicts between operations we would normally do in
__clone() and values passed to __clone()?
- Should we prevent or allow double-write to a readonly property? That
is, if one write happens in __clone(), and the other write happens
automatically due to the property value passed to clone(..).

And to clarify my proposal:
- Everything is the same as in the RFC (except points below)
- Same as in the RFC, the __clone() method is called _after_ the
original object values have been copied over, but _before_ any of the
property values passed as arguments to clone($obj, ...$values) are
assigned.
- Same as in the RFC, the values passed to clone($obj, ...$values) are
assigned automatically after the __clone() method.
- Unlike the RFC, the __clone() method can see (and validate) the
values that were passed to __clone($obj, ...$values)
- Unlike the RFC, the __clone() method can _alter_ the values passed
to __clone($obj, ...$values) before they are assigned.
- As in the RFC, readonly properties can be written only once on
clone. The __clone() method can prevent a double write by unsetting
that key in $values.

Consequence:
By leaving the __clone() method empty, all values are assigned
automatically, as in the RFC.

> where is visibility enforced?

Exactly as in the RFC.
When clone($obj, ...$values) is called, and before __clone() is
invoked, php has to verify which of the properties are legal to be
updated in this way, based on property visibility and readonly status,
and depending on the scope from which it is called.

Actually this raises some questions that I did not think of before:
- After __clone() is invoked, does php need to validate again?
- What happens to private properties that are not accessible from the
scope of the __clone() method? Are they also passed in the $values
array?

>
> Also, WDYT of my simpler proposal itself? Wouldn't it cover all use cases=
?

First, I want to make sure I understand correctly:
- Unlike the RFC, we want to call __clone() _after_ the values from
clone($obj, ...$values) are assigned.
  (I assume this because otherwise the two objects would be identical,
and the original object would be useless)
- Unlike the RFC, we pass the original object as a parameter to __clone().

Tbh I am not sure if the use cases I think of are relevant or not :)
I mostly think of it in terms of functional completeness, without
trying to speculate why a developer would want to do this or that
during __clone().

Let's look at the "benefits" section from your earlier mail.

> The benefits I see:
> - Allow implementing this validation logic you're talking about.

Having __clone() called after the values are assigned, as you propose,
makes it possible to run an integrity check on the object itself. On
the other hand, this may leave us with a short moment of possibly
"bad" property values.
Having __clone() called before the values are assigned means we have
to validate the values array, not the object.

> - Allow to skip deep-cloning of already updated properties (that's a sign=
ificant drawback of the current proposal - deep cloning before setting is a=
 potential perf/mem hog built into the engine) : guarding deep-cloning with=
 a strict comparison would be enough.

With __clone() called after the values are assigned, and with access
to the original object, we can check $this->prop =3D=3D=3D $old->prop to se=
e
whether a specific property was updated (and therefore should not be
deep cloned).
With __clone() called before the values are assigned, and with access
to the values array, we would check isset($values['prop']) instead.
(or really array_key_exists())
In general this would produce the same result, unless a property was
assigned the same value that it had before.

Another question is about "dependent properties", e.g. for lazily
filled calculated values.
With access to the old object we would have to check $this->prop !=3D=3D
$old->prop to then see which dependent properties need to be reset or
recalculated.
With access to the values we would check isset($values['prop']) instead.

> - Allow implementing WeakMap that are able to clone weak-properties as ob=
jects are cloned.

I don't feel informed or qualified to talk about this one..

So, with __clone() called after and with the original object, we
compare old and new when looking for a specific property.
With access to an array of updated (or to be updated) properties, we
could iterate over the changes, or we could check whether the
changelist is empty, which is less obvious to do by comparing old and
new instance.


>
> Note that I don't see the need for operations like in your example (trans=
forming a value while cloning).
> This looks like the job of a setter or a hook instead, not a cloner.

Tbh, most of the classes I wrote that would benefit from a "clone
with" did not really need a __clone() method, because everything in
there was already immutable.
To me, the scenarios where we want both are quite speculative, so this
is why my examples might not be the most realistic.

>
>>
>> >
>> > The benefits I see:
>> > - Allow implementing this validation logic you're talking about.
>> > - Allow to skip deep-cloning of already updated properties (that's a s=
ignificant drawback of the current proposal - deep cloning before setting i=
s a potential perf/mem hog built into the engine) : guarding deep-cloning w=
ith a strict comparison would be enough.
>> > - Allow implementing WeakMap that are able to clone weak-properties as=
 objects are cloned.
>> >
>> > On this last aspect, I think it's new to the discussion but it's somet=
hing I've always found very limiting when using weak-map: let's say some me=
tadata are stored about object $foo in a weakmap, it's currently not possib=
le to track those metadata across clones without using some nasty tricks. I=
f __clone were given the original object, it's be easy to duplicate meta-da=
ta from $foo to $clone.
>> >
>> > I have just one concern significant with adding an argument to __clone=
: it'dbe a BC break to mandate this argument at the declaration level, and =
adding one right now generates an error with current versions of PHP.
>> > However, I think we could (and should if confirmed) provide some FC/BC=
 layer by allowing one to use func_get_args() in __clone. The engine could =
then verify at compile time that __clone has either one non-nullable "objec=
t" argument, or zero.
>>
>> This seems reasonable.
>>
>
> I'd be happy to know what Volker and Tim think about this? I read they ex=
cluded any change to __clone in the RFC, but I think it should still be pos=
sible to discuss this, especially if it provides the path to the desired so=
lution.
>
> Nicolas