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Date: Sat, 22 Jan 2022 10:12:50 -0600
Cc: PHP internals <internals@lists.php.net>
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To: Dominic Grostate <codekestrel@googlemail.com>
Subject: Re: [PHP-DEV] Weak Closures
From: aaron@trowski.com (Aaron Piotrowski)


> On Jan 21, 2022, at 4:31 AM, Dominic Grostate =
<codekestrel@googlemail.com> wrote:
>=20
> Hi Internals,
>=20
> I'd like to express my interest in a possible feature concerning weak
> references. Currently closures created within a class appear to
> contain a reference to the object that created it. This is of course
> necessary in order for the closure to retain the necessary scope.
> However I would like to suggest we have the option for closure to
> weakly reference this object so that when the object is garbage
> collected, the closure too may be rendered null or invalid
> (inspectable).
>=20
> Consider the following example:
> https://gist.github.com/orolyn/7651e4127759aad1736547490baa1394
>=20
> The idea here is that without unset($sample) the loop would run
> forever, because there is a reference to the callback from the object,
> and in turn there is a reference to the object from the main stack.
> With the unset($sample), the idea is that before the callback is even
> called, the reference to the object is destroyed thusly so is the
> reference to callback, and since the callback is only referenced now
> in a WeakMap, the callback will finally be gone.
>=20
> In reality it appears there is a circular reference between the object
> and the callback. I don't know if this is a bug or not, because from
> what I can find PHP was fixed a long time ago to resolve circular
> references.
>=20
> However if this is intentional, I would like the option to make the
> closure weak for example:
>=20
> $c =3D WeakClosure::fromCallable(function () {});
> $c =3D Closure::fromCallable(function () {}, true);
>=20
> Please let me know your thoughts, because maybe there is a way of
> achieving this with PHP as is.
>=20
> Kind regards,
> Dominic
>=20
> --=20
> PHP Internals - PHP Runtime Development Mailing List
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>=20

Hi Dominic,

Implementing a weak closure is possible in user code. We've done so in =
Amp v3: =
https://github.com/amphp/amp/blob/379177aba93518e2df6d626677cbbdc48cc0d8ae=
/src/functions.php#L119-L171

I would be in favor of having this functionality available directly in =
PHP. Amp often uses references to object properties in static closures =
to avoid circular references to $this. There are other ways to =
accomplish this, such as a ref object like =
https://github.com/azjezz/psl/blob/21bf0cd3d6d6055fc88541e9b24f3140bd179b2=
d/src/Psl/Ref.php, but a weak-ref to $this would certainly be more =
convenient.

Cheers,

Aaron Piotrowski