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Subject: Re: [PHP-DEV] [RFC] Readonly property hooks
Date: Mon, 9 Jun 2025 11:51:44 +0200
In-Reply-To: <1e8634d7-ac1a-4025-b4e2-1948aabf5251@app.fastmail.com>
Cc: php internals <internals@lists.php.net>,
 php@nicksdot.dev
To: Larry Garfield <larry@garfieldtech.com>
References: <1e8634d7-ac1a-4025-b4e2-1948aabf5251@app.fastmail.com>
From: claude.pache@gmail.com (Claude Pache)


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> Le 8 juin 2025 =C3=A0 06:16, Larry Garfield <larry@garfieldtech.com> a =
=C3=A9crit :
>=20
> As Nick has graciously provided an implementation, we would like to =
open discussion on this very small RFC to allow `readonly` on backed =
properties even if they have a hook defined.
>=20
> https://wiki.php.net/rfc/readonly_hooks
>=20
> --=20
>  Larry Garfield
>  larry@garfieldtech.com


Hi Larry, Nick,

Last summer, the question of allowing hooks on readonly has been raised =
as part of the RFC =C2=ABProperty hooks improvements=C2=BB, and at that =
time I have raised an objection on allowing the get hook on readonly =
properties and I have suggested for a better design for the main issue =
it was supposed to solve, see https://externals.io/message/124149#124187 =
and the following messages. (The RFC itself was trimmed down to the =
non-controversial part.) I=E2=80=99ll repeat here both my objection and =
my proposal for better design, but more strongly, with the hope that the =
message will be received.

The purpose of readonly properties is (citing the original RFC, =
https://wiki.php.net/rfc/readonly_properties_v2#rationale) to provide =
strong immutable guarantee, i.e.:

```php
class Test {
    public readonly string $prop;
=20
    public function method(Closure $fn) {
        $prop =3D $this->prop;
        $fn(); // Any code may run here.
        $prop2 =3D $this->prop;
        assert($prop =3D=3D=3D $prop2); // Always holds.
    }
}
```

By allowing a get hook on readonly property, you are effectively =
nullifying this invariant. Invariants must be enforced be the engines =
(whenever possible; there is an inevitable loophole until the property =
is initialised), and not left to the discretion of the user. If a get =
hook on readonly property is allowed, a random user will use its =
creativity in order to circumvent the intended invariant (recall: =
immutability). I say =E2=80=9Ccreativity=E2=80=9D, not =E2=80=9Cdumbness=E2=
=80=9D, because you cannot mechanically tell the two apart:

```php
class doc {
    public readonly int page {
        get =3D> $this->page + $this->offset;
    }
    private int $offset =3D 0;
    public function __construct(int $page) {
        $this->page =3D $page;
    }
    public function foo() {
        // $this->offset may be adjusted here
    }
}
```

I know that some people won=E2=80=99t see a problem with that code (see =
the cited thread above), and this is a strong reason not to allow that: =
you cannot trust the user to enforce invariants that they don=E2=80=99t =
understand or are not interested in.

(The objection above is for the `get` hook`; there is no such issue with =
the `set` hook.)

Now, here is the suggestion for a better alternative design, that (1) =
don=E2=80=99t allow to break the invariant of immutability, (2) solve =
the issue of lazy initialisation (which is, I guess, the main purpose of =
the `get` hook on readonly), and (3) also works with nullable =
properties:

Add an additional hook to backed properties, named `init`. When =
attempting to read the value of the backing store, if it is =
uninitialised, then the init hook is triggered, which is supposed to =
initialise it.

=E2=80=94Claude


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<html><head><meta http-equiv=3D"content-type" content=3D"text/html; =
charset=3Dutf-8"></head><body style=3D"overflow-wrap: break-word; =
-webkit-nbsp-mode: space; line-break: after-white-space;"><br =
id=3D"lineBreakAtBeginningOfMessage"><div><br><blockquote =
type=3D"cite"><div>Le 8 juin 2025 =C3=A0 06:16, Larry Garfield =
&lt;larry@garfieldtech.com&gt; a =C3=A9crit :</div><br =
class=3D"Apple-interchange-newline"><div><div>As Nick has graciously =
provided an implementation, we would like to open discussion on this =
very small RFC to allow `readonly` on backed properties even if they =
have a hook =
defined.<br><br>https://wiki.php.net/rfc/readonly_hooks<br><br>-- <br> =
&nbsp;Larry Garfield<br> =
&nbsp;larry@garfieldtech.com<br></div></div></blockquote></div><br><div><b=
r></div><div>Hi Larry, Nick,</div><div><br></div><div>Last summer, the =
question of allowing hooks on readonly has been raised as part of the =
RFC =C2=ABProperty hooks improvements=C2=BB, and at that time I have =
raised an objection on allowing the get hook on readonly properties and =
I have suggested for a better design for the main issue it was supposed =
to solve, see <a =
href=3D"https://externals.io/message/124149#124187">https://externals.io/m=
essage/124149#124187</a>&nbsp;and the following messages. (The RFC =
itself was trimmed down to the non-controversial part.) I=E2=80=99ll =
repeat here both my objection and my proposal for better design, but =
more strongly, with the hope that the message will be =
received.</div><div><br></div><div>The purpose of readonly properties is =
(citing the original RFC, <a =
href=3D"https://wiki.php.net/rfc/readonly_properties_v2#rationale">https:/=
/wiki.php.net/rfc/readonly_properties_v2#rationale</a>)&nbsp;to provide =
strong immutable guarantee, =
i.e.:</div><div><br></div><div>```php</div><div><div>class Test =
{</div><div>&nbsp; &nbsp; public readonly string =
$prop;</div><div>&nbsp;</div><div>&nbsp; &nbsp; public function =
method(Closure $fn) {</div><div>&nbsp; &nbsp; &nbsp; &nbsp; $prop =3D =
$this-&gt;prop;</div><div>&nbsp; &nbsp; &nbsp; &nbsp; $fn(); // Any code =
may run here.</div><div>&nbsp; &nbsp; &nbsp; &nbsp; $prop2 =3D =
$this-&gt;prop;</div><div>&nbsp; &nbsp; &nbsp; &nbsp; assert($prop =3D=3D=3D=
 $prop2); // Always holds.</div><div>&nbsp; &nbsp; =
}</div><div>}</div></div><div>```</div><div><br></div><div>By allowing a =
get hook on readonly property, you are effectively nullifying this =
invariant. Invariants must be enforced be the engines (whenever =
possible; there is an inevitable loophole until the property is =
initialised), and not left to the discretion of the user. If a get hook =
on readonly property is allowed, a random user will use its creativity =
in order to circumvent the intended invariant (recall: immutability). I =
say =E2=80=9Ccreativity=E2=80=9D, not =E2=80=9Cdumbness=E2=80=9D, =
because you cannot mechanically tell the two =
apart:</div><div><br></div><div>```php</div><div><div>class doc =
{</div><div>&nbsp; &nbsp; public readonly int page {</div><div>&nbsp; =
&nbsp; &nbsp; &nbsp; get =3D&gt; $this-&gt;page + =
$this-&gt;offset;</div><div>&nbsp; &nbsp; }</div><div>&nbsp; &nbsp; =
private int $offset =3D 0;</div><div>&nbsp; &nbsp; public function =
__construct(int $page) {</div><div>&nbsp; &nbsp; &nbsp; &nbsp; =
$this-&gt;page =3D $page;</div><div>&nbsp; &nbsp; }</div><div>&nbsp; =
&nbsp; public function foo() {</div><div>&nbsp; &nbsp; &nbsp; &nbsp; // =
$this-&gt;offset may be adjusted here</div><div>&nbsp; &nbsp; =
}</div><div>}</div></div><div>```</div><div><br></div><div>I know that =
some people won=E2=80=99t see a problem with that code (see the cited =
thread above), and this is a strong reason not to allow that: you cannot =
trust the user to enforce invariants that they don=E2=80=99t understand =
or are not interested in.</div><div><br></div><div>(The objection above =
is for the `get` hook`; there is no such issue with the `set` =
hook.)</div><div><br></div><div>Now, here is the suggestion for a better =
alternative design, that (1) don=E2=80=99t allow to break the invariant =
of immutability, (2) solve the issue of lazy initialisation (which is, I =
guess, the main purpose of the `get` hook on readonly), and (3) also =
works with nullable properties:</div><div><br></div><div>Add an =
additional hook to backed properties, named `init`. When attempting to =
read the value of the backing store, if it is uninitialised, then the =
init hook is triggered, which is supposed to initialise =
it.</div><div><br></div><div>=E2=80=94Claude</div><div><br></div></body></=
html>=

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