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Date: Wed, 04 Jun 2025 15:09:29 +0200
To: Nick <php@nicksdot.dev>
Cc: internals@lists.php.net
Subject: Re: [PHP-DEV] Allow hooks in `readonly` promoted properties
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From: tim@bastelstu.be (=?UTF-8?Q?Tim_D=C3=BCsterhus?=)

Hi

Am 2025-06-04 14:19, schrieb Nick:
> Fair. If someone really wants to add random_int(): "well, that 
> assumption doesn't hold anymore, deal” from my side.

Semantically once you involved inheritance it isn't that easy. It is 
allowed to override an “unhooked” property with a hooked property and in 
the “Readonly Amendments” RFC we already decided that inheriting from a 
`readonly` class by a non-`readonly` class should not be valid.

So if you would be allowed to override a readonly unhooked property with 
a hooked property that has a `get` hook that is not a pure function, you 
would make the property effectively mutable, which is something that 
users of the class can't expect. It violates the history property of the 
Liskov substitution principle.

Making this legal might also inhibit engine optimization. Currently if 
you know that a property is readonly you can theoretically optimize:

     if ($object->foo !== null) {
         do_something($object->foo);
     }

into:

     if (($foo = $object->foo) !== null) {
         do_something($foo);
     }

to avoid reading `$object->foo` twice, which for example would need to 
check visibility twice.

>> I believe at the moment that RFC text is all there is. :-)  I don't 
>> know that it's worth opening a discussion without at least a 
>> mostly-done implementation.  Also, Ilija is rather busy on other tasks 
>> at the moment, as am I.  (Unless someone else wants to jump in to 
>> implement it, which would be fine.)
> 
> People often say “you can just do things”. So I did, and tried to 
> contribute the code for your existing RFC text:
> 
> https://github.com/php/php-src/pull/18757
> 
> Can it really be such a little change? I’d appreciate feedback from 
> people more experienced than I am. Thanks!

Your test cases really only scratch the surface of what should be 
tested. You are basically just verifying that it compiles. In fact you 
are not even testing that reassigning the property is disallowed, 
because the test fails due to a visibility error. In fact it appears 
that the `readonly` check comes before the visibility check, which would 
imply that the `readonly` doesn't have an effect: https://3v4l.org/nqgpL

Best regards
Tim Düsterhus