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Date: Sun, 10 Mar 2019 18:35:03 +0000
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Subject: RFC: Locked Classes
From: rowan.collins@gmail.com (Rowan Collins)

Hi all,

I'd like to present a new RFC for "locked classes": classes which 
restrict dynamically adding or removing properties from their instances.

While it can be useful, the ability to set an object property which is 
not part of the class definition can also lead to subtle bugs. Banning 
this for all objects would be a significant and painful breaking change, 
so I propose instead the option to mark a particular class with a new 
keyword, "locked".

An instance of a locked class behaves like any other object, except that:

- Attempting to set a property on the instance which was not declared in 
the class (or inherited from one of its parent classes) will throw an 
error, and the instance will not be modified.
- Attempting to read a property on the instance which was not declared 
(or inherited) will throw an error, rather than raising a Notice and 
evaluating to null.
- Attempting to call unset() on any property of the instance will throw 
an error, and the instance will not be modified.

Note that ECMAScript  / JavaScript includes a similar feature, called 
"sealed objects". However, the proposed modifier applies to classes, and 
"sealed class" has a different meaning elsewhere (e.g. C#, Kotlin), so 
I've chosen "locked class" to avoid confusion.

For further details and examples, please check the RFC at 
https://wiki.php.net/rfc/locked-classes and the tests in the draft 
implementation at https://github.com/php/php-src/pull/3931

Regards,

-- 
Rowan Collins
[IMSoP]