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Date: Sun, 27 Nov 2022 10:50:59 -0600
To: "php internals" <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC] [Discussion] Readonly class amendments
From: larry@garfieldtech.com ("Larry Garfield")

On Sat, Nov 26, 2022, at 6:35 PM, Jordan LeDoux wrote:
> On Sat, Nov 26, 2022 at 3:40 PM Deleu <deleugyn@gmail.com> wrote:
>
>>
>> As I think more about this, there's nothing about the current RFC in this
>> code sample. What's breaking LSP here is the child class doing state
>> modification, not PHP.  To further expand that rationale, PHP allows us to
>> create child classes. Whether that class will be LSP-safe or not is up to
>> us, not up to PHP.
>>
>> However, the point still stands. Allowing child classes to break readonly
>> will make it easier to build code that breaks LSP. The question then
>> becomes: why is this being proposed and is it worth it?
>>
>
> I cannot help but feel that the way `readonly` is being treated is going to
> end up one of those things that is regretted. "Readonly does not imply
> immutability". The fact that very nearly *every* single person who has not
> worked on the RFCs has at some point been confused by this however should
> be very telling.
>
> This comes from two *different* avenues that compound with each other to
> *both* make this design head-scratching to me.
>
> First, in virtually all other technical contexts where the term "readonly"
> is used, it means that the information/data cannot be altered. That is not
> the case with readonly. In PHP, in this implementation, it is not
> "readonly" in the sense that it is used everywhere else for computing, it
> is "assign once".
>
> Second, the English words "read only", particularly to native speakers,
> make this behavior very counterintuitive and confusing. I won't belabor
> that point further.
>
> What "read only" really is, is "constructor initialize only". It honestly
> has nothing to do with "read" as it's implemented.

Not quite.  It really is just write-once.  The idea that you can only do that in the constructor is not in the language; that's been invented by over-eager static analysis tools.  (Everyone should disable that check.)

> I guess I worry that this RFC makes `readonly` even more of a minefield for
> PHP developers, increasing the mental load of using it in code while *even
> further* watering down the benefits it may provide. It's already designed
> in a somewhat counterintuitive way that I feel will be almost completely
> replaced in actual code in the wild by "immutable" if PHP ever gets that.

Working on asymmetric visibility, I have come to agree.  Nikita proposed readonly as a "junior version" of asymmetric visibility, to cover the most common use case without introducing more complexity.  At the time, he was confident that it wouldn't preclude expanding to asymmetric visibility in the future.  Well... I can say with confidence at this point that is not correct, and the design of readonly is causing issues for asymmetric visibility, and for cloning, to the point that (based on feedback in the other thread) we're likely going to for now forbid readonly and a-viz on the same property.

At this point, I think I view readonly as a cautionary tale about the costs of doing "quick and easy" design over something more robust, because the quick-and-easy creates problems down the line that a more thoughtful, holistic view would have avoided.

> LSP doesn't exist because it is some objectively better way of programming
> according to universal laws of entropy or something. It is instead
> important because LSP helps programmers be able to predict the behavior of
> the program they are writing and reduces the short-term memory load
> involved in programming and architecture.
>
> Something that *technically* complies with LSP but makes the program harder
> to predict and increases the mental load of programming violates the
> *purpose* of LSP. We can argue about whether it is technically correct, but
> I feel like that somewhat misses the point: making the language more
> capable, more stable, and more predictable.
>
> In other words, I do not believe it is that important or care to argue
> about whether this RFC violates LSP. It violates the *purpose* of LSP, and
> that's a bigger problem to me personally.
>
> Jordan

At this point, I am inclined to agree.  readonly is wonky enough as is.  Making it semi-LSP (in spirit) just makes it even more wonky.  To flip the earlier sentiment, "readonly is broken enough as is, let's not break it even further."

--Larry Garfield