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Date: Sat, 11 Nov 2017 20:51:49 +0000
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Subject: Re: [PHP-DEV] Constants and Access Modifiers
From: rowan.collins@gmail.com (Rowan Collins)

On 11/11/2017 18:39, Fleshgrinder wrote:
> On 11/6/2017 1:44 AM, Stanislav Malyshev wrote:
>
>>  From this link, it looks like const in Dart has pretty much nothing in
>> common with const in PHP, besides name, so in the interest of avoiding
>> confusion, I would not discuss it in the same topic.
>>
> Yes, Dart has a different understanding of const, which is exactly why I
> posted it for you guys. In the hope that it helps to get more different
> views on the topic. Currently you are too concentrated on how it is
> implemented in PHP at this time, and argue that it is impossible to
> diverge from that path. Which is simply not true, we only have to ensure
> backwards compatibility.

I think the point is that adding "single-assignment variables" (which 
Dart calls "final") or "deeply immutable values" (which Dart calls 
"const") would be a completely different feature which could be added 
side-by-side with what we have now, under a different name. What PHP 
calls "const" (in the context of classes) is neither of those features.


The debate seems to be whether you view class constants as more like 
static properties, or static methods. Given this:

class A { public const Foo = 42; }
echo A::Foo;

Is it equivalent to this (using an imaginary "readonly" modifier)...

class A { public static readonly $foo = 42; }
echo A::$foo;

...or is it equivalent to this (particularly if you imagine an 
optimising compiler that caches / inlines the result)?

class A { public static function foo(): int { return 42; } }
echo A::foo();


The difference is that a field is never "abstract" - it either has a 
value, or it is undefined; you can't add a contract to an interface 
saying "you must have this field" either. A method, on the other hand, 
is assumed to encapsulate something - it's a black box with a contract, 
so defining the contract without any implementation makes sense. (Note 
that I've called $foo a "field" to distinguish it from a "property", as 
C# does: a property with a contract like "$foo { public get; private 
set; }" could indeed be abstract.)

The interesting thing about the above examples is that the static method 
with a fixed return *already works right now*, whereas the readonly 
field doesn't exist; so it makes some sense to say that "const FOO" is 
PHP's way of saying "static readonly $FOO", and have it subject to 
similar semantics.

Regards,

-- 
Rowan Collins
[IMSoP]