On Thu, Nov 23, 2023 at 10:30 PM Deleu <[email protected]> wrote:
>
>
>
> On Thu, Nov 23, 2023 at 5:31 PM Robert Landers <[email protected]>
> wrote:
>>
>> Hello Internals,
>>
>> As you may know, an inherited method cannot reduce the visibility of
>> an overridden method. For example, this results in a fatal error
>> during compilation:
>>
>> class P {
>> public function hello($name = 'world') {
>> echo "hello $name\n";
>> }
>> }
>>
>> class C extends P {
>> private function hello($name = 'world') {
>> parent::hello($name);
>> echo "goodbye $name\n";
>> }
>> }
>>
>> However, we can make certain methods private anyway, namely,
>> constructors (I haven't gone hunting for other built-in methods yet).
>> This is perfectly allowed:
>>
>> class P {
>> public function __construct($name = 'waldo') {
>> echo "hello $name\n";
>> }
>> }
>>
>> class C extends P {
>> private function __construct($name = 'world') {
>> parent::__construct($name);
>> echo "goodbye $name\n";
>> }
>> }
>>
>> To my somewhat trained eye, this appears to violate the Liskov
>> Substitution Principle, for example, this now can have hidden errors:
>>
>> function create(P $class) {
>> return new (get_class($class))();
>> }
>>
>> proven by:
>>
>> $c = (new ReflectionClass(C::class))
>> ->newInstanceWithoutConstructor();
>>
>> create($c);
>>
>> Even though we thought we knew that the constructor was declared public.
>>
>> I'd like to propose an RFC to enforce the covariance of constructors
>> (just like is done for other methods), to take effect in PHP 9, with a
>> deprecation notice in 8.3.x.
>>
>> I'm more than happy to implement it.
>>
>> Does anyone feel strongly about this one way or the other?
>
>
> Constructors are an implementation detail of a specialized class and as such
> they're not subject to LSP because the goal of LSP is to be able to make sure
> that any object of a given type hierarchy can be used to accomplish a certain
> behavior. If you take a step back from PHP's dynamic nature and think about
> LSP from a more pure type system, the fact you're expecting an object of type
> C, but then you completely disregard everything about the object itself and
> dive into it's metadata to build another object, that's the moment you're no
> longer playing by the rules of OOP. It's like those mathematical equations
> that prove that 1 = 2, they all have one thing in common: they end up
> dividing by 0 at some point.
>
> OOP here dictates that you should reach for patterns like Builder, Abstract
> Factory or similar. That way you constraint yourself to the rules of OOP and
> you won't get weird outcomes.
>
> From another point of view, when a type is expected by a function or method,
> all we can expect from it is whatever was defined as the blueprint
> (class/interface) of that object and the __construct() is a special method
> that is not assumed to be part of that blueprint because it's not reasonable
> to do `$object->__construct();` after receiving an object. As such, a
> constructor cannot break LSP because the constructor is not part of the
> object's API from a "receptor" point of view.
>
> I don't have a vote so take my opinion with a bucket of salt, but if I could
> I would definitely vote against such RFC.
>
>
> --
> Marco Deleu
Thanks Marco,
That's an interesting perspective and one I would agree with for the
most part, especially if you take my illustration at face value. Where
it gets weird/breaks down is when you have a class-string, that you
assert is the correct type, and then try to instantiate it:
// from somewhere
$class = "C";
if(is_subclass_of($class, P::class)) {
$example = new $class("world");
}
If PHP didn't offer these built-in methods, then I would fully agree
with you, but it does, which puts it into a weird position where
sometimes a class is substitutable, and in this one special case, it
is not.
Robert Landers
Software Engineer
Utrecht NL
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