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Date: Wed, 6 Aug 2025 23:20:12 +0200
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Subject: Re: [PHP-DEV] Protected inheritance hierarchies
To: Rob Landers <rob@bottled.codes>
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From: jnv@jnvsor.net (Jonathan Vollebregt)

On 8/6/25 1:41 PM, Rob Landers wrote:
> Take for example:
> 
> class Fruit {
>    public string $kind { get => "fruit" }
> }
> 
> class Vegetable extends Fruit {
>    public string $kind { get => "vegetable" }
> }
> 
> function foo(Fruit $fruit) {}
> 
> foo(new Vegetable); // hmmm
> 
> This is a "soft" violation that only makes sense to us humans, but PHP 
> allows it. It requires us humans to realize we are performing an LSP 
> violation and refactor the code so that we don't pass a Carrot to 
> someone expecting a Mango.

Rob, that's not how types work. The only thing guaranteed by this 
definition of $fruit->kind is that it has a getter that returns a 
string. "vegetable" is a string. This isn't a violation at all.

Now if you were able to type specific values in PHP a la psalm/phpstan 
and make something like this:

class Fruit {
   public "apple"|"pear" $kind { get => "fruit" }
}

Well then yeah Vegetable would be a violation, but this would be a 
different (and more specific) type than just string. (And this is why 
you can't extend enums, it would widen a contract)

This isn't a "soft" violation that "PHP allows", this is a well defined 
property of every programming language I know of.