Re: [RFC] [Discussion] Never parameters

From: Date: Fri, 21 Mar 2025 14:32:58 +0000
Subject: Re: [RFC] [Discussion] Never parameters
References: 1 2 3 4 5  Groups: php.internals 
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On 21/03/2025 11:50, Tim Düsterhus wrote:

Am 2025-03-20 21:27, schrieb Matt Fonda:

If an interface adds a method but makes no promises about what parameters
it accepts, then why is it part of the interface in the first place--why
add a method that can't be used?


It would more cleanly allow for userland / PHPDoc-based generics, while still providing some engine-enforced type safety. Consider this example (not sure if I got the syntax completely right):


    /** @template T */
    interface Comparable {
        /** @param T $other */
        public function compareTo(never $other): int;
    }



    /** @implements Comparable<Number> */
    final class Number implements Comparable {
        public function compareTo(Number $other): int { return $this <=> $other; }
    } 


I think I agree with Matt on this: the interface isn't making any usable promises about that method.

In this example, Comparable is a kind of "abstract interface" - in order to actually make use of it, you need to specialise it.

Declaring that a class implements a template interface is like inheriting an abstract method: either you fill in the type parameter ("class Foo implements Comparable<Foo> { ... }"), or the class is also a template ("class Foo<A> implements Comparable<A> { ... }")


I don't think the language should pretend to support something that it doesn't - if the contract is actually enforced by a third-party tool reading docblocks, put the contract in a docblock:


    /**
       * @template T
       * @method compareTo(T $other): int;
       */
    interface Comparable {
    }



    /** @implements Comparable<Number> */
    final class Number implements Comparable {
        public function compareTo(Number $other): int { return $this <=> $other; }
    }



-- 
Rowan Tommins
[IMSoP]

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