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To: Stephen Reay <php-lists@koalephant.com>, PHP Internals
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Thread-Topic: [PHP-DEV] Trait constants
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Date: Sat, 27 Jun 2020 19:09:49 +0000
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Subject: Re: [PHP-DEV] Trait constants
From: tysonandre775@hotmail.com (tyson andre)

> It=92s always struck me as slightly odd that traits don=92t support const=
ants the way classes and interfaces do.=0A=
> I tried to find an explanation of the lack of support in the original RFC=
, and came up empty.=0A=
> =0A=
> A consequent discussion in R11 has led me here.=0A=
> Can anyone working on internals explain why traits don=92t allow constant=
s (either technically or philosophically)?=0A=
> Moreover, what=92s the opinion(s) of the list, on adding support for this=
? Would an RFC be needed? =0A=
=0A=
I'm in favor of adding it. I doubt there's any insurmountable technical obs=
tacles.=0A=
(I work on a static analyzer for php (https://github.com/phan/phan) and don=
't expect technical issues with that proposal)=0A=
I personally find it inconvenient to put constants used by traits in differ=
ent classes/interfaces or in properties.=0A=
=0A=
Other languages allow adding constants to traits:=0A=
=0A=
- Java allows defining constants on interfaces, which allow defining defaul=
t method implementations like PHP traits=0A=
- https://doc.rust-lang.org/edition-guide/rust-2018/trait-system/associated=
-constants.html allows defining constants with values on traits=0A=
  (it also does various other things that impractical for php)=0A=
=0A=
One thing to document/add tests for would be the resolution of `self::MY_CO=
NST`.=0A=
In a trait, `self::method()` may be overridden by the method of the class t=
hat's using the trait.=0A=
I'd expect `self::MY_CONST` to be the same.=0A=
=0A=
```php=0A=
<?php=0A=
trait T {=0A=
    public static function main() { self::foo(); }=0A=
    public static function foo() { echo "In trait\n"; }=0A=
}=0A=
class C {=0A=
    use T;=0A=
    public static function foo() { echo "In override\n"; }=0A=
}=0A=
C::main(); // outputs "In override"=0A=
```=0A=
=0A=
Other thoughts:=0A=
- Should renaming and insteadof also work for constants? I'd assume yes htt=
ps://www.php.net/manual/en/language.oop5.traits.php#language.oop5.traits.co=
nflict=0A=
  (e.g. `use A, B { const A::MY_CONST insteadof B; }`)=0A=
- We already check if there are conflicting values when inheriting constant=
s from an interface and another interface/trait.=0A=
=0A=
Cheers,=0A=
- Tyson=