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Date: Fri, 24 Jun 2022 18:11:55 +0200
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To: Rowan Tommins <rowan.collins@gmail.com>
Cc: PHP Internals List <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC] [Under Discussion] Auto-implement Stringable for
 string backed enums
From: nicolas.grekas+php@gmail.com (Nicolas Grekas)

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> domain SymfonyPermission: string;
> domain AcmePermission: string { 'admin' | 'user' | 'bot' };
> [...]
> Domains can also be considered sets, which you could compare directly,
> and maybe even calculate intersections, unions, etc:
>
> The actual values would be ordinary strings, and type constraints would
> just be checking the value passed against the domain:
>
> Crucially, this solves the described problem of a library accepting an
> infinite (or perhaps just very wide) set of values, and a consuming app
> wanting to constrain that set within its own code.
>
> It's one disadvantage is the typo-proofing and look up availability that
> constants give, but you could always combine the two.
>

Thanks for this idea Rowan, that's really interesting.
I would go one step further and require naming the values in the set.
Borrowing from the syntax of enums, we could have:

set AcmePermission: string {
  case ADMIN = 'admin';
  case USER = 'user';
  case BOT = 'bot';
}

Then AcmePermission::ADMIN would === 'admin' and, as you said,
AcmePermission could be used as a type on functions:

function (AcmePermission $perm): AcmePermission
{
    $perm instanceof AcmePermission; // true

    return $perm;
}

In order to make this work, we would need to be able to autoload the type
AcmePermission.This could be done by the type-checking logic: when checking
$var against AcmePermission and the type is undefined, if $var is an int or
a string, call the autoloader for 'AcmePermission'.

There are other things to consider, like possible changes to reflection,
whether such a set also declares a class like enums do, etc.But overall, I
really like it and I agree with you: this would provide a really good
solution for the problem ppl try to use enums for.

I don't know how we could move forward on that idea. By starting another
thread?

Nicolas

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