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Date: Mon, 29 May 2023 18:43:44 -0700
Cc: PHP internals <internals@lists.php.net>
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To: Andreas Hennings <andreas@dqxtech.net>
Subject: Re: [PHP-DEV] Declaration-aware attributes
From: dusk@woofle.net (Dusk)

On May 29, 2023, at 17:48, Andreas Hennings <andreas@dqxtech.net> wrote:
> Quite often I found myself writing attribute classes that need to fill
> some default values or do some validation based on the symbol the
> attribute is attached to.
> E.g. a parameter attribute might require a specific type on that
> parameter, or it might fill a default value based on the parameter
> name.

+1. This is a substantial limitation in the attribute system.

> Currently I see two ways to do this:
> 1. Do the logic in the code that reads the attribute, instead of the
> attribute class. This works ok for one-off attribute classes, but it
> becomes quite unflexible with attribute interfaces, where 3rd parties
> can provide their own attribute class implementations.
> 2. Add additional methods to the attribute class that take the symbol
> reflector as a parameter, like "setReflectionMethod()", or
> "setReflectionClass()". Or the method in the attribute class that
> returns the values can have a reflector as a parameter.

I see a third way which introduces less "magic":

3.a. Add a method to ReflectionAttribute which retrieves the target of =
the attribute as an appropriate reflection object. (Sadly, the obvious =
name "getTarget" is already taken; I'll call it "getReflectionTarget" =
for now.)

3.b. Add a static method to ReflectionAttribute which, when called =
within an Attribute constructor which is being called by =
ReflectionAttribute::newInstance(), returns the ReflectionAttribute =
object which is being instantiated.

These features could be used together to set a default property on an =
attribute based on its target, e.g.

    #[Attribute(Attribute::TARGET_PROPERTY)]
    class PropertyAnnotation {
      public string $name;
 =20
      public function __construct(?string $name =3D null) {
        $this->name =3D $name ?? =
ReflectionAttribute::underConstruction()->getReflectionTarget()->getName()=
;
      }
    }

Another variant that comes to mind is:

3.b. Add a new flag to attributes which causes the ReflectionAttribute =
object to be passed to the constructor as the first argument, e.g.

    #[Attribute(Attribute::TARGET_PROPERTY | Attribute::WHO_MADE_ME)]
    class PropertyAnnotation {
      public string $name;

      public function __construct(ReflectionAttribute $attr, ?string =
$name =3D null) {
        $this->name =3D $name ?? =
$attr->getReflectionTarget()->getName();
      }
    }

This is a little messier because it can't be used "under the covers" by =
an attribute base class, but it accomplishes the same goals.=