Yay! Example

1_source.php

<?php
@Some(a = 23, b = 42)
class TestClass
{
    public function test($a)
    {

    }
}

2_annotation.yay.php

<?php
macro ·global ·unsafe {
    @·ns()·class(
        ·ls (
            ·chain (T_STRING·field, ·token('='), ·label()·value),
            ·token(',')
        )
        ·fields
    )
    class T_STRING·class_name
} >> {
    new class(new \ReflectionClass(T_STRING·class_name::class)) extends ·class {
        public function __construct(\ReflectionClass $context)
        {
            ·fields ··· {
                $this->T_STRING·field = ·value;
            }
        }
    }
    class T_STRING·class_name
}

3_output.php

<?php
new class(new \ReflectionClass(TestClass::class)) extends Some {
  public function __construct(\ReflectionClass $context)
  {
    $this->a = 23;
    $this->b = 42;
  }
}

class TestClass
{
    public function test($a)
    {

    }
}