rfc:simple-annotations

PHP RFC: Simple Annotations

Introduction

This RFC proposes the introduction of simple value annotations - arbitrary values attached to classes and class members as meta-data, obtainable via reflection.

As an alternative proposal to attributes, this proposal aims to fully leverage existing language features, in order to provide more flexibility, lessen the learning curve, and expose meta-data in a manner that is more immediately useful, without having to build any run-time facilities.

Compared with annotation systems such as Doctrine Annotations, this proposal does not attempt to define or enforce any domain rules - it does not define inheritance semantics, rules about applicable source-code elements, cardinality, or any other rules; these can be defined and implemented by userland packages.

Proposal

The proposed syntax of a single annotation is very simple:

"<<" <php-expression> ">>"

Any valid PHP expression is a valid annotation.

Any number of annotations may be placed in front of any of the following applicable declarations:

  • class, trait and interface declarations
  • function and property declarations in classes/traits/interfaces
  • anonymous function declarations
  • anonymous class declarations (and their members)
  • argument declarations of functions, methods and closures

Annotations are internally collected, for each annotated class or member, in a list which can be obtained via reflection.

The following trivial example annotates a class with a string and an array of numbers:

<< "Hello World" >>
<< [1, 2, 3] >>
class Hello
{}

The following example annotates an entity with a TableName instance, which might be consumed by a database abstraction:

class TableName
{
    public $name;
 
    public function __construct($name) {
        $this->name = $name;
    }
}
 
<< new TableName("users") >>
class User
{
    // ...
}
 
$reflection = new ReflectionClass(User::class);
 
var_dump($reflection->getAnnotations());

Example output:

array(1) {
  [0]=>
  object(TableName)#1 (1) {
    ["name"]=>
    string(5) "users"
  }
}

Annotation expressions are not evaluated until reflection is invoked, and are evaluated only once and internally memoized upon the first call to getAnnotations().

Annotations are Context-free

By design, annotations expressions are evaluated individually in an empty scope - which means there is no access to variables in the parent class, file, local or global scope, e.g. no $this, self or static.

Annotations work consistently regardless of which source element they are applied to, and may be evaluated without first creating an object instance.

Annotations that do require context should explicitly ask for that context - for example, you could use an anonymous function, a callable, or an anonymous class, to provide context via dependency injection.

Reflection API

The following classes will have an added getAnnotations() method:

* ClassReflection * ReflectionFunctionAbstract (ReflectionFunction and ReflectionMethod) * ReflectionProperty * ReflectionParameter

The getAnnotations() method has the following signature:

  public function getAnnotations($filter = null) : array

The optional $filter argument, if given, filters the returned list of annotations as follows:

  • If one of string, int, float, bool are given, filters annotations using is_int(), is_string(), etc.
  • If a fully-qualified class-name is given, filters annotations using instanceof

If null is given (default) all annotations are returned.

These methods do not take into account inheritance - annotations belong to the actual *declaration*, not to an abstract *member*, and as such, traversing parent classes, interfaces, etc. is up to the consumer.

Backward Incompatible Changes

None.

Proposed PHP Version(s)

Next PHP 7.x.

RFC Impact

To SAPIs

TODO

To Existing Extensions

TODO

To Opcache

TODO

Open Issues

Make sure there are no open issues when the vote starts!

Unaffected PHP Functionality

Annotations are a new feature - it does not affect any existing functionality.

Out of Scope

It has been suggested that this RFC should reserve certain names for compiler directives, such as (for instance) the memoization-directive supported by Hack. This proposal does not reserve any such names, because (as others pointed out during that discussion) these are not meta-data, but rather directives for the compiler, and such features ought to be supported directly by keywords or syntax rather than by magical meta-data.

Proposed Voting Choices

TODO State whether this project requires a 2/3 or 50%+1 majority (see voting)

Patches and Tests

There is a draft with no available implementation at this time.

Implementation

TODO After the project is implemented, this section should contain

  1. the version(s) it was merged to
  2. a link to the git commit(s)
  3. a link to the PHP manual entry for the feature

References

Rejected Features

None.

rfc/simple-annotations.txt · Last modified: 2017/09/22 13:28 (external edit)