PHP: rfc:functional-interfaces

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====== PHP RFC: Functional Interfaces ======
  * Version: 0.1
  * Date: 2016-04-17
  * Author: krakjoe
  * Status: Declined
  * First Published at: http://wiki.php.net/rfc/functional-interfaces

===== Introduction =====
A functional interface is an interface which declares only one abstract method, a familiar example is Countable:

<code php>
<?php
interface Countable {
    public function count();
}
?>
</code>

Such interfaces are also known as SAM (Single Abstract Method) interfaces.

While the language has a few examples of functional or SAM interfaces, the ecosystem has many more.

===== Proposal =====
A closure is able to provide a way to implement a functional interface:

<code php>
<?php
interface IFoo {
    public function method() : int;
}

$cb = function () implements IFoo : int {
    return 42;
};
</code>

There is enough information in the code above for the engine to reason that $cb should implement IFoo, and obviously be a Closure.

The engine generates the appropriate class entry using the closure as the only public method, having easily determined the correct name for that method (there is, and can only be, one possible candidate).

This is extremely powerful, because Closures have lexical scope, and so unlike an anonymous class, can access the private properties and other symbols where the Closure is declared.

The code below is not good code, it's not the most efficient version of the code that could exist. It serves to show the difference between implementing a functional interface using a closure, and provides a comparison with anonymous classes:

[[https://3v4l.org/b4AWq/rfc#rfc-func_interfaces|Functional Interfaces - Counter Example]]

<code php>
<?php
class Foo {
	private $bar = [];

	public function fill($limit = 100) {
		for ($i = 0; $i < $limit; $i++) {
			$this->bar[] = mt_rand($i, $limit);
		}
	}

	public function getEvenCounter() : Countable {
		return function () implements Countable {
			$counter = 0;
			foreach ($this->bar as $value) {
				if ($value % 2 === 0)
					$counter++;
			}
			return $counter;
		};
	}

	public function getOddCounter() : Countable {
		return function () implements Countable {
			$counter = 0;
			foreach ($this->bar as $value) {
				if ($value % 2 !== 0) {
					$counter++;
				}
			}
			return $counter;
		};
	}
}

$foo = new Foo();

$even = $foo->getEvenCounter();
$odd = $foo->getOddCounter();

$it = 0;

while (++$it<10) {
	$foo->fill(50);
	var_dump(
		count($even),
		count($odd));
}
?>
</code>

The same code using anonymous classes:

<code php>
<?php
class Foo {
	private $bar = [];

	public function fill($limit = 100) {
		for ($i = 0; $i < $limit; $i++) {
			$this->bar[] = mt_rand($i, $limit);
		}
	}

	public function getEvenCounter() : Countable {
		return new class($this->bar) implements Countable {
			public function __construct(&$bar) {
				$this->bar =& $bar;
			}

			public function count() {
				$counter = 0;
				foreach ($this->bar as $value) {
					if ($value % 2 === 0)
						$counter++;
				}
				return $counter;
			}

			private $bar;
		};
	}

	public function getOddCounter() : Countable {
		return new class($this->bar) implements Countable {
			public function __construct(&$bar) {
				$this->bar =& $bar;
			}

			public function count() {
				$counter = 0;
				foreach ($this->bar as $value) {
					if ($value % 2 !== 0) {
						$counter++;
					}
				}
				return $counter;
			}

			private $bar;
		};
	}
}

$foo = new Foo();

$it = 0;

$even = $foo->getEvenCounter();
$odd = $foo->getOddCounter();

while (++$it<10) {
	$foo->fill(50);

	var_dump(
		count($even),
		count($odd));
}
?>
</code>

The anonymous class version:

  * must use referencing, or fetch a new Countable object on each iteration, 
  * is extremely verbose
  * must set dependencies in the constructor
  * has no support for lexical scope

The functional interface version:

  * is sparse
  * is easier to reason about
  * does not require the use of references
  * supports lexical scope

Functional interface support does not change the definition of an interface, and only reuse the definition of a Closure.

===== Receiving and Invoking Functional Interfaces =====

The implementation of a functional interface is an instance of Closure, and the interface it was declared to implement, it has the behaviour of both.

The implementation would have the following formal definition:

<code>final class {Interface}\0{closure} extends Closure implements Interface</code>

Such that the following is always true:

<code>$instance instanceof Interface && $instance instanceof Closure</code>

[[https://3v4l.org/mOJXI/rfc#rfc-func_interfaces|Functional Interfaces - Receiving and Invoking]]

<code php>
<?php
interface ILog {
	public function log(string $message, ... $args) : void;
}

class Foo {
	public function __construct(ILog $logger) {
		$this->logger = $logger;
	}

	public function thing() {
		$this->logger->log("thing");
	}
}

$logger = function (string $message, ... $args) implements ILog : void {
	printf("{$message}\n", ... $args);
};

$foo = new Foo($logger);
$foo->thing();

$logger("next thing");
</code>

This means that the receiver (%%Foo::__construct%%) can receive, and consumer (Foo::thing) can invoke the interface as if it were a normal object, while the creator of $logger, who must know it is a Closure, can still invoke it as a Closure.

Both methods of invocation are valid in both receiving and declaring contexts.

===== Error Conditions =====
The following conditions will cause compiler errors:

[[https://3v4l.org/65W6i/rfc#rfc-func_interfaces|Functional Interfaces - Compiler Error 1]]

<code php>
<?php
interface IFoo {
    public function method1();
    public function method2();
}

function () implements IFoo {};
?>
</code>

<code>Fatal error: cannot implement non functional interface IFoo in /in/65W6i on line 7</code>

//Reason: IFoo cannot be considered a functional interface, because it contains more than one abstract method.//

[[https://3v4l.org/qLbPv/rfc#rfc-func_interfaces|Functional Interfaces - Compiler Error 2]]

<code php>
<?php
interface IFoo {
    public function foo();
}
interface IBar extends IFoo {
    public function bar();
}

function () implements IBar {};
</code>

<code>Fatal error: cannot implement non functional interface IBar in /in/qLbPv on line 9</code>

//Reason: Although IBar only declares one abstract method, it extends IFoo and so contains two abstract methods//

[[https://3v4l.org/WT98N/rfc#rfc-func_interfaces|Functional Interfaces - Compiler Error 3]]

<code php>
<?php
abstract class Foo {
    abstract public function bar();
}

function () implements Foo {};
</code>

<code>Fatal error: cannot implement non interface Foo in /in/WT98N on line 6</code>

//Reason: Although Foo contains only one abstract method, it is not an interface//

[[https://3v4l.org/MMuD0/rfc#rfc-func_interfaces|Functional Interfaces - Compiler Error 4]]

<code php>
<?php
new class {
    public function __construct() {
        function () implements self {};
    }
};
</code>

<code>Fatal error: functional interface cannot implement self in /in/MMuD0 on line 4</code>

//Reason: Although self is a valid scope in that context, self, parent, and static, can never be interfaces//

[[https://3v4l.org/2AiUV/rfc#rfc-func_interfaces|Functional Interfaces - Compiler Error 5]]

<code php>
<?php
interface IFoo {
	public static function method();
}

function () implements IFoo {};
</code>

<code>Fatal error: cannot create non static implementation of static functional interface IFoo in /in/2AiUV on line 6</code>

//Reason: The compiler would raise less specific errors later on//

[[https://3v4l.org/o9gIB/rfc#rfc-func_interfaces|Functional Interfaces - Compiler Error 6]]

<code php>
<?php
interface IFoo {
	public function method();
}

static function () implements IFoo {};
</code>

<code>Fatal error: cannot create static implementation of non static functional interface IFoo in /in/o9gIB on line 6</code>

//Reason: The compiler would raise less specific errors later on//

===== Syntax Choices =====

Interface and return type reversed:

<code>function (string $arg) use($thing) : int implements Interface</code>

It looks as if ''int'' is somehow implementing ''Interface''.

Interface before arguments:

<code>function implements Interface (string $arg) use($thing) : int {}</code>

The arguments list looks as if it somehow applies to ''Interface''.

Interface after arguments and before use:

<code>function (string $arg) implements Interface use($thing) : int {}</code>

This looks as if ''Interface'' somehow uses ''$thing''.

===== Vote =====

Voting started on May 15th, ended May 29th 2016.

<doodle title="Accept functional interfaces? (2/3+1 majority required)" auth="krakjoe" voteType="single" closed="true">
   * Yes
   * No
</doodle>

===== Backward Incompatible Changes =====
N/A

===== Proposed PHP Version(s) =====
7.1

===== RFC Impact =====

==== To Existing Extensions ====
The API to create functional interface implementations is exported by Zend, and part of the already exported Closure API.

==== To Opcache ====
Opcache may need a trivial patching.

===== Future Scope =====
When the concept of functional interfaces is implemented, it may be worth discussing the coercion, or explicit cast of callables.

===== Proposed Voting Choices =====
2/3 majority required, simple yes/no vote proposed.

===== Patches and Tests =====

https://github.com/php/php-src/pull/1866

===== 3v4l =====

3v4l have been kind enough to provide testing facilities for this patch.

===== References =====

[[http://news.php.net/php.internals/92404|php.internals]]
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