Closures and $this

Hi,

since a few months have passed since the last discussion on this topic
and perhaps people had time to gather some experience with the current
closure implementation in PHP 5.3 I'd like to restart the debate on
$this in closures and object extension.

Foreword: The discussion should center on the future behaviour of PHP 6
ONLY. Whether or not a backport is even possible (binary compability
etc.) or should even be done should be topic of a separate discussion
and should NOT influence the decision. The ONLY goal of this
discussion should be to agree on a SANE way of implementing $this for
closures for PHP 6.

I've updated the original RFC I wrote a tiny bit (I didn't change much):

http://wiki.php.net/rfc/closures/object-extension

The basic outline is the following:

• In the first section I explain the general issue.
• In the second section I show the proposals that were made on
  internals@ before I wrote the RFC.
• In the third section I compare the approaches and explain why
  the approaches (B) and (D) are inconsistent.
• In the fourth section I propose a new approach (bindTo) that
  was only briefly discussed after the original RFC.

Please read the complete RFC and try to understand the points I'm trying
to make. If something is unclear, please ask first. In the past
discussion I had the impression that a lot of people understood only
partial aspects of the problem which made the discussion extremely
noisy. I believe that is a disservice to the issue. However, now we have
the huge advantage of NOT having an immanent deadline for a release. I
hope this will enable a consensus on this issue.

Discuss away!

Regards,
Christian

Hi.

Personally, I beleive that (A) approach is the best: "bind $this to
the object scope at creation" and never change it, issue error when
$this is used where not available. It also seems to me like this way
it could be implemented with better performance, am I right?

Dynamically changing $this introduces confusion. You can use
$this->foo or $this->bar() inside a closure, but when $this
dynamically changes - you cannot be sure that "foo" property or "bar"
method is still there. Furthermore, dynamic approach introduces even
more mess when thinking about availability of private/protected class
members.

Consider this code:

class Foo {
private $closure;
function __construct()
{
/* It seems natural to me that, doing this assignment from within the class,
one can use private members inside the closure */
$this->closure = function() { $this->privateMethod(); };
}
function setClosure($closure)
{
$this->closure = $closure;
}
function getClosure()
{
return $this->closure;
}
}

$foo = new Foo();
$foo->getClosure()->__invoke();// #CALL-1
$foo->setClosure(function() {
/*
But is it good to use private members here? It seems like it would
break encapsulation,
and the information that the class was designed to hide - is now
made available in public access,
although indirect.
*/
$this->privateMethod();
});
$foo->getClosure()->__invoke();// #CALL-2

In the end, I would like PHP to issue error when $this is used in
#CALL-2 but to continue working on #CALL-1.

The (A) approach seems to me more predictable, more simple, it has
less information the developer needs to keep in mind when using
closures.

2009/11/17 Mathieu Suen mathieu.suen@easyflirt.com:

Chris Stockton a écrit :

Hello,

On Tue, Nov 17, 2009 at 2:59 AM, Mathieu Suen
mathieu.suen@easyflirt.com wrote:

Christian Seiler a écrit :

Hi,

since a few months have passed since the last discussion on this topic
and perhaps people had time to gather some experience with the current
closure implementation in PHP 5.3 I'd like to restart the debate on
$this in closures and object extension.

I believe that (D) wins my vote, and it convinces me twice. Once
because I believe it is the most intuitive for users (A), and twice
because I believe it is also the most useful (C) for users. In my
opinion It seems the most "PHP" way.

-Chris

(D) is quite inconstant:

$block = $foo->closur;

$foo->closur();
$block();

This would produce 2 different output.
For (C) here is my objection:

Suppose that we are in a MVC pattern.
In a controller on could do:

$model->onFailDo(function () {$this->reportError()});
$model->save();

In the model side:

public function onFailDo($block)
{
       $this->signalFailure = $block
}

public function save()
{
       //... Something wrong happen
       return $this->signalFailure();
}

If you dynamically bind the $this you are obliged to treat the error
reporting in the model which is not a good idea. Even worst you can't even
catch up the error in the controller.
It also mean that the closure is getting tightly coupled with anyone who is
using it. And passing closure to different object is like spreading
dependency over all the system.

-- Mathieu Suen

--

--
Regards,
Victor

Hi!

If php-people really would like rebinding $this, I beleive this should
be done via method like Closure->bindTo() - to make the fact of
rebinding clear in the code.

However, there is a problem with rebinding $this - that was not
mentioned yet, I think.

This can be a problem, but since PHP is not a compiled language, this
problem already exists - all method names are only checked in runtime
anyway, so it doesn't matter if you rebind the closure or not. Of
course, with rebinding you can easily shoot yourself in the foot by
calling non-existing method - but so you can with $foo->$bar() or $foo()
or a number of other ways. So I think if this construct is expressed in
a way that makes the intent clear (i.e., no implicit rebinding) then it
would be OK.

Stanislav Malyshev, Zend Software Architect
stas@zend.com http://www.zend.com/
(408)253-8829 MSN: stas@zend.com

Hello.

Jelrik, I could easily show a real-world example where using $this in
closures could help. Consider Person class which has to verify that
email address is unique.
Suppose we have validation code in a single place and
filter_var_array() is used to check all the data:

class Person {
..
function validate()
{
filter_var_array(array(
'email' => array('filter' => FILTER_CALLBACK, 'callback' =>
$this->getEmailValidator())
));
}
private function getEmailValidator()
{
return function($email) {
// here, $this->id property is used, not only $email argument
}
}
}

When $this is available in closures, everything is ok. When you don't
have $this - you end up with messy decisions like:

private function getEmailValidator()
{
$self = $this;
return function($email) use ($self) {
// here, $self->getId() method is used
}
}

.. or:

function validate()
{
filter_var_array(array(
// $this->validateEmail() should be public - breaks encapsulation
'email' => array('filter' => FILTER_CALLBACK, 'callback' =>
array($this, 'validateEmail')
));
}

Of course, this is a special case - but I could easily imagine
ActiveRecord class in which validation is based on a similar approach.

This is why $this in closures would be useful for me - however I
definitely would like $this not to change ever.

2009/11/19 Jelrik van Hal jelrikvh@jelrikvh.nl:

Victor Bolshov wrote:

Hi.

Personally, I beleive that (A) approach is the best: "bind $this to
the object scope at creation" and never change it, issue error when
$this is used where not available. It also seems to me like this way
it could be implemented with better performance, am I right?

Dynamically changing $this introduces confusion. You can use
$this->foo or $this->bar() inside a closure, but when $this
dynamically changes - you cannot be sure that "foo" property or "bar"
method is still there. Furthermore, dynamic approach introduces even
more mess when thinking about availability of private/protected class
members.

Consider this code:

class Foo {
 private $closure;
 function __construct()
 {
   /* It seems natural to me that, doing this assignment from within the
class,
      one can use private members inside the closure */
   $this->closure = function() { $this->privateMethod(); };
 }
 function setClosure($closure)
 {
   $this->closure = $closure;
 }
 function getClosure()
 {
   return $this->closure;
 }
}

$foo = new Foo();
$foo->getClosure()->__invoke();// #CALL-1
$foo->setClosure(function() {
 /*
 But is it good to use private members here? It seems like it would
break encapsulation,
 and the information that the class was designed to hide - is now
made available in public access,
 although indirect.
 */
 $this->privateMethod();
});
$foo->getClosure()->__invoke();// #CALL-2

In the end, I would like PHP to issue error when $this is used in
#CALL-2 but to continue working on #CALL-1.

The (A) approach seems to me more predictable, more simple, it has
less information the developer needs to keep in mind when using
closures.

2009/11/17 Mathieu Suen mathieu.suen@easyflirt.com:

Chris Stockton a écrit :

Hello,

On Tue, Nov 17, 2009 at 2:59 AM, Mathieu Suen
mathieu.suen@easyflirt.com wrote:

Christian Seiler a écrit :

Hi,

since a few months have passed since the last discussion on this topic
and perhaps people had time to gather some experience with the current
closure implementation in PHP 5.3 I'd like to restart the debate on
$this in closures and object extension.

I believe that (D) wins my vote, and it convinces me twice. Once
because I believe it is the most intuitive for users (A), and twice
because I believe it is also the most useful (C) for users. In my
opinion It seems the most "PHP" way.

-Chris

(D) is quite inconstant:

$block = $foo->closur;

$foo->closur();
$block();

This would produce 2 different output.
For (C) here is my objection:

Suppose that we are in a MVC pattern.
In a controller on could do:

$model->onFailDo(function () {$this->reportError()});
$model->save();

In the model side:

public function onFailDo($block)
{
      $this->signalFailure = $block
}

public function save()
{
      //... Something wrong happen
      return $this->signalFailure();
}

If you dynamically bind the $this you are obliged to treat the error
reporting in the model which is not a good idea. Even worst you can't
even
catch up the error in the controller.
It also mean that the closure is getting tightly coupled with anyone who
is
using it. And passing closure to different object is like spreading
dependency over all the system.

-- Mathieu Suen

--

Hi,

Victor, I do wholeheartedly agree with you about the mess things will be
when allowing $this to refer to different things in different contexts of
calling the same closure (B, C and D): I think it'll prove very difficult to
keep track of which call will end up doing exactly what in which
environment.

What I'm not really aware of are any big advantages of rebinding $this to
the local environment of the calling code: isn't it just making $this/things
unnecessarily difficult?

Also, I'm wondering why people think there should be a $this inside a
closure. As stated in the RFC/Wiki, closures are not thought out to be used
alongside OO-code. Why would we want to couple them? I see the use of
closures, but not as OO-related tools using $this. What is the problem with
proposal (0), leaving it as it is now?

-jelrikvh

--

--
Regards,
Victor

Victor, I do wholeheartedly agree with you about the mess things will be
when allowing $this to refer to different things in different contexts of
calling the same closure (B, C and D): I think it'll prove very difficult to
keep track of which call will end up doing exactly what in which
environment.

I think any mechanism for rebinding $this (whether automagical or
explicit) will be too, well, weird. I can imagine a system of
rebinding $this being both consistent and useful, but it adds too much
complexity: to understand what's going on, you not only need to
understand closures, but you need to know how they interact with PHP's
OO system, you need to know what rebinding is (it doesn't happen
anywhere else that I can think of), and you need to know how the
rebinding system works. JavaScript's approach actually fairly
confusing--and this is despite the fact that the language has always
had it.

Also, I'm wondering why people think there should be a $this inside a
closure. As stated in the RFC/Wiki, closures are not thought out to be used
alongside OO-code. Why would we want to couple them? I see the use of
closures, but not as OO-related tools using $this. What is the problem with
proposal (0), leaving it as it is now?

I do think it would be nice if function() use ($this) { ... } worked,
and bound $this in the same way that it bound any other variable,
instead of producing "Cannot use $this as lexical variable". This
would eliminate the need to do:

$obj = $this;
$fn = function() use ($obj) {
// use $obj as if it were $this
}

Was this how things used to work, before

http://wiki.php.net/rfc/closures/removal-of-this

happened?

Michael

--
http://beebo.org