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Date: Thu, 30 Jun 2022 11:17:19 +0100
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Subject: Re: [PHP-DEV] [RFC] Short Closures 2, aka auto-capture take 3
From: rowan.collins@gmail.com (Rowan Tommins)

On 30/06/2022 10:20, Robert Landers wrote:
> I think their suggested code would work (at least currently in PHP) by
> the simple fact they would increase the reference count on that
> object/resource until they set it as null. However, with the
> "optimization," the reference count will never be incremented and thus
> fail to work as defined.


No, the captured value is tied to the lifetime of the closure itself, 
not the variable inside the closure.

$some_resource = acquire_some_resource();
// refcount=1 (outer $some_resource)

$fn = function() use ($some_resource) {
     $some_resource = null;
}
// refcount=2 (outer $some_resource, closure $fn)

$fn();
// during execution, refcount is 3 (outer $some_resource, closure $fn, 
local $some_resource)
// once the local variable is written to, the refcount goes back to 2 
(outer $some_resource, closure $fn)

unset($some_resource);
// refcount=1 (closure $fn)

$fn();
// the captured variable always starts with its original value, 
regardless of how many times you execute the function
// during execution, refcount is now 2 (closure $fn, local $some_resource)
// after execution, refcount is still 1 (closure $fn)

unset($fn);
// only now does the refcount go down to 0 and trigger the destructor


The only way for it to work would be using capture by reference (not 
supported by the proposed short syntax):

$some_resource = acquire_some_resource();
// refcount=1: simple variable

$fn = function() use (&$some_resource) {
     $some_resource = null;
}
// refcount=1: a reference set with 2 members (outer $some_resource, 
closure $fn)

$fn();
// during execution, we have a reference set with 3 members (outer 
$some_resource, closure $fn, local $some_resource)
// the assignment assigns to this reference set, changing the value 
referenced by all 3 members
// refcount on the resource drops from 1 to 0, triggering the destructor

$fn();
// because it was captured by reference, the initial value of 
$some_resource in the closure has now changed


Regards,

-- 
Rowan Tommins
[IMSoP]