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Date: Thu, 30 Jun 2022 09:19:05 +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 29/06/2022 23:31, Dan Ackroyd wrote:
> Imagine some code that looks like this:
>
> // Acquire some resource e.g. an exclusive lock.
> $some_resource = acquire_some_resource();
>
> $fn = fn () {
>      // Free that resource
>      $some_resource = null;
> }
>
> // do some stuff that assumes the exclusive
> // lock is still active.
>
> // call the callback that we 'know' frees the resource
> $fn();
>
> That's a not unreasonable piece of code to write


For that to work, it would require the variable to be captured by 
reference, not value. Writing to a variable captured by value, like 
writing to a parameter passed by value, is just writing to a local variable.


In fact, the "optimisation" is in my opinion a critical part of the 
semantics, to avoid the opposite problem:

// Acquire some resource e.g. an exclusive lock.
$some_resource = acquire_some_resource();

$fn = fn () {
     // Use a variable that happens to have the same name
     // A naive implementation would see $some_resource mentioned, and 
capture it
     // Over-writing the local variable here makes no difference; the 
closure still holds the value for next time
     $some_resource = 'hello';
}

// Free what we believe is the last pointer, to trigger the destructor
unset($some_resource);

// If $some_resource gets captured, it can only be released by 
destroying the closure
unset($fn);


Regards,

-- 
Rowan Tommins
[IMSoP]