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Date: Thu, 30 Jun 2022 12:28:50 +0200
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To: Robert Landers <landers.robert@gmail.com>
Cc: Rowan Tommins <rowan.collins@gmail.com>, internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC] Short Closures 2, aka auto-capture take 3
From: guilliam.xavier@gmail.com (Guilliam Xavier)

On Thu, Jun 30, 2022 at 11:20 AM Robert Landers
<landers.robert@gmail.com> wrote:
>
> On Thu, Jun 30, 2022 at 10:19 AM Rowan Tommins <rowan.collins@gmail.com> wrote:
> >
> > 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]
> >
> > --
> > PHP Internals - PHP Runtime Development Mailing List
> > To unsubscribe, visit: https://www.php.net/unsub.php
> >
>
> > For that to work, it would require the variable to be captured by
> > reference, not value.
>
> 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.
>
> --
> PHP Internals - PHP Runtime Development Mailing List
> To unsubscribe, visit: https://www.php.net/unsub.php
>

No offense, but why don't you just try it? Please see equivalents of:
  - Dan's code: https://3v4l.org/51jXY => doesn't "work"
  - Dan's code with capture by reference (as said by Rowan):
https://3v4l.org/JoUVi => "works"
  - Rowan's code: https://3v4l.org/7ZVv3 => shows the "problem" with capture

PS: I see that Rowan just replied with refcount explanations. I agree
(but am sending this anyway)

Regards,

-- 
Guilliam Xavier