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Re: [perl #79908] my $x; sub(){$x} makes a constant even if$xchanges

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From:
Dr.Ruud
Date:
December 1, 2010 23:24
Subject:
Re: [perl #79908] my $x; sub(){$x} makes a constant even if$xchanges
Message ID:
20101201183830.21133.qmail@lists-nntp.develooper.com
On 2010-12-01 00:24, Abigail wrote:
> On Tue, Nov 30, 2010 at 10:34:36PM +0100, Dr.Ruud wrote:
>> On 2010-11-29 10:22, Dave Mitchell wrote:
>>> On Sun, Nov 28, 2010 at 05:16:28PM -0800, Father Chrysostomos wrote:

>>>> I know this is what constant.pm used to use, but it causes a bug:
>>>
>>> What do you think is the bug? It seems to be behaving the way I expect.
>>>
>>>> $ perl -MO=Deparse -e' BEGIN{my $x = 5; *foo = sub(){$x}; $x=6} print foo'
>>>> sub BEGIN {
>>>>       my $x = 5;
>>>>       *foo = sub () {
>>>>           $x;
>>>>       }
>>>>       ;
>>>>       $x = 6;
>>>> }
>>>> print 5;
>>
>> The compiler should patch the constsub at the '$x = 6' line.
>
> If the compiler can detect that afterwards $x won't change - but you
> need to be able to solve the halting problem to do that in general.
> For instance,
>
>    BEGIN {
>       my $x = 5;
>       *foo = sub () {$x};
>       *bar = sub {$x = shift};
>    }
>
> the compiler could only patch the constsub if it knows bar() won't
> be called.

And that is not '$x = CONSTANT', so that should then be a compile error.

-- 
Ruud

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