Weakening captures using Sub::Quote - perl

Question

I'd like to weaken captured variables in the code generated by Sub::Quote. For example, here's the non-quoted alternative:
use 5.10.0;
use Scalar::Util qw[ weaken ];
{
my $s = 'foo';
my $x = sub { say $s };
weaken( my $y = $x );
my $bar = sub { &$y };
&$bar;
$x = undef;
&$bar
}
and the output:
foo
Can't use an undefined value as a subroutine reference [...]
And here's my Sub::Quote attempt:
use 5.10.0;
use Sub::Quote;
use Scalar::Util qw[ weaken ];
{
my $s = 'foo';
my $x = sub { say $s };
weaken( my $y = $x );
my $bar = quote_sub( '&$y', { '$y' => \$y } );
&$bar;
$x = undef;
&$bar;
}
and the output:
foo
foo
Obviously the captured $y isn't weakened. Is there a way of altering the generated code to weaken captured variables?
The documentation is sparse, and the Sub::Quote implementation is complex; I'm fairly convinced this isn't possible with the current code, but I'd love to be shown to be wrong.

Answer 1

my $bar = quote_sub( '&$y', { '$y' => \$y } );
is roughly the same as
my $bar = eval(q{ my $y = $y; sub { &$y } });
(It does more, but those bits are irrelevant to this question). As you can see, that creates a new strong reference to the sub[1].
As a workaround, you could add a layer of indirection:
my $bar = eval(q{ my $y_ref = \$y; sub { &{ $$y_ref } } });
This can be achieved by using:
my $bar = quote_sub( '&{$$y_ref}', { '$y_ref' => \\$y } );
There wouldn't be any problems if the $y created by Sub::Quote was an alias for your $y. This can be achieved using Data::Alias or an experimental feature introduced in 5.22.
This can be demonstrated using the following:
{
package Sub::Quote;
my $sub = sub {
my ($from, $captures, $indent) = #_;
join(
'',
"use feature qw( refaliasing );\n",
"no warnings qw( experimental::refaliasing );\n",
map {
/^([\#\%\$])/
or croak "capture key should start with \#, \% or \$: $_";
(' ' x $indent).qq{\\my ${_} = \\${1}{${from}->{${\quotify $_}}};\n};
} keys %$captures
)
};
no warnings qw( redefine );
*capture_unroll = $sub;
}
my $bar = quote_sub( '&$y', { '$y' => \$y } );
You could talk to the module's maintainer about adding an option that would cause the use of aliasing.
When you create a copy of a (strong or weak) reference, it's a strong reference.