Data::Dumper incomplete reference relationship

[damluk]

Perl: 5.36
Module: Data::Dumper

The following Dump output correctly shows the relationship between $foo and $bar:

root@fa394195f07f:~# cat good.pl 
use Data::Dumper;
my $foo = [42];
my $bar = [\$foo];
CORE::say Data::Dumper->new([$foo, $bar], [qw[foo bar]])->Indent(0)->Dump;
root@fa394195f07f:~# perl good.pl 
$foo = [42];$bar = [\$foo];

However, when $foo is not a reference, the relationship is lost:

root@fa394195f07f:~# cat bad.pl 
use Data::Dumper;
my $foo = 42;
my $bar = [\$foo];
CORE::say Data::Dumper->new([$foo, $bar], [qw[foo bar]])->Indent(0)->Dump;
root@fa394195f07f:~# perl bad.pl 
$foo = 42;$bar = [\42];

[damluk]

root@fa394195f07f:~# cat workaround.pl 
package Dumper {
  use parent 'Data::Dumper';
  sub new {
    my ($class, $names) = (shift, pop);
    $class->SUPER::new(\@_, $names);
  }
}
my $foo = 42;
my $bar = [\$foo];
CORE::say Dumper->new($foo, $bar, [qw[foo bar]])->Indent(0)->Dump;
root@fa394195f07f:~# perl workaround.pl 
$foo = 42;$bar = [\$foo];

[tonycoz]

The problem is the entries in the first array ref are copies of $foo and $bar, so that step loses the link between them.

D::D handles this reasonably for references, but 42 in the array ref is no different to any other 42, consider if it were 0 or 1 instead.

For the class case, @_ contains aliases to $foo and $bar so that link remains.

You can also handle this with an anonymous sub:

my @names = qw(foo bar);
CORE::say sub { Data::Dumper->new(\@_, \@names)->Indent(0)->Dump; }->($foo, $bar);

[jkeenan]

@tonycoz, is there any need for clarification of documentation here? (Otherwise, it seems this ticket is closable.)

[tonycoz]

@tonycoz, is there any need for clarification of documentation here? (Otherwise, it seems this ticket is closable.)

I'm not sure.

The "working" case may be an accident, unlike new() the Values() method makes no attempt to save the aliasing of the values, copying them instead:

my @names = qw(foo bar);
CORE::say sub { Data::Dumper->new([], \@names)->Values(\@_)->Indent(0)->Dump; }->($foo, $bar);
# $foo = 42;$bar = [\42];

I think the Seen() method is the blessed way to get what the OP is after here, but it looks broken.

I tried supplying Seen({ '\\$foo' => \$foo, '\\$bar' => \$bar }) which sort of matches the result of Seen() on the "working" case, but Seen() mangles it, adding extra $.

So there may be a real bug here.

[damluk]

The variables to be dumped are not always known at compile time. If ($values, $names) = [$foo, $bar], [qw[foo bar]]) is only known at run time, one cannot use the alias trick unless the Data::Dumper call is evaled with an expanded list of variables. It would be better to have a mode of operation that can handle this case without eval. For example, the current call

Data::Dumper->new([$foo, $bar], [qw[foo bar]])

could be the implicit mode that is equivalent to this explicit one,

Data::Dumper->new([$foo, $bar], [qw[foo bar]], deref => 0)

while the following mode would have the opportunity to give correct results for the bad.pl use case:

Data::Dumper->new([\$foo, \$bar], [qw[foo bar]], deref => 1)