How to watch particular hash key for changing its value?

Question

I have a hash, e.g. $hash->{'foo'}{'bar'}.

I want to call Carp::cluck in any place where value of bar key changed.

How to do that ? Is there any ready module on CPAN that can do that trick ?

Answer 1

my $hash = { foo => { bar => 1 } };
Internals::SvREADONLY( $hash->{foo}{bar}, 1 );
$hash->{foo}{bar} = 2;

produces

Modification of a read-only value attempted at -e line 4.

But that's a fatal error, and it doesn't include a trace (unless Carp::Always is used).

I would recommend adding set magic to the scalar.

use Carp            qw( cluck );
use Variable::Magic qw( wizard cast );

my $wizard = wizard(
   set => sub {
      cluck("Warning: Modification of a read-only value attempted");
   },
);

my $hash = { foo => { bar => 1 } };
cast( $hash->{foo}{bar}, $wizard );
$hash->{foo}{bar} = 2;

produces

Warning: Modification of a read-only value attempted at -e line 6.
        main::__ANON__(SCALAR(0x4200c90), undef) called at -e line 12
        eval {...} called at -e line 12

The same can be accomplished with tie, but it would be more expensive. (Tied variables are built upon magic.)

Answer 2

Tie::Trace almost gets you there.

use Tie::Trace 'watch';
my $hash = { foo => { bar => "original value" } };
watch $hash->{foo}{bar};
sub f1 { f2() }
sub f2 { f3() }
sub f3 { $hash->{foo}{bar} = "new value" }
f1();

Output:

'new value' at watch.pl line 6

You can make the output produce a full stack trace by importing Carp::Always or by monkey patching the Tie::Trace::_carpit function or with a $SIG{__WARN__} handler like

$SIG{__WARN__} = sub {
    if (caller(0) eq 'Tie::Trace') {
        # warning is from Tie::Trace
        Carp::cluck(@_);
    } else {
        CORE::warn(@_);
    }
};
...