Question

I have a Perl script that’s trying to set some configured DateTime and DateTime::Duration instances as Readonly constants. But I see strange behavior when trying to do math on these objects if they are Readonly. Here’s a minimal example:

#!/usr/bin/perl -w

use strict;
use warnings;

use DateTime;
use Readonly;

Readonly my $X => DateTime->now;
my $x = DateTime->now;

Readonly my $Y => DateTime::Duration->new( days => 3 );
my $y = DateTime::Duration->new( days => 3 );

my $a = $X - $Y;
my $b = $x - $y;

print "$a\n";
print "$b\n";

On my system (Perl 5.10.0 on OSX) this displays:

$ ./datetime_test.pl 
Argument "2011-07-12T20:36:08" isn't numeric in subtraction (-) at ./datetime_test.pl line 15.
-4305941629
2011-07-09T20:36:08

So it looks like making the DateTime and the DateTime::Duration Readonly causes them to function incorrectly. Is this a bug? Or am I using Readonly wrong? I’ve also tried Readonly::Scalar and Readonly::Scalar1, and both behave the same way.

Answer 1

The problem is that they’re objects (references), not normal scalars. You would need to Readonly the values contained in the references, not the references themselves; but this turns out to be tricky. Something like this appears to work:

use Readonly;
use DateTime;

# you can't just say "Readonly %$dt"; here at least, it dies on blessed refs
sub makeRO {
  my $dt = shift;
  while (my ($k, $v) = each %$dt) {
    Readonly $dt->{$k} => $v;
  }
}

my $x = DateTime::Duration->new(days => 3);
makeRO($x);
my $y = DateTime::Duration->new(days => 3);

my $a = $x - $y;
# print "$a\n"; # this isn't overloaded; you'll get "DateTime::Duration=HASH(...)"
print $a->days, "\n";