Question

Consider the following Perl code.

#!/usr/bin/perl

use strict;
use warnings;

$b="1";

my $a="${b}";

$b="2";

print $a;

The script obviously outputs 1. I would like it to be whatever the current value of $b is.

What would be the smartest way in Perl to achieve lazy evaluation like this? I would like the ${b} to remain “unreplaced” until $a is needed.

Answer 1

I’m more interested in knowing why you want to do this. You could use a variety of approaches depending on what you really need to do.

You could wrap up the code in a coderef, and only evaluate it when you need it:

use strict; use warnings;

my $b = '1';
my $a = sub { $b };
$b = '2';
print $a->();

A variant of this would be to use a named function as a closure (this is probably the best approach, in the larger context of your calling code):

my $b = '1';
sub print_b
{
    print $b;
}

$b = '2';
print_b();

You could use a reference to the original variable, and dereference it as needed:

my $b = '1';
my $a = \$b;
$b = '2';
print $$a;