Recently, I had an interesting concept pop into my mind. Is it possible to use constants as package methods, and if so, is there any compelling reason why people don't do that more often? A little research showed me that yes, it is possible to use constants as package methods, so now I open the question up to my fellow monks:

Is it a BAD THING(tm) to use constants as package methods?

Pros:

• It's a constant. It can't be changed.
• But it can still be overridden by derived classes.
  • Another 'use constant' line
  • A subroutine that computes a value. Not necessarily a constant.
• Can use either indirect or OO invocation syntax.
• The meaning is explicit in the 'use constant' line.

Cons:

• Lose inlining due to method lookup
• No interpolation in strings, without the standard constant trick.( ie. @{$obj->VALUE} )
• Derived classes MUST consistently call as a method invocation, not as a subroutine

Example:

test.pl

#!perl
use strict;
use warnings;

use My::Package;
use My::Package::Derived;

local $\ = "\n";

my $item1 = new My::Package;
my $item2 = new My::Package::Derived;

print $item1->SOME_VALUE;
print $item1->SOME_DERIVED_VALUE;
print $item1->ANOTHER_DERIVED_VALUE;

print '-' x 30;

print $item2->SOME_VALUE;
print $item2->SOME_DERIVED_VALUE;
print $item2->ANOTHER_DERIVED_VALUE;
[download]

My/Package.pm

package My::Package;
use strict;
use warnings;

use constant SOME_VALUE => 12;
use constant SOME_DERIVED_VALUE => 14;
use constant ANOTHER_DERIVED_VALUE => 21;

sub new {
    bless {}, ref($_[0]) || $_[0];
}

1;
[download]

Derived.pm

package My::Package::Derived;
use strict;
use warnings;
use My::Package;

our (@ISA) = qw[My::Package];

use constant SOME_DERIVED_VALUE => 15;

sub ANOTHER_DERIVED_VALUE() {
    int(rand(100)+1);
}

1;
[download]

References

• constant
• my $var = ''; vs. use constant VAR => '';
• What's the Right Way to declare constants in packages?
• Constant Functions

2002-03-13 Edit by Corion : Fixed title (almost) per request

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