Perl Hacks

Hack #57. Name Your Anonymous Subroutines

Trade a little anonymity for expressivity.

Despite the apparently oxymoronic name, "named anonymous subroutines" are an undocumented feature of Perl. Originally described by "ysth" on Perl Monks, these are a wonderful feature.

Suppose your program merrily runs along with a carefree attitude—but then dies an ugly death:

Denominator must not be zero! at anon_subs.pl line 11
      main::__ANON__(0) called at anon_subs.pl line 17

What the heck is main::__ANON__(0)? The answer may be somewhere in code such as:

use Carp;

sub divide_by
{
    my $numerator = shift;
    return sub
    {
        my $denominator = shift;
        croak "Denominator must not be zero!" unless $denominator;
        return $numerator / $denominator;
    };
}

my $seven_divided_by = divide_by(7);
my $answer           = $seven_divided_by->(0);

In this toy example, it's easy to see the problem. However, what if you're generating a ton of those divide_by subroutines and sending them all throughout your code? What if you have a bunch of subroutines all generating subroutines (for example, if you've breathed too deeply the heady fumes of Mark Jason Dominus' Higher Order Perl book)? Having a bunch of subroutines named __ANON__ is very difficult to debug.

Tip

$seven_divided_by is effectively a curried version of divide_by( ). That is, it's a function that already has one of multiple arguments bound to it. There's a piece of random functional programming jargon to use to impress people.

The Hack

Creating an anonymous subroutine creates ...