Question

Given a variable containing a string that represents the name of a package, how do I call a specific subroutine of the package?

Here’s the closest thing I have figured out:

package MyPackage;

sub echo {
    print shift;
}

my $package_name = 'MyPackage';
$package_name->echo('Hello World');

1;

The problem with this code is the subroutine is called as a class method; the package name is passed in as the first argument. I want to invoke the subroutine from the package name without a special first argument being implicitly passed in.

Answer 1

Perl method calls are just regular subroutines, which get the invocant as the first value.

use strict;
use warnings;
use 5.10.1;

{
  package MyPackage;
  sub new{ bless {}, shift } # overly simplistic constructor (DO NOT REUSE)
  sub echo{ say @_ }
}

my $package_name = 'MyPackage';
$package_name->echo;

my $object = $package_name->new();
$object->echo; # effectively the same as MyPackage::echo($object)

MyPackage
MyPackage=HASH(0x1e2a070)

---

If you want to call a subroutine without an invocant, you will need to call it differently.

{
  no strict 'refs';
  ${$package_name.'::'}{echo}->('Hello World');
  &{$package_name.'::echo'}('Hello World');
}

# only works for packages without :: in the name
$::{$package_name.'::'}{echo}->('Hello World');

$package_name->can('echo')->('Hello World');

• The can method returns a reference to the subroutine that would be called if it had been called on the invocant. The coderef can then be used separately.

  my $code_ref = $package_name->can('echo');
  $code_ref->('Hello World');
  

  There are some caveats to using can:

  • can may be overridden by the package, or any class from which it inherits.
  • The package that defines a method may be different than the invocant.

  This may actually be the behaviour you’re looking for though.

• Another approach is to use something called a symbolic reference.

  {
    no strict 'refs';
    &{ $package_name.'::echo' }('Hello World');
  }
  

  Using symbolic references is usually not recommended. Part of the problem is that it is possible to accidently use a symbolic reference where you didn’t intend on using one. This is why you can’t have use strict 'refs'; in effect.

  This may be the simplest way to do what you want to do though.

• If you don’t want to use a symbolic reference you could use the Stash.

  $MyPackage::{echo}->('Hello World');
  $::{'MyPackage::'}{echo}->('Hello World');
  
  $main::{'MyPackage::'}{echo}->('Hello World');
  $main::{'main::'}{'MyPackage::'}{echo}->('Hello World');
  $main::{'main::'}{'main::'}{'main::'}{'MyPackage::'}{echo}->('Hello World');
  

  The only problem with this is that you would have to split $package_name on ::

  *Some::Long::Package::Name::echo = \&MyPackage::echo;
  
  $::{'Some::'}{'Long::'}{'Package::'}{'Name::'}{echo}('Hello World');
  
  sub get_package_stash{
    my $package = shift.'::';
    my @package = split /(?<=::)/, $package;
    my $stash = \%:: ;
    $stash = $stash->{$_} for @package;
    return $stash;
  }
  get_package_stash('Some::Long::Package::Name')->{echo}('Hello World');
  

  This isn’t that big of a problem though. After a quick look on CPAN you find Package::Stash.

  use Package::Stash;
  my $stash = Package::Stash->new($package_name);
  my $coderef = $stash->get_symbol('&echo');
  $coderef->('Hello World');
  

  (The Pure Perl version of Package::Stash uses symbolic references, not the Stash)

---

It’s even possible to make an alias of the subroutine/method, as if had been imported from a module that was using Exporter:

*echo = \&{$package_name.'::echo'};
echo('Hello World');

I would recommend limiting the scope of the alias though:

{
  local *echo = \&{$package_name.'::echo'};
  echo('Hello World');
}

This is an exception, where you can use a symbolic reference with strict 'refs' enabled.