lexical::underscore3pm

Usage

   use 5.010;
   use lexical::underscore;
   use Test::More;
   
   sub is_uppercase {
      my $var = @_ ? shift : ${lexical::underscore()};
      return $var eq uc($var);
   }
   
   my $thing = 'FOO';
   my $works = 0;
   
   given ( $thing ) {
      when ( is_uppercase ) { $works++ }
   }
   
   ok($works);
   done_testing();

Overview

Starting with Perl 5.10, it is possible to create a lexical version of the Perl default variable $_. Certain Perl constructs like the "given" keyword automatically use a lexical $_ rather than the global $_.

It is occasionallly useful for a sub to be able to access its caller's $_ variable regardless of whether it was lexical or not. The "(_)" sub prototype is the official way to do so, however there are sometimes disadvantages to this; in particular it can only appear as the final required argument in a prototype, and there is no way of the sub differentiating between an explicitly passed argument and $_.

This caused me problems with Scalar::Does, because I wanted to enable the "does" function to be called as either:

   does($thing, $role);
   does($role);  # assumes $thing = $_

With "_" in the prototype, $_ was passed to the function at the end of its argument list; effectively "does($role, $thing)", making it impossible to tell which argument was the role.

Enter "lexical::underscore" which allows you to access your caller's lexical $_ variable as easily as:

   ${lexical::underscore()}

You can access lexical $_ further up the call stack using:

   ${lexical::underscore($level)}

If you happen to ask for $_ at a level where no lexical $_ is available, you get the global $_ instead.

This module does work on Perl 5.8 but as there is no lexical $_, always returns the global $_.

Copyright and licence

This software is copyright (c) 2012, 2014 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

Disclaimer of warranties

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.