Toby Inkster

NAME

lexical::underscore - access your caller's lexical underscore

SYNOPSIS

   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();

DESCRIPTION

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 $_.

Technical Details

The lexical::underscore function returns a scalar reference to either a lexical $_ variable somewhere up the call stack (using PadWalker magic), or to the global $_ if there was no lexical version.

Wrapping lexical::underscore in ${ ... } dereferences the scalar reference, allowing you to access (and even assign to) it.

BUGS

Please report any bugs to http://rt.cpan.org/Dist/Display.html?Queue=lexical-underscore.

SEE ALSO

PadWalker.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2012 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.




Hosting generously
sponsored by Bytemark