NAME
   Sub::Lazy - defer calculating subs until necessary

SYNOPSIS
      use strict;
      use Test::More;
      use Sub::Lazy;

      my $it_happens = 0;

      sub double :Lazy {
         $it_happens++;  # side-effect

         my $n = shift;
         return $n * 2;
      }

      my $eight = double(4);

      # The 'double' function hasn't been executed yet.
      is($it_happens, 0);

      # The correct answer was calculated.
      is($eight, 8);

      # The 'double' function was executed when necessary.
      is($it_happens, 1);

      done_testing;

DESCRIPTION
   Sub::Lazy allows you to mark subs as candidates for lazy evaluation. Good
   candidates for lazy evaluation:

   *   Have no side-effects. They don't alter global variables; they don't
       make use of any closed-over lexical variables; they don't do IO or
       make system calls.

   *   Are only called in scalar context. This module always imposes a scalar
       context on subs. (Of course the sub can return an arrayref.)

   The actual work is done by Data::Thunk. Data::Thunk is awesome but it does
   have its limitations. It's not completely transparent (if you try hard
   enough, you can tell the difference between a value that has not been
   calculated yet and one that has) and it will sometimes be over-eager to
   calculate a value. But it's probably the best solution for lazy scalars on
   CPAN, so I've reused it rather than writing a half-arsed replacement for
   it.

   This module defines an atttribute (`:Lazy`) to allow you to wrap a sub
   with Data::Thunk, making the whole business a little easier.

   If your function is known to always return an instance of a particular
   class, then you can specify that:

      sub get_manager :Lazy(class=>Person) {
         ...;
      }

   Sub::Lazy will then use Data::Thunk's `lazy_object` feature, which allows
   Data::Thunk to further postpone evaluation of the sub in some cases.

   You can even patch in further details about the object you are returning:

      sub get_manager :Lazy(class=>Person,job_title=>"Manager") {
         ...;
      }

   Now `get_manager(...)->job_title` will return "Manager" without needing to
   evaluate `get_manager`.

ENVIRONMENT
   Setting the `PERL_SUB_LAZY_DISABLE` environment variable to true allows
   you to disable the effects of this module. Subs will be run eagerly. This
   environment variable needs to be set *prior* to loading Sub::Lazy. It is a
   global off switch.

   `Sub::Lazy::ENABLED`
       Checks the status of the global off switch.

BUGS
   Please report any bugs to
   <http://rt.cpan.org/Dist/Display.html?Queue=Sub-Lazy>.

SEE ALSO
   Data::Thunk.

AUTHOR
   Toby Inkster <[email protected]>.

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.