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Toby Inkster

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 <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.