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.
