NAME

       Scalar::Defer - Lazy evaluation in Perl

SYNOPSIS

           use Scalar::Defer; # exports 'defer', 'lazy' and 'force'

           my ($x, $y);
           my $dv = defer { ++$x };    # a deferred value (not memoized)
           my $lv = lazy { ++$y };     # a lazy value (memoized)

           print "$dv $dv $dv"; # 1 2 3
           print "$lv $lv $lv"; # 1 1 1

           my $forced = force $dv;     # force a normal value out of $dv

           print "$forced $forced $forced"; # 4 4 4

DESCRIPTION

       This module exports two functions, "defer" and "lazy", for constructing values that are
       evaluated on demand.  It also exports a "force" function to force evaluation of a deferred
       value.

   defer {...}
       Takes a block or a code reference, and returns a deferred value.  Each time that value is
       demanded, the block is evaluated again to yield a fresh result.

   lazy {...}
       Like "defer", except the value is computed at most once.  Subsequent evaluation will
       simply use the cached result.

   force $value
       Force evaluation of a deferred value to return a normal value.  If $value was already a
       normal value, then "force" simply returns it.

   is_deferred $value
       Tells whether the argument is a deferred value or not. (Lazy values are deferred too.)

       The "is_deferred" function is not exported by default; to import it, name it explicitly in
       the import list.

NOTES

       Deferred values are not considered objects ("ref" on them returns 0), although you can
       still call methods on them, in which case the invocant is always the forced value.

       Unlike the "tie"-based Data::Lazy, this module operates on values, not variables.
       Therefore, assigning another value into $dv and $lv above will simply replace the value,
       instead of triggering a "STORE" method call.

       Similarily, assigning $dv or $dv into another variable will not trigger a "FETCH" method,
       but simply propagates the deferred value over without evaluationg.  This makes it much
       faster than a "tie"-based implementation -- even under the worst case scenario, where it's
       always immediately forced after creation, this module is still twice as fast than
       Data::Lazy.

CAVEATS

       Bad things may happen if this module interacts with any other code which fiddles with
       package 0.

       Performance of creating new deferred or lazy values can be quite poor under perl 5.8.9.
       This is due a bugfix since 5.8.8, where re-blessing an overloaded object caused bad
       interactions with other references to the same value.  5.8.9's solution involves walking
       the arenas to find all other references to the same object, which can cause "bless" (and
       thus "defer" in Scalar::Defer to be up to three orders of magnitude slower than usual.
       perl 5.10.0 and higher do not suffer from this problem.

SEE ALSO

       Data::Thunk, which implements "lazy" values that can replace itself upon forcing, leaving
       a minimal trace of the thunk, with some sneaky XS magic in Data::Swap.

AUTHORS

       Audrey Tang <cpan@audreyt.org>

COPYRIGHT

       Copyright 2006, 2007, 2008, 2009 by Audrey Tang <cpan@audreyt.org>.

       This software is released under the MIT license cited below.

   The "MIT" License
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       software and associated documentation files (the "Software"), to deal in the Software
       without restriction, including without limitation the rights to use, copy, modify, merge,
       publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
       to whom the Software is furnished to do so, subject to the following conditions:

       The above copyright notice and this permission notice shall be included in all copies or
       substantial portions of the Software.

       THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
       INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
       PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
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       DEALINGS IN THE SOFTWARE.