Scalar::Defer - Calculate values on demand use Scalar::Defer; # exports 'defer' and 'lazy' 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 This module exports two functions, "defer" and "lazy", for building 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 normal value, then "force" simply returns it. 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 into $dv and $lv above will simply replace the value, instead of triggering a "STORE" method call. Also, thanks to the "overload"-based implementation, this module is about 2x faster than Data::Lazy. Audrey Tang <cpan@audreyt.org> Copyright 2006 by Audrey Tang <cpan@audreyt.org>. Permission is hereby granted, free of charge, to any person obtaining a copy of this 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 fur- nished 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, FIT- NESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE X CONSORTIUM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.