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.
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 <
[email protected]>
COPYRIGHT
Copyright 2006, 2007, 2008 by Audrey Tang <
[email protected]>.
This software is released under the MIT license cited below.
The "MIT" License
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 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 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.
