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https://github.com/bestpractical/scalar-defer


https://github.com/bestpractical/scalar-defer

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README 

        
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 



COPYRIGHT

    Copyright 2006, 2007, 2008 by Audrey Tang .



    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.