Module Version: 0.01

# NAME

Hash::Lazy - A Hash implementation with lazy evaluation feature

# SYNOPSIS

Here's a fibonacci number calucator, recursively defined, memoized:

use Hash::Lazy;
my \$fib = Hash { my (\$h, \$k)= @_; return \$h->{\$k-1} + \$h->{\$k-2} };
\$fib->{0} = 0;
\$fib->{1} = 1;

say \$fib->{10}; # 55

# DESCRIPTION

Hash::Lazy is a way to have a lazy evaluated hash in your program. Unlike other Hash::* modules, it doesn't work on real hash variables (those with % sigil). Read on to see how it works.

This module exports a keyword Hash by default. If you cannot import this keyword in your current namespace for any reasons, you can re-named to 'lazy_hash' (or any other names or your choice) by saying:

use Hash::Lazy Hash => { -as => 'lazy_hash' };

This Hash keyword construct and returns an object that acts exactly like a normal hash reference. It requies a code block as its only argument. The block will be the builder to build the value of the wanted key. Therefore, the current hash object and the key will be the block arguments. This block need to either returns the value, or sets it back to \$hash.

# returns it
\$next = Hash {
my (\$hash, \$key) = @_;
return \$key + 1;
};

# or assign it
\$next = Hash {
my (\$hash, \$key) = @_;
\$hash->{\$key} = \$key + 1;
};

The later takes advantages of the former if you do both.

The returned object is just like a hash reference, therefore it's safe to assign values to it, or clear it:

# Clear all previously calculated values, and resets the seed.
%\$fib = ();
\$fib->{0} = 1;
\$fib->{1} = 1;

Notice that it's not ok to copy it to a hash:

%fib = %\$fib;

This basically freeze the magic-ness and the resulting %fib becomes static. The builder will not be called through any access to the %fib hash. Please be aware of this if you intend to do so.

The example code of fibonacci number above are also available at examples/fib.pl in the distribution tarball, or at http://cpansearch.perl.org/src/GUGOD/Hash-Lazy-0.01/examples/fib.pl.

# AUTHOR

Kang-min Liu <gugod@gugod.org>

Other lazy evaluation modules on CPAN, like Scalar::Lazy, Tie::Array::Lazy, Variable::Lazy, Data::Lazy, Data::Thunk, Scalar::Defer, Object::Lazy.

The Ruby Hash class constructor http://www.ruby-doc.org/core/classes/Hash.html.

The Perl 6 version of fibonacciy number calculation is really awesome, http://en.wikibooks.org/wiki/Fibonacci_number_program#Perl_6.

Copyright (c) 2009, Kang-min Liu <gugod@gugod.org>.

This is free software, licensed under:

The MIT (X11) License

# DISCLAIMER OF WARRANTY

BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR, OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENSE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

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