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Algorithm::C3(3)      User Contributed Perl Documentation     Algorithm::C3(3)

       Algorithm::C3 - A module for merging hierarchies using the C3 algorithm

	 use Algorithm::C3;

	 # merging a classic diamond
	 # inheritance graph like this:
	 #    <A>
	 #   /	 \
	 # <B>	 <C>
	 #   \	 /
	 #    <D>

	 my @merged = Algorithm::C3::merge(
	     sub {
		 # extract the ISA array
		 # from the package
		 no strict 'refs';
		 @{$_[0] . '::ISA'};

	 print join ", " => @merged; # prints D, B, C, A

       This module implements the C3 algorithm. I have broken this out into
       it's own module because I found myself copying and pasting it way too
       often for various needs. Most of the uses I have for C3 revolve around
       class building and metamodels, but it could also be used for things
       like dependency resolution as well since it tends to do such a nice job
       of preserving local precedence orderings.

       Below is a brief explanation of C3 taken from the Class::C3 module. For
       more detailed information, see the "SEE ALSO" section and the links

   What is C3?
       C3 is the name of an algorithm which aims to provide a sane method
       resolution order under multiple inheritance. It was first introduced in
       the language Dylan (see links in the "SEE ALSO" section), and then
       later adopted as the preferred MRO (Method Resolution Order) for the
       new-style classes in Python 2.3. Most recently it has been adopted as
       the 'canonical' MRO for Perl 6 classes, and the default MRO for Parrot
       objects as well.

   How does C3 work.
       C3 works by always preserving local precedence ordering. This
       essentially means that no class will appear before any of it's
       subclasses. Take the classic diamond inheritance pattern for instance:

	   /   \
	 <B>   <C>
	   \   /

       The standard Perl 5 MRO would be (D, B, A, C). The result being that A
       appears before C, even though C is the subclass of A.  The C3 MRO
       algorithm however, produces the following MRO (D, B, C, A), which does
       not have this same issue.

       This example is fairly trivial, for more complex examples and a deeper
       explanation, see the links in the "SEE ALSO" section.

       merge ($root, $func_to_fetch_parent, $cache)
	   This takes a $root node, which can be anything really it is up to
	   you. Then it takes a $func_to_fetch_parent which can be either a
	   CODE reference (see SYNOPSIS above for an example), or a string
	   containing a method name to be called on all the items being
	   linearized. An example of how this might look is below:

		 package A;

		 sub supers {
		     no strict 'refs';
		     @{$_[0] . '::ISA'};

		 package C;
		 our @ISA = ('A');
		 package B;
		 our @ISA = ('A');
		 package D;
		 our @ISA = ('B', 'C');

	     print join ", " => Algorithm::C3::merge('D', 'supers');

	   The purpose of $func_to_fetch_parent is to provide a way for
	   "merge" to extract the parents of $root. This is needed for C3 to
	   be able to do it's work.

	   The $cache parameter is an entirely optional performance measure,
	   and should not change behavior.

	   If supplied, it should be a hashref that merge can use as a private
	   cache between runs to speed things up.  Generally speaking, if you
	   will be calling merge many times on related things, and the parent
	   fetching function will return constant results given the same
	   arguments during all of these calls, you can and should reuse the
	   same shared cache hash for all of the calls.	 Example:

	     sub do_some_merging {
		 my %merge_cache;
		 my @foo_mro = Algorithm::C3::Merge('Foo', \&get_supers, \%merge_cache);
		 my @bar_mro = Algorithm::C3::Merge('Bar', \&get_supers, \%merge_cache);
		 my @baz_mro = Algorithm::C3::Merge('Baz', \&get_supers, \%merge_cache);
		 my @quux_mro = Algorithm::C3::Merge('Quux', \&get_supers, \%merge_cache);
		 # ...

       I use Devel::Cover to test the code coverage of my tests, below is the
       Devel::Cover report on this module's test suite.

	------------------------ ------ ------ ------ ------ ------ ------ ------
	File			   stmt	  bran	 cond	 sub	pod   time  total
	------------------------ ------ ------ ------ ------ ------ ------ ------
	Algorithm/		  100.0	 100.0	100.0  100.0  100.0  100.0  100.0
	------------------------ ------ ------ ------ ------ ------ ------ ------
	Total			  100.0	 100.0	100.0  100.0  100.0  100.0  100.0
	------------------------ ------ ------ ------ ------ ------ ------ ------

   The original Dylan paper

   The prototype Perl 6 Object Model uses C3

   Parrot now uses C3

   Python 2.3 MRO related links

   C3 for TinyCLOS

       Stevan Little, <>

       Brandon L. Black, <>

       Copyright 2006 by Infinity Interactive, Inc.


       This library is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

perl v5.16.2			  2013-08-25		      Algorithm::C3(3)

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