NAME¶
Algorithm::C3 - A module for merging hierarchies using the C3 algorithm
SYNOPSIS¶
use Algorithm::C3;
# merging a classic diamond
# inheritance graph like this:
#
# <A>
# / \
# <B> <C>
# \ /
# <D>
my @merged = Algorithm::C3::merge(
'D',
sub {
# extract the ISA array
# from the package
no strict 'refs';
@{$_[0] . '::ISA'};
}
);
print join ", " => @merged; # prints D, B, C, A
DESCRIPTION¶
This module implements the C3 algorithm. I have broken this out into it's own
module because I found myself copying and pasting it way too often for various
needs. Most of the uses I have for C3 revolve around class building and
metamodels, but it could also be used for things like dependency resolution as
well since it tends to do such a nice job of preserving local precedence
orderings.
Below is a brief explanation of C3 taken from the Class::C3 module. For more
detailed information, see the "SEE ALSO" section and the links
there.
What is C3?¶
C3 is the name of an algorithm which aims to provide a sane method resolution
order under multiple inheritance. It was first introduced in the language
Dylan (see links in the "SEE ALSO" section), and then later adopted
as the preferred MRO (Method Resolution Order) for the new-style classes in
Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl
6 classes, and the default MRO for Parrot objects as well.
How does C3 work.¶
C3 works by always preserving local precedence ordering. This essentially means
that no class will appear before any of it's subclasses. Take the classic
diamond inheritance pattern for instance:
<A>
/ \
<B> <C>
\ /
<D>
The standard Perl 5 MRO would be (D, B, A, C). The result being that
A
appears before
C, even though
C is the subclass of
A. The
C3 MRO algorithm however, produces the following MRO (D, B, C, A), which does
not have this same issue.
This example is fairly trivial, for more complex examples and a deeper
explanation, see the links in the "SEE ALSO" section.
FUNCTION¶
- merge ($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);
# ...
}
CODE COVERAGE¶
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/C3.pm 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
------------------------ ------ ------ ------ ------ ------ ------ ------
SEE ALSO¶
The original Dylan paper¶
- <http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
The prototype Perl 6 Object Model uses C3¶
- <http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
Parrot now uses C3¶
- <http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
- <http://use.perl.org/~autrijus/journal/25768>
- <http://www.python.org/2.3/mro.html>
- <http://www.python.org/2.2.2/descrintro.html#mro>
C3 for TinyCLOS¶
- <http://www.call-with-current-continuation.org/eggs/c3.html>
AUTHORS¶
Stevan Little, <stevan@iinteractive.com>
Brandon L. Black, <blblack@gmail.com>
COPYRIGHT AND LICENSE¶
Copyright 2006 by Infinity Interactive, Inc.
<
http://www.iinteractive.com>
This library is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.