7 use Scalar::Util 'blessed';
11 # this is our global stash of both
12 # MRO's and method dispatch tables
13 # the structure basically looks like
17 # MRO => [ <class precendence list> ],
19 # orig => <original location of method>,
20 # code => \&<ref to original method>
26 # use these for debugging ...
27 sub _dump_MRO_table { %MRO }
32 # skip if the caller is main::
33 # since that is clearly not relevant
34 return if $class eq 'main';
35 return if $TURN_OFF_C3;
36 # make a note to calculate $class
44 # this will not run under the following
47 # - require Class::C3;
48 # - eval "use Class::C3"
49 # in all those cases, you need to call
50 # the initialize() function manually
54 # why bother if we don't have anything ...
55 return unless keys %MRO;
56 _calculate_method_dispatch_tables();
57 _apply_method_dispatch_tables();
58 %next::METHOD_CACHE = ();
62 # why bother if we don't have anything ...
63 return unless keys %MRO;
64 _remove_method_dispatch_tables();
65 %next::METHOD_CACHE = ();
70 # clean up the %MRO before we re-initialize
71 $MRO{$_} = undef foreach keys %MRO;
75 ## functions for applying C3 to classes
77 sub _calculate_method_dispatch_tables {
78 foreach my $class (keys %MRO) {
79 _calculate_method_dispatch_table($class);
83 sub _calculate_method_dispatch_table {
86 my @MRO = calculateMRO($class);
87 $MRO{$class} = { MRO => \@MRO };
90 # we do @MRO[1 .. $#MRO] here because it
91 # makes no sense to interogate the class
92 # which you are calculating for.
93 foreach my $local (@MRO[1 .. $#MRO]) {
94 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
95 # skip if already overriden in local class
96 next unless !defined *{"${class}::$method"}{CODE};
98 orig => "${local}::$method",
99 code => \&{"${local}::$method"}
100 } unless exists $methods{$method};
103 # now stash them in our %MRO table
104 $MRO{$class}->{methods} = \%methods;
107 sub _apply_method_dispatch_tables {
108 foreach my $class (keys %MRO) {
109 _apply_method_dispatch_table($class);
113 sub _apply_method_dispatch_table {
116 foreach my $method (keys %{$MRO{$class}->{methods}}) {
117 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
121 sub _remove_method_dispatch_tables {
122 foreach my $class (keys %MRO) {
123 _remove_method_dispatch_table($class);
127 sub _remove_method_dispatch_table {
130 foreach my $method (keys %{$MRO{$class}->{methods}}) {
131 delete ${"${class}::"}{$method};
135 ## functions for calculating C3 MRO
137 # this function is a perl-port of the
138 # python code on this page:
139 # http://www.python.org/2.3/mro.html
144 # remove all empty seqences
145 my @nonemptyseqs = (map { (@{$_} ? $_ : ()) } @seqs);
146 # return the list if we have no more no-empty sequences
147 return @res if not @nonemptyseqs;
148 my $cand; # a canidate ..
149 foreach my $seq (@nonemptyseqs) {
150 $cand = $seq->[0]; # get the head of the list
152 foreach my $sub_seq (@nonemptyseqs) {
153 # XXX - this is instead of the python "in"
154 my %in_tail = (map { $_ => 1 } @{$sub_seq}[ 1 .. $#{$sub_seq} ]);
156 # jump out as soon as we find one matching
157 # there is no reason not too. However, if
158 # we find one, then just remove the '&& last'
159 $nothead++ && last if exists $in_tail{$cand};
161 last unless $nothead; # leave the loop with our canidate ...
162 $cand = undef; # otherwise, reject it ...
164 die "Inconsistent hierarchy" if not $cand;
166 # now loop through our non-empties and pop
167 # off the head if it matches our canidate
168 foreach my $seq (@nonemptyseqs) {
169 shift @{$seq} if $seq->[0] eq $cand;
178 [ $class ], # the class we are linearizing
179 (map { [ calculateMRO($_) ] } @{"${class}::ISA"}), # the MRO of all the superclasses
180 [ @{"${class}::ISA"} ] # a list of all the superclasses
184 package # hide me from PAUSE
190 use Scalar::Util 'blessed';
192 our $VERSION = '0.03';
197 my @label = (split '::', (caller(1))[3]);
198 my $label = pop @label;
199 my $caller = join '::' => @label;
201 my $class = blessed($self) || $self;
203 goto &{ $METHOD_CACHE{"$class|$caller|$label"} ||= do {
205 my @MRO = Class::C3::calculateMRO($class);
208 while ($current = shift @MRO) {
209 last if $caller eq $current;
214 foreach my $class (@MRO) {
215 last if (defined ($found = *{$class . '::' . $label}{CODE}));
218 die "No next::method '$label' found for $self" unless $found;
232 Class::C3 - A pragma to use the C3 method resolution order algortihm
238 sub hello { 'A::hello' }
248 sub hello { 'C::hello' }
254 # Classic Diamond MI pattern
263 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
265 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
267 D->can('hello')->(); # can() also works correctly
268 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
272 This is currently an experimental pragma to change Perl 5's standard method resolution order
273 from depth-first left-to-right (a.k.a - pre-order) to the more sophisticated C3 method resolution
278 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
279 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
280 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
281 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
282 default MRO for Parrot objects as well.
284 =head2 How does C3 work.
286 C3 works by always preserving local precendence ordering. This essentially means that no class will
287 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
295 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
296 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
297 (D, B, C, A), which does not have this same issue.
299 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
300 the L<SEE ALSO> section.
302 =head2 How does this module work?
304 This module uses a technique similar to Perl 5's method caching. During the INIT phase, this module
305 calculates the MRO of all the classes which called C<use Class::C3>. It then gathers information from
306 the symbol tables of each of those classes, and builds a set of method aliases for the correct
307 dispatch ordering. Once all these C3-based method tables are created, it then adds the method aliases
308 into the local classes symbol table.
310 The end result is actually classes with pre-cached method dispatch. However, this caching does not
311 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
312 your classes to be effectively closed. See the L<CAVEATS> section for more details.
314 =head1 OPTIONAL LOWERCASE PRAGMA
316 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
317 the regular install since lowercase module names are considered I<"bad"> by some people. However I
318 think that code looks much nicer like this:
328 But hey, it's your choice, thats why it is optional.
334 =item B<calculateMRO ($class)>
336 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
340 This can be used to initalize the C3 method dispatch tables. You need to call this if you are running
341 under mod_perl, or in any other environment which does not run the INIT phase of the perl compiler.
344 This can B<not> be used to re-load the dispatch tables for all classes. Use C<reinitialize> for that.
346 =item B<uninitialize>
348 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
349 style dispatch order (depth-first, left-to-right).
351 =item B<reinitialize>
353 This effectively calls C<uninitialize> followed by C<initialize> the result of which is a reloading of
354 B<all> the calculated C3 dispatch tables.
356 It should be noted that if you have a large class library, this could potentially be a rather costly
361 =head1 METHOD REDISPATCHING
363 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
364 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
365 method along the C3 linearization. This is best show with an examples.
367 # a classic diamond MI pattern ...
381 sub foo { 'B::foo => ' . (shift)->next::method() }
386 sub foo { 'C::foo => ' . (shift)->next::method() }
391 sub foo { 'D::foo => ' . (shift)->next::method() }
393 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
395 A few things to note. First, we do not require you to add on the method name to the C<next::method>
396 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
397 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
399 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
400 not automatically use the current C<@_>.
404 Let me first say, this is an experimental module, and so it should not be used for anything other
405 then other experimentation for the time being.
407 That said, it is the authors intention to make this into a completely usable and production stable
408 module if possible. Time will tell.
410 And now, onto the caveats.
414 =item Use of C<SUPER::>.
416 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
417 However, it's use in conjuntion with this module is very much not recommended, and in fact very
418 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
419 more details on it's usage above.
421 =item Changing C<@ISA>.
423 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
424 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
425 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
426 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
427 in F<t/20_reinitialize.t> for more information.
429 =item Adding/deleting methods from class symbol tables.
431 This module calculates the MRO for each requested class during the INIT phase by interogatting the symbol
432 tables of said classes. So any symbol table manipulation which takes place after our INIT phase is run will
433 not be reflected in the calculated MRO. Just as with changing the C<@ISA>, you will need to call
434 C<reinitialize> for any changes you make to take effect.
444 You can never have enough tests :)
450 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this module's test suite.
452 ---------------------------- ------ ------ ------ ------ ------ ------ ------
453 File stmt bran cond sub pod time total
454 ---------------------------- ------ ------ ------ ------ ------ ------ ------
455 Class/C3.pm 99.2 93.3 66.7 96.0 100.0 92.8 96.3
456 ---------------------------- ------ ------ ------ ------ ------ ------ ------
457 Total 99.2 93.3 66.7 96.0 100.0 92.8 96.3
458 ---------------------------- ------ ------ ------ ------ ------ ------ ------
462 =head2 The original Dylan paper
466 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
470 =head2 The prototype Perl 6 Object Model uses C3
474 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
478 =head2 Parrot now uses C3
482 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
484 =item L<http://use.perl.org/~autrijus/journal/25768>
488 =head2 Python 2.3 MRO related links
492 =item L<http://www.python.org/2.3/mro.html>
494 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
498 =head2 C3 for TinyCLOS
502 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
508 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
510 =head1 COPYRIGHT AND LICENSE
512 Copyright 2005 by Infinity Interactive, Inc.
514 L<http://www.iinteractive.com>
516 This library is free software; you can redistribute it and/or modify
517 it under the same terms as Perl itself.