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
142 my $class_being_merged = $seqs[0]->[0];
145 # remove all empty seqences
146 my @nonemptyseqs = (map { (@{$_} ? $_ : ()) } @seqs);
147 # return the list if we have no more no-empty sequences
148 return @res if not @nonemptyseqs;
150 my $cand; # a canidate ..
151 foreach my $seq (@nonemptyseqs) {
152 $cand = $seq->[0]; # get the head of the list
154 foreach my $sub_seq (@nonemptyseqs) {
155 # XXX - this is instead of the python "in"
156 my %in_tail = (map { $_ => 1 } @{$sub_seq}[ 1 .. $#{$sub_seq} ]);
158 # jump out as soon as we find one matching
159 # there is no reason not too. However, if
160 # we find one, then just remove the '&& last'
161 $nothead++ && last if exists $in_tail{$cand};
163 last unless $nothead; # leave the loop with our canidate ...
165 $cand = undef; # otherwise, reject it ...
167 die "Inconsistent hierarchy found while merging '$class_being_merged':\n\t" .
168 "current merge results [\n\t\t" . (join ",\n\t\t" => @res) . "\n\t]\n\t" .
169 "mergeing failed on '$reject'\n" if not $cand;
171 # now loop through our non-empties and pop
172 # off the head if it matches our canidate
173 foreach my $seq (@nonemptyseqs) {
174 shift @{$seq} if $seq->[0] eq $cand;
183 [ $class ], # the class we are linearizing
184 (map { [ calculateMRO($_) ] } @{"${class}::ISA"}), # the MRO of all the superclasses
185 [ @{"${class}::ISA"} ] # a list of all the superclasses
189 package # hide me from PAUSE
195 use Scalar::Util 'blessed';
197 our $VERSION = '0.03';
202 my @label = (split '::', (caller(1))[3]);
203 my $label = pop @label;
204 my $caller = join '::' => @label;
206 my $class = blessed($self) || $self;
208 goto &{ $METHOD_CACHE{"$class|$caller|$label"} ||= do {
210 my @MRO = Class::C3::calculateMRO($class);
213 while ($current = shift @MRO) {
214 last if $caller eq $current;
219 foreach my $class (@MRO) {
220 last if (defined ($found = *{$class . '::' . $label}{CODE}));
223 die "No next::method '$label' found for $self" unless $found;
237 Class::C3 - A pragma to use the C3 method resolution order algortihm
243 sub hello { 'A::hello' }
253 sub hello { 'C::hello' }
259 # Classic Diamond MI pattern
268 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
270 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
272 D->can('hello')->(); # can() also works correctly
273 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
277 This is currently an experimental pragma to change Perl 5's standard method resolution order
278 from depth-first left-to-right (a.k.a - pre-order) to the more sophisticated C3 method resolution
283 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
284 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
285 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
286 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
287 default MRO for Parrot objects as well.
289 =head2 How does C3 work.
291 C3 works by always preserving local precendence ordering. This essentially means that no class will
292 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
300 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
301 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
302 (D, B, C, A), which does not have this same issue.
304 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
305 the L<SEE ALSO> section.
307 =head2 How does this module work?
309 This module uses a technique similar to Perl 5's method caching. During the INIT phase, this module
310 calculates the MRO of all the classes which called C<use Class::C3>. It then gathers information from
311 the symbol tables of each of those classes, and builds a set of method aliases for the correct
312 dispatch ordering. Once all these C3-based method tables are created, it then adds the method aliases
313 into the local classes symbol table.
315 The end result is actually classes with pre-cached method dispatch. However, this caching does not
316 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
317 your classes to be effectively closed. See the L<CAVEATS> section for more details.
319 =head1 OPTIONAL LOWERCASE PRAGMA
321 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
322 the regular install since lowercase module names are considered I<"bad"> by some people. However I
323 think that code looks much nicer like this:
333 But hey, it's your choice, thats why it is optional.
339 =item B<calculateMRO ($class)>
341 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
345 This can be used to initalize the C3 method dispatch tables. You need to call this if you are running
346 under mod_perl, or in any other environment which does not run the INIT phase of the perl compiler.
349 This can B<not> be used to re-load the dispatch tables for all classes. Use C<reinitialize> for that.
351 =item B<uninitialize>
353 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
354 style dispatch order (depth-first, left-to-right).
356 =item B<reinitialize>
358 This effectively calls C<uninitialize> followed by C<initialize> the result of which is a reloading of
359 B<all> the calculated C3 dispatch tables.
361 It should be noted that if you have a large class library, this could potentially be a rather costly
366 =head1 METHOD REDISPATCHING
368 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
369 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
370 method along the C3 linearization. This is best show with an examples.
372 # a classic diamond MI pattern ...
386 sub foo { 'B::foo => ' . (shift)->next::method() }
391 sub foo { 'C::foo => ' . (shift)->next::method() }
396 sub foo { 'D::foo => ' . (shift)->next::method() }
398 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
400 A few things to note. First, we do not require you to add on the method name to the C<next::method>
401 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
402 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
404 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
405 not automatically use the current C<@_>.
409 Let me first say, this is an experimental module, and so it should not be used for anything other
410 then other experimentation for the time being.
412 That said, it is the authors intention to make this into a completely usable and production stable
413 module if possible. Time will tell.
415 And now, onto the caveats.
419 =item Use of C<SUPER::>.
421 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
422 However, it's use in conjuntion with this module is very much not recommended, and in fact very
423 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
424 more details on it's usage above.
426 =item Changing C<@ISA>.
428 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
429 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
430 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
431 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
432 in F<t/20_reinitialize.t> for more information.
434 =item Adding/deleting methods from class symbol tables.
436 This module calculates the MRO for each requested class during the INIT phase by interogatting the symbol
437 tables of said classes. So any symbol table manipulation which takes place after our INIT phase is run will
438 not be reflected in the calculated MRO. Just as with changing the C<@ISA>, you will need to call
439 C<reinitialize> for any changes you make to take effect.
449 You can never have enough tests :)
455 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.
457 ---------------------------- ------ ------ ------ ------ ------ ------ ------
458 File stmt bran cond sub pod time total
459 ---------------------------- ------ ------ ------ ------ ------ ------ ------
460 Class/C3.pm 99.2 93.3 66.7 96.0 100.0 92.8 96.3
461 ---------------------------- ------ ------ ------ ------ ------ ------ ------
462 Total 99.2 93.3 66.7 96.0 100.0 92.8 96.3
463 ---------------------------- ------ ------ ------ ------ ------ ------ ------
467 =head2 The original Dylan paper
471 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
475 =head2 The prototype Perl 6 Object Model uses C3
479 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
483 =head2 Parrot now uses C3
487 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
489 =item L<http://use.perl.org/~autrijus/journal/25768>
493 =head2 Python 2.3 MRO related links
497 =item L<http://www.python.org/2.3/mro.html>
499 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
503 =head2 C3 for TinyCLOS
507 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
513 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
515 =head1 COPYRIGHT AND LICENSE
517 Copyright 2005 by Infinity Interactive, Inc.
519 L<http://www.iinteractive.com>
521 This library is free software; you can redistribute it and/or modify
522 it under the same terms as Perl itself.