7 our $VERSION = '0.15_01';
12 eval "require mro"; # XXX in the future, this should be a version check
14 die $@ if $@ !~ /locate/;
15 # eval "require Class::C3::XS";
17 # die $@ if $@ !~ /locate/;
18 eval "require Algorithm::C3; require Class::C3::next";
27 # this is our global stash of both
28 # MRO's and method dispatch tables
29 # the structure basically looks like
33 # MRO => [ <class precendence list> ],
35 # orig => <original location of method>,
36 # code => \&<ref to original method>
38 # has_overload_fallback => (1 | 0)
43 # use these for debugging ...
44 sub _dump_MRO_table { %MRO }
47 # state tracking for initialize()/uninitialize()
48 our $_initialized = 0;
52 # skip if the caller is main::
53 # since that is clearly not relevant
54 return if $class eq 'main';
56 return if $TURN_OFF_C3;
57 mro::set_mro($class, 'c3') if $C3_IN_CORE;
59 # make a note to calculate $class
61 $MRO{$class} = undef unless exists $MRO{$class};
67 %next::METHOD_CACHE = ();
68 # why bother if we don't have anything ...
69 return unless keys %MRO;
71 mro::set_mro($_, 'c3') for keys %MRO;
76 $MRO{$_} = undef foreach keys %MRO;
78 _calculate_method_dispatch_tables();
79 _apply_method_dispatch_tables();
85 # why bother if we don't have anything ...
86 %next::METHOD_CACHE = ();
87 return unless keys %MRO;
89 mro::set_mro($_, 'dfs') for keys %MRO;
92 _remove_method_dispatch_tables();
97 sub reinitialize { goto &initialize }
99 ## functions for applying C3 to classes
101 sub _calculate_method_dispatch_tables {
102 return if $C3_IN_CORE;
104 foreach my $class (keys %MRO) {
105 _calculate_method_dispatch_table($class, \%merge_cache);
109 sub _calculate_method_dispatch_table {
110 return if $C3_IN_CORE;
111 my ($class, $merge_cache) = @_;
113 my @MRO = calculateMRO($class, $merge_cache);
114 $MRO{$class} = { MRO => \@MRO };
115 my $has_overload_fallback = 0;
118 # we do @MRO[1 .. $#MRO] here because it
119 # makes no sense to interogate the class
120 # which you are calculating for.
121 foreach my $local (@MRO[1 .. $#MRO]) {
122 # if overload has tagged this module to
123 # have use "fallback", then we want to
125 $has_overload_fallback = ${"${local}::()"}
126 if defined ${"${local}::()"};
127 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
128 # skip if already overriden in local class
129 next unless !defined *{"${class}::$method"}{CODE};
130 $methods{$method} = {
131 orig => "${local}::$method",
132 code => \&{"${local}::$method"}
133 } unless exists $methods{$method};
136 # now stash them in our %MRO table
137 $MRO{$class}->{methods} = \%methods;
138 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
141 sub _apply_method_dispatch_tables {
142 return if $C3_IN_CORE;
143 foreach my $class (keys %MRO) {
144 _apply_method_dispatch_table($class);
148 sub _apply_method_dispatch_table {
149 return if $C3_IN_CORE;
152 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
153 if $MRO{$class}->{has_overload_fallback};
154 foreach my $method (keys %{$MRO{$class}->{methods}}) {
155 if ( $method =~ /^\(/ ) {
156 my $orig = $MRO{$class}->{methods}->{$method}->{orig};
157 ${"${class}::$method"} = $$orig if defined $$orig;
159 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
163 sub _remove_method_dispatch_tables {
164 return if $C3_IN_CORE;
165 foreach my $class (keys %MRO) {
166 _remove_method_dispatch_table($class);
170 sub _remove_method_dispatch_table {
171 return if $C3_IN_CORE;
174 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
175 foreach my $method (keys %{$MRO{$class}->{methods}}) {
176 delete ${"${class}::"}{$method}
177 if defined *{"${class}::${method}"}{CODE} &&
178 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
183 my ($class, $merge_cache) = @_;
185 return @{mro::get_linear_isa($class)} if $C3_IN_CORE;
187 return Algorithm::C3::merge($class, sub {
201 Class::C3 - A pragma to use the C3 method resolution order algortihm
207 sub hello { 'A::hello' }
217 sub hello { 'C::hello' }
223 # Classic Diamond MI pattern
232 # initializez the C3 module
233 # (formerly called in INIT)
234 Class::C3::initialize();
236 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
238 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
240 D->can('hello')->(); # can() also works correctly
241 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
243 =head1 SPECIAL NOTE FOR 0.15_01
245 To try this with the experimental perl core c3 patch,
246 download a recent copy perl-current:
248 http://mirrors.develooper.com/perl/APC/perl-current-snap/perl-current@30943.tar.bz2
250 apply the enclosed c3.patch, and install this perl:
252 sh Configure -Dusedevel -Dprefix=/where/I/want/it -d -e && make && make test && make install
254 then try your C3-using software against this perl + Class::C3 0.15_01.
258 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
259 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
263 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
264 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
265 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
266 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
267 default MRO for Parrot objects as well.
269 =head2 How does C3 work.
271 C3 works by always preserving local precendence ordering. This essentially means that no class will
272 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
280 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
281 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
282 (D, B, C, A), which does not have this same issue.
284 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
285 the L<SEE ALSO> section.
287 =head2 How does this module work?
289 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
290 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
291 gathers information from the symbol tables of each of those classes, and builds a set of method
292 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
293 then adds the method aliases into the local classes symbol table.
295 The end result is actually classes with pre-cached method dispatch. However, this caching does not
296 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
297 your classes to be effectively closed. See the L<CAVEATS> section for more details.
299 =head1 OPTIONAL LOWERCASE PRAGMA
301 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
302 the regular install since lowercase module names are considered I<"bad"> by some people. However I
303 think that code looks much nicer like this:
313 But hey, it's your choice, thats why it is optional.
319 =item B<calculateMRO ($class)>
321 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
325 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
326 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
327 use C3. Here is a quick code example:
331 # ... Foo methods here
336 # ... Bar methods here
340 Class::C3::initialize(); # now it is safe to use Foo and Bar
342 This function used to be called automatically for you in the INIT phase of the perl compiler, but
343 that lead to warnings if this module was required at runtime. After discussion with my user base
344 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
345 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
346 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
347 your own INIT method which calls this function.
351 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
352 clear the MRO cache first.
354 =item B<uninitialize>
356 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
357 style dispatch order (depth-first, left-to-right).
359 =item B<reinitialize>
361 This is an alias for L</initialize> above.
365 =head1 METHOD REDISPATCHING
367 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
368 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
369 method along the C3 linearization. This is best show with an examples.
371 # a classic diamond MI pattern ...
385 sub foo { 'B::foo => ' . (shift)->next::method() }
390 sub foo { 'C::foo => ' . (shift)->next::method() }
395 sub foo { 'D::foo => ' . (shift)->next::method() }
397 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
399 A few things to note. First, we do not require you to add on the method name to the C<next::method>
400 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
401 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
403 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
404 not automatically use the current C<@_>.
406 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
407 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
409 $self->next::method(@_) if $self->next::can;
411 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
412 The previous example could be simply written as:
414 $self->maybe::next::method(@_);
416 There are some caveats about using C<next::method>, see below for those.
420 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
421 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
422 whatever your needs might be.
424 But there are still caveats, so here goes ...
428 =item Use of C<SUPER::>.
430 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
431 However, it's use in conjuntion with this module is very much not recommended, and in fact very
432 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
433 more details on it's usage above.
435 =item Changing C<@ISA>.
437 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
438 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
439 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
440 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
441 in F<t/20_reinitialize.t> for more information.
443 =item Adding/deleting methods from class symbol tables.
445 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
446 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
447 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
448 changes you make to take effect.
450 =item Calling C<next::method> from methods defined outside the class
452 There is an edge case when using C<next::method> from within a subroutine which was created in a different
453 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
454 will not work correctly:
456 *Foo::foo = sub { (shift)->next::method(@_) };
458 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
459 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
460 uses C<caller> to find the name of the method it was called in, it will fail in this case.
462 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
463 assign a name to an anonymous subroutine for you. Simply do this:
465 use Sub::Name 'subname';
466 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
468 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
469 manage to find a workaround for it, so until someone gives me a working patch this will be a known
470 limitation of this module.
476 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
479 ---------------------------- ------ ------ ------ ------ ------ ------ ------
480 File stmt bran cond sub pod time total
481 ---------------------------- ------ ------ ------ ------ ------ ------ ------
482 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
483 ---------------------------- ------ ------ ------ ------ ------ ------ ------
484 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
485 ---------------------------- ------ ------ ------ ------ ------ ------ ------
489 =head2 The original Dylan paper
493 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
497 =head2 The prototype Perl 6 Object Model uses C3
501 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
505 =head2 Parrot now uses C3
509 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
511 =item L<http://use.perl.org/~autrijus/journal/25768>
515 =head2 Python 2.3 MRO related links
519 =item L<http://www.python.org/2.3/mro.html>
521 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
525 =head2 C3 for TinyCLOS
529 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
533 =head1 ACKNOWLEGEMENTS
537 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
538 and finding many bugs and providing fixes.
540 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
541 calls inside C<eval> and anon-subs.
543 =item Thanks to Robert Norris for adding support for C<next::can> and
544 C<maybe::next::method>.
550 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
552 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
554 =head1 COPYRIGHT AND LICENSE
556 Copyright 2005, 2006 by Infinity Interactive, Inc.
558 L<http://www.iinteractive.com>
560 This library is free software; you can redistribute it and/or modify
561 it under the same terms as Perl itself.